1 CHAPTER 1 INTRODUCTION 1.1 Background
In present scenario, the supply of electricity in Nepal lags way behind the actual demand of the nation. So, an ambitious plan to harness 10000 MW of electric potential in period of 10 years has been proposed by the government. In order to achieve the plan so proposed, expertise at high level and core competence is required among local manufacturers and technical experts. Such requirements have brought technological expertise in the country to their toes to develop excellence in their field. Therefore, Kathmandu University came up with an idea to fulfil the bridge of technological expertise in hydro turbines sector with establishment of Turbine Testing Laboratory (TTL), which is currently in construction phase within the University premises, (Turbine Testing Laboratory, 2009).
Kathmandu University has intended to establish a turbine testing laboratory for the purpose of education, research and development in close cooperation with indigenous industries. To meet this formidable task, Kathmandu University has entered into a technical cooperation with Norwegian University for Science and Technology (NTNU)/Hydropower Lab and NORAD as major sponsor. The Turbine Testing Laboratory is aimed to come in operation by mid 2011, (Turbine Testing Laboratory, 2009).
1.2 Purpose, Project Goals and Success Criteria 1.2.1 Purpose
The purposes of establishing a Turbine Testing Laboratory at Kathmandu Industry are listed below:
Purpose 1:build competence and knowledge within the hydropower sector of Nepal
Teaching/learning facility
Industrial courses
2 Purpose 2: motivate research
Development of efficient turbines able to withstand sand erosion.
Development of turbine and pump technology
Maintenance of turbines
Purpose 3: provide a meeting place for the industry and university
for research and student projects for the industry
open doors for collaborative research with national and international universities and research institutions,
1.2.2 Success Criteria
The project goals and long time success criteria of establishment of Turbine Testing Laboratory at Kathmandu University are:
KU’s own planning and design capacity is enhanced to the extent that a similar future project can be implemented in a 50% shorter time period with at least the same quality The five year goal after the completion of the project are:
Testing and certification of at least 3 numbers of mini or micro turbines produced in Nepal for internal or external parties
At least 5 students at Master level use the TTL for their research.
At least 1 student actively uses TTL for his/her PhD work.
Undertake a minimum of 3 industrial courses or professional courses for researchers.
Publish a minimum of 3 articles in internationally recognized technical magazines. (Turbine Testing Laboratory, 2009)
3 1.3 Organization
Figure 1.1 Organizational charts, TTL, KU
Project Steering Committee
Chairman Registrar Dean SOE,
Reps from CED, Acc Dept, Proj Man
Appoint management Approve plans and budget
Rep
Project Management
Proj Man T Skeie Assist Proj Man R Shrestha
Implement project according to plans and budget
Civil contractor
Access road Landscaping Building Electrical wiringHydro-Mech.
Contractor
Pipes System Weir EOT Crane Design teamCivil/structural: CED Bajracharya Electro-mech: D Bista, R Skrestha
Hydro-mech: R.Shrestha
Design of structures and equipment
Accounting Dept Fin Man K Baral
Account reports
External advisors NTNU Prof Dalhaug, Brandåstrø
Advise on technical matters includ design Assist in testing and commissioning
Training
Electro- Mech.
Contractor
Pumps Frequency Controllers Control Panel4 CHAPTER 2
TECHNICAL SPECIFICATION OF THE COMPANY 2.1 Lab specification
30 metres open system head
150 metres closed system head
⁄ maximum flow
300 kW maximum testing capacity
capacity lower ( lab) reservoir
capcity upper reservoir
5000 kg EOT crane capacity
2.2 Block diagram of Turbine Testing Lab
5 2.3 Schematic diagram of Turbine Testing Lab
Figure 2.3: Schematic diagram of Turbine Testing Lab
In figure 2.3, the numbers 1 to 10 represent the multi turn valve with electric actuator and
a = Pump a
b = Pump b
c = upper reservoir
d = lower (lab) reservoir
j = drain valve 2.4 Standarization of Tests
Turbine testing lab will follow International Electrotechnical Comission (IEC-60193) standard for conducting model tests.
2.5 On going research and development activities
Two PhD and four masters students are currently working for the new design of Francis Turbine.
Fault analysis of 12 MW pelton runner of Khimti Hydropower plant is under progress.
Patnership with ESAP/RRE for suitable projects is under discussion.
Extension of Renewable Nepal project for establsihing local hydropower manufacturing company is under planning phase.
6 CHAPTER 3
TRAINING DETAILS 3.1 Training Methodology
3.1.1 Introduction
Introduction to the Turbine Testing lab (TTL)
Purpose of lab 3.1.2 Work Assigned
Literature review
Design review
Field survey and documentation
Design and fabrication of head measurement prototype
Test of the prototype
3.1.3 Recommendation for selection 3.1.4 Observation
Observation of work in progress to build the lab
Observation of drawings of the lab
Materials and Equipments used in the lab
Management of the Technical personnels 3.1.5 Work Accomplished
Literature review Weirs
Head measurement mechanism
Flow Meters and calibration procedure
Design review
Weir installed in the lab
Field survey and documentation
Design and fabrication of head measurement prototype
Test of the prototype
Recommendation for selection 3.1.6 Documentation
7 CHAPTER 4
WORK ACCOMPLISHED DETAILS 4.1 LITERATURE REVIEW ON WEIR
4.1.1 Background
The weir is one of the oldest structures used to measure the flow of water in open channels. Several rating equations were developed for standard rectangular contracted weirs by different investigators. Generally, the data of each investigator are within +1.5 to +2.5 percent with respect to their individual equations, but comparisons of the various equations differ as much as several percent (King and Brater, 1976; Ackers et al., 1978).
In the past, user organizations selected an equation, called it standard, and specified construction requirements and limitations of use. However, Kindsvater and Carter (1959) developed an improved method for computing rates of flow through rectangular, thin-plate weirs. Their method also applies to fully side suppressed, partially contracted, and fully contracted rectangular weirs. Kulin and Compton (1975) discuss the method and equation for rating fully contracted V-notch weirs with any angle between 25 degrees and 100 degrees. This method also rates partially contracted 90-degree, V notch weirs.
The Kindsvater approach accounts for velocity of approach effects and the accompanying variation of discharge coefficient caused by changes of effective width and head. This method is preferred for calibrating or rating rectangular and triangular weirs. Also, this method will correct for excess approach velocity in standard weirs. Thus, this newer approach will accurately recalibrate some of the older weirs that are no longer operating as standard, as well as some that never were standard.
4.1.2 Definition of Weirs
A measuring weir is simply an overflow structure built perpendicular to an open channel axis to measure the rate of flow of water. Inspecting and checking the critical parts of weir structures for degradation and improper operation are easy.
