Level Sensor: Design and Laboratory Test

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195

Level Sensor: Design and Laboratory Test

Nirupam and Dr. S. S. Solanki

Department of Electronics and Communication Engineering Birla Institute of Technology, Mesra, Ranchi, Jharkhand, India-835215

[email protected], [email protected]

Abstract— This paper presents a low cost water level sensor for dam, tank & reservoirs. The proposed sensor design has a simple, reliable and low cost design concept. A reflected float mechanism has been introduced in the design to reflect the incident wave (signal). For precise measurement, the presented design has been calibrated and tested for level measurement up to 225 cm with a high resolution. The accuracy of the setup is 0.1cm and it can be improved further as per the requirement, by simply varying the circuit parameters. To clearly present the design concept, the complete step by step processes for the design realization have been mentioned.

Index Terms—Source, detector, dam’s monitoring, liquid level sensor.

I. INTRODUCTION

Dams are artificial barrier which have high potential risk of eventual collapse causing catastrophic consequences to the environment and especially to humans. Dam water level is generally measured manually, which adds manual measurement errors. Manual measurement is also not effective due to various problems such as difficulties in reaching the measurement site, human error, low resolution of the measuring instrument, etc. In the view of above, many automatic water level measurement systems using mechanical sensors such as resistive [1, 2], capacitive [2] or magnetic sensor [3], have been already reported in literature. Still, these sensors have to do direct contact with water that makes their life span shorter due to chemical and physical constraints such as corrosion, pressure under water and so on. Sensors like resistive and magnetic sensors can measure only the water level at some points while capacitive sensor can measure the water level at all points. However, capacitive sensor is affected by water composition and can affect the measurement result. Seeing the above design requirements, this paper makes following contributions:

1) A sensor design for water level measurement is presented.

2) Design offers precise measurement of the water level, without direct contact with water, which increases its life time.

3) Further, the microcontroller is used as a data processor and controller to other electronic components utilized in the design presented.

Rest of the paper is organized as follows. Section III presents the study of conventional sensor