A properly built and operated weir of a given shape has a unique depth of water at the measuring station in the upstream pool for each discharge. Thus, weirs can be rated with respect to an upstream head relative to the crest elevation versus discharge, and equations or tables which
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apply to the particular shape and size weir can be generated. The crest overflow shape governs how the discharge varies with head measurement.
4.1.3 Principle
The discharge over thin plate weirs is a function of the head on the weir, the size and the shape of the discharge area, and an experimentally determined co-efficient which takes into account the head on the weir, the geometrical properties of the weir and approach channel and the dynamical properties of the water.
4.1.4 Installation 4.1.4.1 Selection of site
This type of weir to be used for discharge measurement is determined in part by nature of the proposed measuring site. Under some conditions of design and use, weirs shall be located in rectangular flumes or in weirs boxes which stimulate flow conditions in rectangular flumes. Under other conditions, weirs may be located in natural channels as well as flumes or weirs boxes with no significant difference in measurement accuracy. Specific site-related requirements of the installation are described in 6.3
4.1.4.2 Installation conditions 4.1.4.2.1 General
Weir discharge is critically influenced by the physical characteristics of the weir and the weir channel. Thin-plate weirs are especially dependent on installation features which control the velocity distribution conformance with standard specifications.
4.1.4.2.2 Weir
Thin plate weirs shall be vertical and perpendicular to the walls of the channel. The intersection of the weir plate with the walls and floor of the channel shall be watertight and firm, and the weir shall be capable of withstanding the maximum flow without distortion or damage.
Stated practical limits associated with different discharge formulae such as minimum width, minimum weir height, minimum head, and maximum values of h/p and b/B (where h is the measured head, p is the height of crest relative to floor, b is the measured width of the notch and
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B is the width of the approach channel), are factors which influence both the selection of weir type and the installation.
4.1.4.2.3 Approach Channel
For the purpose of this international standard the approach channel is that portion of the weir channel which extends upstream from the weir a distance not less than ten times the width of the nappe at maximum head. If the weir is located in a weir box, the length of the box shall be equal to the specified length of the approach channel.
The flow in the approach channel shall be uniform and steady, with the velocity distribution approximating that in a channel of sufficient length to develop normal (resistance- controlled) flow in smooth, straight channels. Figure 1 shows measured normal velocity distributions in rectangular channels, upstream from the influence of a weir. Baffles and flow straighteners can be used to stimulate normal velocity distribution, but their location with respect to the weir shall be not less than the minimum length prescribed for the approach channel.
The influence of approach-channel velocity distribution on weir flow increases as h/p and b/B increase in magnitude. If a weir installation unavoidably results in a velocity distribution which is appreciably uniform, the possibility of error in calculated discharge should be checked by means of an alternative discharge-measuring method for a representative range of discharges. 4.1.4.2.4 Downstream channel
The shape and size of the channel downstream from the weir is of no significance, but the level of the water in the downstream channel shall be a sufficient vertical distance below the crest to ensure free, fully ventilated discharges, Free( no submerged) discharge is ensured when the discharge is independent of the downstream water level. Fully ventilated discharge is ensured when the air pressure on the lower surface of the nappe is fully atmospheric.
4.1.4.2.5 Measurement of head 4.1.4.2.5.1 Head measuring devices
In order to obtain discharge measurement accuracies specified for the standard weirs, the head on the weir shall be measured with a laboratory-grade hook gauge, point gauge, manometer, or other gauge of equivalent accuracy. For a continuous record of head variations, precise float
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gauges and servo operated point gauges can be used. Staff and tape gauges can be used when less accurate measurements are acceptable.
4.1.4.2.5.2 Stilling Well
For the exceptional case where surface velocities and disturbances in the approach channel are negligible, the headwater level can be measured directly (for example, by means of a point gauge mounted over the headwater surface). Generally, however, to avoid water-level variations caused by waves, turbulence or vibration, the headwater level should be measured in stilling well.
Stilling well is connected to the approach channel by means of a suitable conduit, equipped if necessary with throttle valve to damp oscillations. At the channel end of the conduit, the connection is made to floor or wall piezometers or a static tube located at the head measurement section.
4.1.4.2.5.3 Head Measurement section
The head measurement section shall be located a sufficient distance upstream from the weir to avoid the region of surface drawdown caused by the formation of the nappe. On the other hand it shall be sufficiently close to the weir that the energy loss between the head measurement section and the weir is negligible. For the weirs included in this International standard the location of the head measurement section will be satisfactory if it is at a distance equal to 4 t0 5 times the maximum head (4 to 5 h max) upstream from the water.
If high velocities occur in the approach channel or if water-surface disturbances or irregularities occur at the head-measurement section because of high values of h/p or b/B, it may be necessary to install several pressure intakes to ensure that the head measured in stilling well is the average of the heads at the several measurement points.
4.1.4.2.5.4 Head-gauge Zero
Accuracy of head measurement is critically dependent upon the determination of the head-gauge datum or gauge zero, which is defined as the gauge reading corresponding to the level of the weir crest( rectangular weirs) or the level of the vertex of the notch (triangular -notch weirs). When necessary, the gauge zero shall be checked. Numerous acceptable methods of determining the gauge zero are in use. Typical methods are described in subsequent clauses dealing specifically with rectangular and triangular weirs.
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Because of surface tension, the gauge zero cannot be determined with sufficient accuracy by reading the head gauge with the water in the approach channel drawn down to the apparent crest (or notch) level.
4.1.4.2.5.5 Maintenance
Maintenance of the weir and the weir channel is necessary to ensure accurate measurements. The approach channel shall be kept free of silt, vegetation and obstructions which might have deleterious effects on the flow conditions specified for the standard installation. The downstream channel shall be kept free of obstructions which might cause submergence or inhibit full ventilation of the nappe under all conditions of flow.
The weir plate shall be kept clean and firmly secured. In the process of cleaning, care shall be taken to a avoid damage to the crest or notch, particularly the upstream edges and surfaces. Construction specifications for these most sensitive features should be reviewed before maintenance is under taken.
Head- measurement piezometers, connecting conduits and the stilling well shall be cleaned and checked foe leakage. The hook or point gauge, manometer, float or other instruments used to measure the head shall be checked periodically to ensure accuracy.
4.1.5 Types of weir
4.1.5.1 Rectangular thin plate weirs 4.1.5.1.1 Types
The rectangular thin plate weir is a general classification in which the rectangular-notch weir is the basic form and the full-width weir is a limiting case. A diagrammatic illustration of the basic weir form is shown in figure 2 with intermediate values of b/B and h/p. When b/B is 1.0 that is when the width of the weir (b) id equal to the width of the channel at the weir section (B), the weir is of full-width type (also referred to as a “suppressed” weir, because its nappe lacks side contractions.
4.1.5.1.2 Specifications for the standard weir
The basic weir form consists of a rectangular notch in a vertical, thin plate. The plate shall be plane and rigid and perpendicular to the walls and the floor of the approach channel. The
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upstream face of the plate shall be smooth (in the vicinity of the notch it shall be equivalent in surface finish to that of rolled sheet-metal).
The vertical bisector of the notch shall be equidistant from the two walls of the channel. The crest surface of the notch shall be horizontal, plane surface, which shall form a sharp edge at its intersection with the upstream face of the weir plate. For the limiting case of the full-width weir, the crest of the weir shall extend to the walls of the channel, which in the vicinity of the crest shall be plane and smooth.
To ensure that the upstream edges of the crest and the sides of the notch are sharp, they shall be machined or filled, perpendicular to the upstream face of the weir plate, free of burrs or scratches and untouched by abrasive cloth or paper. The downstream edges of the notch shall be chamfered if the plate is thicker than the maximum allowable width of the notch surface. The surface of the chamfer shall make an angle of not less than Pi/4 radians (45 degree) with the crest and side surfaces of the notch. The weir plate in the vicinity of the notch preferably shall be made of corrosion-resistant metal; but if it is not, all specified smooth surfaces and sharp edges shall be kept coated with a thin, protective film( for example, oil, wax, silicone) applied with a soft cloth.
4.1.5.1.3 Specifications for installations
The specifications stated in 6.3 shall apply. In general, the weir shall be located In straight , horizontal, rectangular, approach channel if possible. However, if the effective opening of the notch is so small in comparison with the area of the upstream channel that the approach velocity is negligible; the shape of the channel is not significant. In any case, the flow in the approach channel shall be uniform and steady.
If the width of the weir is equal to the width of the channel at the weir section (i.e. a full-width weir), the sides of the channel upstream from the plane of the weir shall be vertical, plane, parallel and smooth (equivalent in surface finish to that of rolled sheet metal). The sides of the channel above the crest of a full-width weir shall extend at least 0.3 h max downstream from the plane of the weir. Fully ventilated discharge shall be ensured as specified in 6.3.4.
The approach channel floor shall be smooth, flat and horizontal when the height of the crest relative to the floor (p) is small and /or h/p is large. For rectangular weirs, the floor should be
13
smooth, flat and horizontal, particularly when p is less than 0.1 m and/or h max /p is greater than 1. Additional conditions are specified in connection with the recommended discharge formulae. 4.1.5.1.4 Specifications for head measurement
4.1.5.1.4.1 Determination of gauge zero
The head gauge datum or gauge zero shall be determined with great care, and it shall be checked when necessary. A typical, acceptable method of determining the gauge zero for rectangular weirs is described as follows
a) Still water in the approach channel is drawn to a level below the weir crest.
b) A temporary hook gauge is mounted over the approach channel, a short distance upstream from the weir crest
c) A precise machinists’ level is placed with its axis horizontal, with one end lying on the weir crest and the other end on the point of the temporary hook gauge (the gauge having been adjusted to hold the level in the position). The reading of the temporary gauge is recorded;
d) The temporary hook gauge is lowered to the water surface in the approach channel and its reading is recorded. The permanent gauge is adjusted to read the level in the stilling well, and this reading is recorded;
e) The computed difference between the two readings of the temporary gauge is added to the reading of the permanent gauge. The sum is the gauge zero for the permanent gauge. 4.1.5.1.5 Discharge formulae- general
Recommended discharge formulae for rectangular thin-plate weirs are presented in two categories:
a) Formulae for the basic weir form ( all values of b/B); b) Formulae for full width weirs(b/B = 1.0)
Common symbols used in the formulae are defined as follows:
Q is the volume rate of flow, in cubic meters per second; C is the coefficient of discharge (non-dimensional);
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b is the measured width of the notch, in meters; B is the width of the approach channel in meters h is the measured height , in meters;
p is the height of the crest relative to the floor, in meters.
(Note: Additional symbols are defined following their first occurrence in a formula)
a) Formulae for the basic weir form (all values of b/B) 4.5.1.5.1 Kindsvater- Carter formulae
The Kindsvater-Carter formula for the basic weir form is
……… (1) In which
is the coefficient of discharge; is the effective width;
is the effective head; Evaluation of , AND
Figure 4 shows experimentally determined values of as a function of h/p for representation values of b/B. Values of for immediate values of b/B can be determined by interpolation. The coefficient of discharge has been determined by experiment as a function of two variables from the formula
………... ……. (2)
The effective width and head are defined by the equations
………...…….. (3) ……….. (4)
15
In which and are experimentally determined quantities in meters, which compensate for the combined effects of viscosity and surface tension.
Figure 5 shows values of , which have been experimentally determined as a function of b/B Experiments have shown that can be taken to have a constant value of 0.001 m for weirs constructed in strict conformance with recommended specifications.
Formulae For
For specific values of b/B the relationship between and h/p has been shown by experiment (see figure 4) to be of the linear form,
Thus for the values of b/B sown on figure 4 formulae for can be written as follows:
……….. (5) ………... (6) ……… ……….. (7) ……… ……….. (8) ………... (9) ……… .(10) ……… …….. (11) ……… ……. (12)
(Note: For intermediate values of b/B, formulae for can be determined satisfactorily by interpolation)
16 Practical limitations on h/p, h, b and p
Practical limits are placed on h/p because head-measurement difficulties and errors result from surges and waves which occur in the approach channel at larger values for h/p. Limits are placed on h to avoid “clinging nappe” phenomenon which occurs at very low heads. Limits are placed on b because of uncertainties regarding the combined effects of viscosity and surface tension represented by the quantity at very small values of b. Limits are placed on p and B-b to avoid the instabilities which result from eddies that from in the corners between the channel boundaries and the weir when values of p and B-b are small.
For conservative practice, limitations applicable to use of the Kindsvater- Carter formula are: a) h/p shall not be greater than 2.5;
b) h shall be not less than 0.03m; c) b shall be not less than 0.15m; d) p shall be not less than 0.10m;
e) either (B-b)/2 = 0 (full width weir) or (B-b)/2 shall not be less than 0.10m ( concentrated weir)
4.1.5.1.5.2 SIA formula
The SIA formula for the basic weir form is:
………. (13)
In which
………. (14)
Practical limitations applicable to the use of the SIA formula are a) h/p shall be not greater than 1.0;
b) b/B shall be not less than 0.3;
c) h shall be not less than 0.025 B/b and not greater than 0.80m; d) p shall be not less than 0.30m
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………. (15)
Formulae for full width weirs (b/B = 1.0)
In addition to formulae (5) and (15), which represent the limiting case of b/B = 1.0 in the Kindsvater-Carter and SIA formulae for weirs of the basic form, the following formulae are recommended for b/B = 1.0 only.
4.1.5.1.5.3 Rehbock Formula (1929)
The Rehbock formula in the form proposed in 1929 is of the effective head variety:
………. ..(16)
In which
……….. (17)
……….. (18) Practical limitations applicable to the use of the Rehbock formula are:
a) h/p shall be not greater than 1.0; b) h shall be between 0.03 and 0.75m; c) b shall be not less than 0.30m; d) p shall be not less than 0.10m; 4.1.5.1.5.4 IMFT Formula
The IMFT formula for full-width weir is:
………(19)
In which
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In which is the average velocity in the approach channel, , in which is the area of the flow at the head-measurement section.
Because is a function of Q, it must be computed by successive approximations. Practical limitations applicable to the use of the IMFT formula are:
a) h/p shall be not greater than 2.5; b) h shall be not less than 0.03m; c) b shall be not less than 0.20m; d) p shall be not less than 0.10m;
4.1.5.1.6 Accuracy of discharge coefficient-rectangular weirs
The accuracy of discharge measurements made with a rectangular thin-plate weir depends primarily on the accuracy of the head and width measurements and on the applicability of the discharge formula and coefficients used. If great care is exercised in meeting the construction, installation, and operational conditions specified in this international standard uncertainties (at 95% confidence level) attributable to the coefficients of discharge will be not greater than 1.5% for values of h/p less than 1.0, not greater than 2% for values of h/p between 1.0 and 1.5 and not greater than 3% for values of h/p between 1.5 and 2.5. The specified uncertainties are applicable only if the additional restrictions on values of h, b, p, h/p and (B-b)/2 given in 9.6 and 9.7 are applied. The combination of all uncertainties which contribute significantly to the uncertainty of discharge measurements is treated in clause 11. Examples of estimated uncertainties in measured discharge are given in clause 12.
4.1.5.2 Triangular Notch thin plate weir 4.1.5.2.1 Specifications for the standard weir
The triangular-notch thin plate weir consists of a V-shaped notch in a vertical, thin plate. A diagrammatic illustration of the triangular-notch weir is shown in figure-6. The weir plate shall be plane and rigid and perpendicular to the walls and floor of the channel. The upstream face of the plate shall be smooth (in the vicinity of the notch it shall be equivalent in surface finish to that of rolled sheet-metal).
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The bisector of the notch shall be vertical and equivalent from the two walls of the channel. The surfaces of the notch shall be plane surfaces, which shall form sharp edges at their intersection with the upstream face of the weir plate. The width of the notch surfaces, measured perpendicular to the face of the plate, shall be between 1 to 2 mm.
To ensure that the upstream edges of the notch are sharp, they shall be machined or filed, perpendicular to the upstream face of the plate, free of blurs or scratches and untouched by abrasive cloth or paper. The downstream edges of the notch shall be chamfered if the weir plate is thicker than the maximum allowable width of the notch surface. The surface of the chamfer shall make an angle of not less than π/4 radians (45) with the surfaces of the notch preferably shall be made of corrosion-resistant metal; but if it is not, all specified smooth surfaces shall be kept coated with a a thin protective film ( for example oil, wax, silicone) applied with a soft cloth.
4.1.5.2.2 Specifications for the installation
The specification stated in 6.2 shall apply. In general, the weir shall be located in a straight, horizontal, rectangular channel if possible. However, if the effective opening of the notch is so small in comparison with the area of the upstream channel that the approach velocity is negligible; the shape of the channel is not significant. In any case, the flow in the approach channel shall be uniform and steady, as specified in 6.3.3.
If the top width of the nappe at maximum head is large in comparison with the width of the channel, the channel walls shall be straight, vertical and parallel. If the height of the vertex relative to the level of the floor is small in comparison with the maximum head , the channel floor shall be smooth, flat and horizontal. In general the approach channel should be smooth, straight and rectangular when B/b max is less than 3 and/or h max/p is greater than 1. Additional conditions are specified in connection with the recommended discharge formulae.
4.1.5.2.3 Specifications for head measurement 4.1.5.2.3.1 Determination of notch angle
Precise head measurement for triangular-notch weirs require that the notch angle (angle included between sides of the notch) measured accurately. One of several satisfactory methods is described as follows:
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a) Two true disks of different, micro metered diameters are placed in the notch with their edges tangent to the sides of the notch.
b) The vertical distance between the centers for two corresponding edges) of the two disks is measured with a micrometer caliper.
c) The notch angle α is twice the angle whose sine is equal to the difference between the radii of the disks divided by the distance between the centers of the disks.
4.1.5.2.3.2 Determination of gauge zero
The head-gauge datum or gauge zero shall be determined with great care, it shall be checked when necessary. A typical acceptable method of determining the gauge zero for triangular-notch weirs is described as follows:
a) Still water in the approach channel is drawn to a level below the vertex of the notch. b) A temporary hook gauge is mounted over the approach channel, with its point a short
distance upstream from the vertex of the notch.
c) A true cylinder of known (micro-metered diameter is placed with its axis horizontal, with one end resting in the notch and the other end balanced on the point of the temporary hook gauge. A machinists’ level is placed on the top of the cylinder precisely horizontal. The reading of the temporary gauge is recorded.
d) The temporary hook gauge is lowered to the water surface in the approach channel and the reading is recorded. The permanent gauge is adjusted to read the level in the stilling well, and this reading is recorded.
e) The distance (Y) from the bottom of the cylinder to the vertex of the notch is computed with the known value of notch angle (α) and the radius (r) of the cylinder i.e.
This distance is then subtracted from the reading recorded in c), the result being the reading of the temporary gauge at the vertex of the notch.
f) The difference between the computed reading in e) and the reading of the temporary in d) is added to the reading of the permanent gauge in d). The sum is the gauge zero for the permanent gauge.
An advantage of this method is that it refers the gauge zero to the geometrical vertex which is defined by the sides of the notch.
21 4.1.5.2.4 Discharge Formulae- General
Recommended discharge formulae for triangular notch thin-plate weirs are presented in two categories:
a) Formula for all notch between π/9 and 5π/9 radians (20° and 100°); b) Formulae for specific notch angles (fully contracted weirs
Common symbols used in the formulae are defined as follows: Q is the volume rate of flow, in cubic meters per second; C is the coefficient of discharge (non-dimensional);
g is the acceleration due to gravity, in meters per second squared;
α is the notch angle, i.e., the angle included between the sides of the notch, in degrees; h is the measured head in metres
(Note: Additional special symbols are defined following their first occurrence in a formula)
Formula for notch angles between π/9 and 5π/9 radians (20° and 100°) The Kindsvater Shen formula for triangular notch weir is:
……….. (21) in which
is the coefficient of discharge is the effective head
The coefficient of discharge has been determined by experiment as a function of three variables (see figure 7)
……….. (22)
In which
p is the height of the vertex of the notch with respect to the floor of the approach channel B is the width of the approach channel
22 is defined by the equation,
……… (23)
In which is an experimentally determined quantity, in .meters, which compensates for the combined effects of viscosity and surface tension.
Evaluation of and
For triangular weirs with notch angle α equal to π/2 radians (90°), figure 7 shows experimentally determined values of for a wide range of values of h/p and p/B. For α = π/2 radians (90°), has been shown to have a constant value of 0.00085 for a corresponding range of values of h/p and p/B.
For notch angles other than π/2 radians (90°), experimental data are insufficient to define as a function of h/p and p/B. However, for weir notches which are small relative to the area of the approach channel, the velocity of approach is negligible and the effects of h/p and p/b are also negligible. For this condition (the so-called “fully-contracted” condition), figure 8 shows experimentally determined values of as a function alone. Corresponding values of are shown in figure 9.
Practical limitations on α, h/p, p/B, h and p
For reasons related to hazards of measurement-error and lack of experimental data, the following practical limits are applicable to the use of the kindsvater-Shen formula:
a) α shall be between π/9 and 5π/9 radians ( 20° and 100°);
b) h/p shall be limited to the range shown on figure 7 for α = π/2 radians (90°); h/p shall be not greater than 0.35 for other values of α;
c) p/B shall be limited to the range shown on figure 7 for α = π/2 radians (90°); p/B shall be between 0.10 and 1.5 for other values of α.
d) h shall be not less than 0.06m e) p shall be not less than 0.09m.
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Formula for specific notch angles (fully contracted-weir) BSI formula for three related angles
This formula is for notch angles which have a special geometric relationship to each other; a) tangent α/2 =1 (α = π/2 radians or 90°);
b) tangent α/2 = 0.50 (α = 0.9273 radian or 53° 8′); c) tangent α/2 = 0.25 (α = 0.4899 radian or 28° 4′); The BSI discharge formula is:
………(24)
And the experimentally determined values of C and Q for the condition of “full contraction” are shown in tables 1, 2 and 3.
Practical limitations applicable to the use of this formula are: a) h/p shall be not greater than 0.4;
b) h/B shall be not greater than 0.2; c) h shall be between 0.05 and 0.38m; d) p shall be not less than 0.45m; e) B shall be not less than 1.0m;
Accuracy of discharge coefficients- Triangular-notch weirs
The accuracy of discharge measurements made with a triangular-notch thin-plate weir depends primarily on the accuracy of the head and notch-angle measurements and on the applicability of the discharge formula and coefficients used. If great care is exercised in meeting the construction, installation, and operational conditions specified in this international standard, uncertainties (at 95% confidence level) attributable to the coefficients of discharge will be not greater than 1.0%.
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4.1.6 Criteria for the selection of standard weirs and flumes
The essential criteria for selection from among the standard weirs and flumes are given below: 4.1.6.1 Available difference in water levels
Thin-plate weirs and free over-falls require a sufficient difference between upstream and downstream water levels which will ensure free, fully ventilated flow under conditions of maximum discharge.
Broad crested weirs may be used with relatively smaller differences in water level; triangular-profile weirs and standing wave flumes may be used with even smaller differences in water level. For all types of weirs and flumes included in this international standard the discharge should be free or independent of the downstream water level.
4.1.6.2 Accuracy of measurement
The accuracy in a single determination, of discharge depends upon the estimation of the component uncertainties involved but approximate range of uncertainties for the weirs and flumes (at 95% confidence levels) are as follows
Rectangular thin-plate weirs (full width and notch): 1 to 4%
Triangular notch weirs (notch angles between π/9 and 5π/9 radians or 20° to 100°): 1 to 2%.
Broad crested weirs: 3 to 5%
Triangular-profile weirs: 2 to 5%
Standing-wave flumes: 2 to 5%
Free over-fall: 5 to 10%
Deviations from the standard construction, installation or use may result in larger measurement errors. The larger figures given above are recommended conservative values for use under conditions of strict conformance with standard specifications. The smallest values can be obtained only for weirs under vigorous control, such as may be built and installed in well-equipped laboratories. Under field conditions, thin plate weirs are specially subject to errors caused by natural hazards.
25 4.1.6.3 Dimensions and shape of open channel
Rectangular full-width weirs and notch weirs (both rectangular and triangular), of large size relative to the size of the approach channel, should be located in vertical-walled level-floored rectangular channels, or in weir boxes of rectangular channels, or in weir boxes of rectangular cross-section for a distance extending upstream not less than 10 times the width of the nappe at maximum head. For thin plate weirs of small size relative to the size of the approach channel, especially if the velocity of approach is negligible, the size and shape of the channel is of no importance.
Broad crested weirs are best used in rectangular channels, but they can be used with good accuracy in non-rectangular channels if a smooth, rectangular approach channel extends upstream from the weir a distance not than twice the maximum head.
Flumes can be used in channels of any shape if flow conditions in the approach channel are reasonably uniform and steady.
For weirs and flumes of all types the size and shape of the downstream channel are of no significance except that they permit free, fully ventilated flow under all conditions of use.
4.1.6.4 Flow conditions in the approach channel
For weirs of all types, flow in the approach channel shall be sub-critical, uniform and steady. Ideally, especially for relatively high velocities of approach, the velocity distribution should approximate that in a channel of sufficient length to develop normal (resistance-controlled) flow in straight, smooth channels. For relatively low velocities of approach and for flumes, flow conditions in the channel are of less importance. In short channels and weir boxes, baffles and flow-straighteners may be used to stimulate normal velocity distribution. Care should be taken to ensure that erosion and/or deposition upstream of the weir or flume do not significantly alter the velocity of approach or velocity distribution to the measurement structure. Sub-critical flow is ensured when
26 In which
is the average velocity in the approach channel on meters per second; g is the acceleration due to gravity in meters per second squared; A is the cross-sectional area of the channel, in square meters;
is the width of the channel at the water surface, in meters. 4.1.6.5 Flow with sediment load
For flows with suspended load, the use of thin-plate weirs should be avoided because the crest edge may be damaged or worn by the suspended materials. On streams with bed load, use of measurement structures which significantly reduce the stream velocity is not recommended as it may result in changing deposition-scour dependent on flow regime. Flumes will generally perform than weirs on streams with sediment load.
4.1.6.6 Flow with floating debris
Broad crested weirs, triangular profile weirs, standing wave flumes and free overfall structures will normally pass floating debris more effectively than thin-plate weirs. The use of the triangular notch (V-notch) weir in particular should be avoided unless a debris trap is installed upstream.
4.1.6.7 Magnitude of discharge to be measured
For reasons related to accuracy and construction, thin plate weirs are best used for the measurement of relatively small discharges. Broad-crested weirs, triangular-profile weirs and flumes are best used for large discharges.
4.1.6.8 Range of discharge to be measured
For best overall accuracy over a wide range of small discharges, a triangular-notch (v-notch) weir should be used in preference to a rectangular-notch or rectangular full-width weir. For a wide range of larger discharges, a trapezoidal-throat or U-throat flumes should be used in preference to a broad-crested weir, free over-fall, rectangular-throated flume or triangular-profile weir.
27 4.1.6.9 Construction
Thin plate weirs must be constructed with precision tools under machine shop conditions. Flumes, broad crested weirs, triangular-profile weirs and free over-falls can be constructed satisfactorily in the field. In all cases, great care must be exercised in making the structures conform to standard specifications.
Broad crested weirs, triangular weirs, free over-falls and flumes are inherently stronger and more easily maintained under conditions of high heads in large channels.
4.2 Literature review on Weir head measurement mechanism 4.2.1 Measurement of Head (Introduction)
Selecting the proper water measurement device for a particular site or situation is not an easy task. Many site-specific factors and variables must be considered and weighed. In addition, each system has unique operational requirements and concerns. Reliable estimates on future demands of the proposed system and knowledge of the immediate measurement needs are beneficial. Factors influencing the accuracy of a single flow-rate measurement were considered and the importance of accurate measurement of upstream sill referenced head was discussed in this chapter. In fact, the measurement of head is so important that the success or failure of the measuring structure often depends almost entirely upon the effectiveness of the gage, sensor, or recorder used.
When we use the term sill-referenced head, we mean that the head is measured with respect to the invert of the control section of the structure-i.e., the section at which the flow passes through critical depth. This control section is located in the flume throat at a distance of about L/3 from the downstream edge of the sill. In the direction of flow, the top of the sill (weir crest or invert of flume throat) must be truly level. If minor undulations in the elevation of the sill occur along its length, we recommend that the level at the control section be used as the sill-reference level rather than taking the average along the length of the sill. If the sill is intended to be horizontal in the direction perpendicular to the flow, then the average level across the width of the sill at the control section should be used as the sill-reference level.
The gauging or head-measurement station should be located sufficiently far upstream from the structure to avoid the area of water surface drawdown, yet it should be close enough for the
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energy loss between the gauging station and the structure to be negligible. This means it will be located at a distance between two and three times from the leading edge of the sill or at
from the beginning of the converging transition, whichever is greater (Figure 7.1)
If only occasional flow measurements are required, the water level at the gauging station can be measured by a vertical or an inclined gage installed in the approach channel. If continuous flow records are needed, or if the flow rate is to be transmitted electronically to a distant location, a water level transducer and/or an automatic recorder will be needed. Regardless of the type of head-measurement device used, it should be located to one side of the approach channel to minimize its interference with the flow approaching the structure.
Figure 4.1 Schematic diagram showing general terminology and location of gaging station and Control.
4.2.2 Types of Head Measuring Devices 4.2.2.1 Gages
When continuous measurements of flow rate are not needed, or in channels where the fluctuation of flow is gradual, periodic readings on a calibrated physical gage may be satisfactory. Depending on the type of flume and the required accuracy of the head reading (see previous section), a point gage, dipstick, or staff gage may be used.
29 4.2.2.2 Point gage
A point gage is the most accurate head-measurement instrument (error of 0.1 mm). Its use is normally restricted to research facilities. The point gage is always used in combination with a stilling well. The point gage consists of a pointed, graduated rod suspended above the water surface and raised or lowered in relation to a fixed measurement scale, often including a vernier scale to increase measurement accuracy. The rod is lowered until the point just touches the water surface, and the vertical position of the point is then read from the vemier scale.
4.2.2.3 Dipstick
A dip stick is essentially a stick or rod that is calibrated to indicate level. It is inserted into the water in the stilling well until the end of the stick rests on a base corresponding to the exact sill-reference level of the structure. Usually the bottom of the tank is used to ensure that the dip stick is inserted to the correct depth. Reading the scale on the dip stick indicates the level measurement. The stilling well used in combination with a dipstick should have a sufficiently large diameter so that the stick does not raise the water level upon insertion. Even then, the stick should be inserted slowly until it rests on its reference point. A dipstick can supply very accurate information on head (error of 0.001 m). Most portable RBC flumes use a hardwood dipstick that is directly marked in flow rate units.
A lead line acts in the same way as a dip stick. A steel measuring tape with a weight attached, the lead line can be used in most places that the dip stick can. Since the lead line can be rolled up into a smaller, compact unit, it is often easier to handle than a dip stick.
4.2.2.4 Sight Glasses and Gage Glasses
The sight glass is an important method for visually determining level. The sight glass is a transparent tube of glass or plastic mounted outside the vessel and connected to the vessel with pipes. The liquid level in the sight glass matches the level of liquid in the process tank.
In process systems that contain a liquid under high pressure a reflex sight glass is used. This device is armored, to permit it to tolerate higher temperatures and higher pressures. Gage glasses are typically glass covered ports in a vessel that make it possible to observe the level of the substance in the vessel. Many gage glasses will have a scale mounted on the tank that allows the level to be read.
30 4.2.2.5 Staff Gage
A staff gage should be placed in such a manner that the water level can be read from the canal bank and so that its surface can be cleaned by the observer. For earthen channels, the gage can be mounted vertically on a support structure placed in the flowing stream. The support structure should not interfere with the flow of water through the flume throat or over the weir crest, and it should not catch floating debris.
For concrete-lined canals, the gage can be mounted directly on the canal wall. For sloping canal walls, the length indicated on the gage will be greater than the corresponding vertical water depth. The relative slope lengths versus vertical lengths for the most commonly used side slopes are shown in Figure 4.2
Within an irrigation project, it is convenient to mark the gages of structures in Liter/s, m3/s, ft3/s, or other units of discharge rather than in head units. Once the gage has been mounted and checked, this eliminates the possibility of using the wrong rating tables. Direct read-out gages can also be used on movable weirs.
With the software one can calculate a basic rating table showing discharge versus head (one discharge value in each line of the table). The software will also provide the vertical gage marking distances for a direct reading gage. In contrast, the ditch rider’s rating table can be printed with either the vertical gage marking distances or the distances along a sloped canal bank, allowing the use of a standard linear gage installed on a slope. The wall gage module of the software also can provide the gage dimensions and produce wall gages for mounting directly against the slope of the approach channel. For this gage, the marks need not be more than about 3 or 4 cm apart, since interpolation between marks will give reasonable accuracy. For example, on the gage shown, there is a 2.5 cm difference in elevation (4.5 cm along the sloped wall gage) between 2.20 and 2.40 m3/s. Interpolation between these marks by eye is relatively easy. With experience, an observer can easily read the discharge to within ±4% of the true discharge.
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Figure 4.2 Multipliers for layout of an inclined page
Figure 4.3 The inclined gage is mounted against the right canal bank. The gage is labeled with discharge units.
Most permanent gages are constructed from paltes of enameled steel, cast aluminum, or polyester. Baked enamel steel gages with linear scales are available from commercial sources. These gages will last for a very long time. Gages marked in discharge units can be custom-ordered in large quantities, but are considerably more expensive. Spray enamel paints with UV protection can also be used to make gages on steel. These are not as durable as the baked enamel, but are considerably less expensive. Gages in discharge units can also be made by dtamping
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aluminum but stock with a hammer, chisel, or metal-stamping discs. These gages require periodic cleaning, so they must be accessible.
4.2.2.6 Automatic recorders and water level sensors
Automatic water-level recorders create a permanent a record of the variation of water surface elevations as a function of time. A sensor converts the water level into physical motion and/or an electrical signal that can be recorded on paper, magnetic tape, or other electronic form by the recorder (data logger). Automatic recorder systems have several advantages over ordinary gages:
In channels with daily fluctuations of flow, continuous records provide the most accurate means of determining the daily average and total flow.
The entire hydrograph is recorded with the maximum and minimum water levels as a function of time. This provides data on the reaction time of the channel system to upstream changes in flow.
Observations can be made at remote places where observers are not available, or in locations that are not accessible under all conditions.
A number of meteorological instrument manufacturers produce a variety of commercially sensors and recorder. In some cases, the sensor and recorder systems are integrated together into a single system, while in other cases the sensor and recorder are separate devices. The type of sensor chosen for the site can have important ramifications for the design of the structure and appurtenances. Some of the most common types of sensors and their important characteristics are described below.
4.2.2.7 Submerged Pressure Transducers
Pressure transducers convert the hydrostatic pressure of water at a given depth into an electrical signal that can be recorded. Transducers are available in many different configurations that exploit a variety of properties of different materials or devices to accomplish this conversion. Submerged pressure transducers can be suspended in a stilling well or installed in a protective pipe which is perforated to admit water. The transducer is fastened in place, submerged below the minimum expected water level. To produce an output of gage pressure (i.e., referenced to ambient atmospheric pressure), the transducer is vented to the atmosphere via a vent tube integrated into the cable carrying the electronic output signal of the transducer. The free end of
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the vent tube should terminate in the instrument enclosure, and a desiccant should be used to prevent water vapor entry into the vent tube, as this can lead to corrosion of the transducer and errors in its output. A flexible bladder can be used as a desiccant replacement, provided that expansion and contraction of the bladder does not change the pressure (i.e., the pressure inside and outside the bladder must be the same)
Advantages of pressure transducers are
The relative simplicity of installation, since a stilling well is not required, and their accuracy, which can range from ±1.0 to ±0.1 percent of the maximum range that can be measured by the transducer.
Accuracy and cost are generally proportional. Disadvantages include
The need to maintain the desiccant .pack associated with the vent tube and the requirement to protect the transducer from freezing or remove it from service during the winter.
The most significant calibration issue for pressure transducers is avoiding drift of their output at zero pressure, since this can lead to relatively large percentage errors in flow rate at minimum discharge conditions.
Fouling of the opening to the transducer can also be a problem. 4.2.2.8 Pressure Bulb
This instrument consists of a flexible bulb that is placed in a perforated metal container for protection and connected by an air tube to a mechanical pressure gage and recorder or to a pressure transducer with an electronic output. The container and flexible bulb are fixed in place below the minimum water level to be recorded. Any change in water level changes the pressure inside the system and thus is recorded.
Advantages of this recorder are
The container and bulb do not require a stilling well and the distance between the bulb and the recorder may be up to 50 m (1 75 ft).Hence, the installation of the system is simple and relatively cheap while the recorder can be placed at a suitable location.
34 The major disadvantage of the pressure bulb is
The error in the recorded water level is generally ±2% of the maximum range that can be measured by the recorder. If this range, for example, is 1.0 m, the error in recorded head is ±0.02 m for all heads. As a result, at the minimum flow condition the measured flow rate can be rather inaccurate. Also, system leaks can cause operational failure.
Despite these disadvantages, the pressure bulb is very suitable for relatively temporary installations and sites at which the greatest accuracy is not necessary. A regular calibration between the staff-gage reading and the recorded water level is required for this type of instrument to maintain sufficient accuracy.
4.2.2.9 Bubblers
This instrument consists of a tube that is usually fastened with its open end at least 0.05 m below the lowest water level to be recorded. The tube is connected to a supply of air from a cylinder of compressed air or a small compressor and to a pressure gage or a pressure transducer plus a recorder. Air flows very slowly from the open end of the tube, and the pressure required to overcome the head of water above the end of the pipe is measured and recorded. The method by which the pressure is measured and recorded may be similar to that of the pressure bulb or may involve recent electronic devices. The advantages and disadvantages are somewhat similar to the pressure transducer system already described, except that the transducer is not submerged, so it need not be removed in freezing weather and there is less scaling and fouling of the transducer. Not submerging the transducer has proven to dramatically improve the reliability of bubbler systems compared to submerged pressure sensors.
Relatively long transmitting distances can be achieved with the bubbler system. Installations of 300 m have been used. On these long lines, it is best to use two small 3-mm inside-diameter tubes for economy and accuracy. One tube carries the bubble air supply from the source to the bubble outlet at 3 to 5 bubbles per second. The second tube is attached as a branch line as near as practical to the bubble outlet, preferably within 2 to 5 m. This second tube then senses the bubble pressure at the desired distance. Because there is essentially no flow in the sensing line after stabilization, there are no appreciable friction losses. On the source line, even 3 to 5 bubbles per second cause a significant pressure drop in several hundred meters, and thus the source line cannot also serve as the sensing line at long distances. Thermal gradients and pressure change in
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the sensing line may become significant only if large vertical distances are encountered. Figure 7.3 illustrates the schematic arrangement of a remote bubble gage.
Transmission distance is limited primarily by the allowable response time needed for a change in flow to be detected. The gage becomes more sluggish with increasing sensing line length because larger volumes of air must be moved to achieve a new stable pressure reading. For 300 m of 3-mm line (inside diameter), stability is usually reached in several seconds, depending on the volume sensing requirements of the pressure sensing gage. For example, a large-bore manometer requires more volume shift than a small pressure gage, but the manometer may be more sensitive.
Figure 4.4 Schematic for a remote recording bubbler system that is not sensitive to transmission distances up to 300 m.
Self-contained bubbler systems have been developed in recent years that integrate a small pressure compressor and optional pressure tank, the transducer, and associated electronics into a low-power unit that can easily function continuously on solar power (Figure 4.5). Another variation on the bubbler concept is the double-bubbler, in which bubbles are delivered alternately through two different tubes that terminate a fixed vertical distance apart in the water column. If the same transducer is used to sense the atmospheric pressure and the pressure in each tube, one can compensate for changes in the transducer calibration, producing a more accurate
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measurement. Accuracy of commercially available bubbler systems has improved in recent years and can now be on the order of ±0.003 m (±0.001 ft).
4.2.2.10 Ultrasonic level sensors
Ultrasonic level sensors are mounted above the water surface and determine the position of the free surface by measuring the transit time of an acoustic pulse that travels from the sensor down to the water surface and is reflected back up to the sensor. To achieve useful accuracy, the sensor must be temperature compensated, since the speed of sound in air varies with temperature. -Ultrasonic level sensors can be installed with or without a stilling well; a stilling well is preferred because it reduces waves on the water surface that can reduce the measurement accuracy. Details of the particular sensor should be considered when designing the stilling well, as the acoustic signal transmitted by the sensor radiates out in a cone pattern. The signal thus may be reflected back up to the sensor off the walls of the stilling well (especially if the walls are rough), causing the sensor to measure this distance rather than the distance to the water surface. Conversely, installing the sensor directly above a relatively small diameter smooth-walled pipe that extends down into the water works well with some sensors. This is because the acoustic signal is not reflected back up the pipe due to the flat angle of incidence of the acoustic signal with the pipe wall.
Figure 4.5 A self-contained bubbler water level sensor. (Courtesy Digital Control Corporation, Largo, Florida, USA)
37 Advantages of ultrasonic level sensors are
The relative ease of installation and the fact that they do not physically contact the water surface, making them a good choice for sites with pollutes or corrosive waters.
Disadvantages are
They have only moderate accuracy
Even when temperature-compensated, are affected by temperature gradients that may exist in the air space between the sensor and the water surface. Temperature gradients can be extreme in many sites, especially in stilling wells located in the daytime sun where the top of the stilling well or instrument enclosure can reach temperatures of 60°C or higher.
They also require periodic maintenance to ensure a clear path between the sensor and the water surface; spider webs beneath the sensor have been known to cause erroneous measurements.
4.2.2.11 Electrical sensors 4.2.2.11.1 Capacitance
A capacitor consists of two plates separated from each other by an insulating material called a dielectric. In applications involving capacitance measuring devices, one side of the process container acts as one plate and an immersion electrode is used as the other. The dielectric is either air or the material in the vessel. The dielectric varies with the level in the vessel. This variation produces a change in capacitance that is proportional to level. Thus, level values are inferred from the measurement of changes in capacitance, which result from changes in the level. Capacitance type level measurement devices offer many advantages.
Simple in design, they contain no moving parts and require minimal maintenance.
The availability of corrosive resistant probes is also an advantage. Capacitive level measurement devices have these limitations
Measurement is subject to error caused by temperature changes affecting the dielectric constant of the material.
If the probes should become coated with a conductive material, errors in measurement may occur.
38 4.2.2.11.2 Conductivity
A material's ability to conduct electric current can also be used to detect level. This method is typically used for point measurement of liquid interfaces of relatively high conductivity. Conductivity applications are usually limited to alarm devices and on/off control systems. A common arrangement is two electrodes positioned at the top in a tank. One extends to a minimum level and the other is positioned so that its lower edge is at the maximum level. The tank is grounded and functions as the common or third electrode. Usually, a stilling well is provided to ensure that the interface is not disturbed and to prevent false measurement.
The advantages of conductivity method include
Low cost and simple design
As well as the fact that there are no moving parts in contact with the process material. These advantages make this type of system an effective method of detecting and indicating level for many water-based materials.
There are limitations to the conductivity method which as follows:
The first is process substance must be conductive.
Second, only point detection measurements can be obtained.
The possibility of sparking also makes this method prohibitive for explosive or flammable process substances.
4.2.2.11.3 Resistance
Resistance type level detectors use the electrical relationship between resistance and current flow to accurately measure level. The most common design uses a probe consisting of two conductive strips. One strip has a gold-plated steel base; the other is an elongated wire resistor. The strips are connected at the bottom to form a complete electrical circuit. The upper ends of the strips are connected to a low voltage power supply. The probe is enclosed in a flexible plastic sheath which isolates the strips from the process material. As the level of the process material rises, the hydrostatic pressure forces the resistance strips together up to the interface. This action shorts the circuit below the interface level, and total resistance is reduced proportionately. Resistance sensing devices can be used for liquid-gas interfaces and for slurries or solids. As with the other
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electrical level sensors discussed, resistance-type level detectors require relatively little maintenance.
4.2.2.12 Float-operated recorder
Float-operated recorders have been one of the most commonly used instruments for measuring water level because of their relatively low cost, good accuracy ( , and wide availability. The instrument consists of a float of sufficiently large diameter, which is attached to a tape or cable that passes around the float wheel of a recorder and then to a counter weight The float rises and falls with the water level, and its movement rotates the float wheel and thus is recorded. To function properly, the float must be located in standing water. Thus, a stilling well is required on all field installations (see Section 4.6).
Care should be taken to ensure that when the float is rising its counterweight does not lodge on top of the float but keeps well above it or passes the float. If a high degree of accuracy is required, the counterweight should not be permitted to become submerged over part of the operating range since this will change the submergence of the float and thus affect the recorded water level. This systematic error may be prevented by
Locating the counterweight inside a separate watertight and water-free pipe.
Mounting two different-sized drums on the axle of the recorder. The larger diameter drum serves to coil up the float wire and the small diameter drum coils up the Counterweight wire, yielding reduced movement of the counterweight relative to the float. The drums require a spiral groove for coiling up several turns of wire, otherwise there is an error due to coiling of cable on top of itself. Tapes cannot be used with this method.
Extending the stilling-well pipe to such a height that the counterweight does not touch the float wheel at low stage nor the water surface at the maximum expected stage.
Most of the earlier recorders relied on the friction drive of a cable on the float wheel of the recorder. To improve the accuracy of the head measurements, we recommend that a recorder be equipped with a calibrated float tape that passes over the float wheel. The float and counterweight should be attached to the ends of the tape by ring connectors. If the recorder is not equipped with a tape index pointer, one should be attached either to the shelter-house floor or to
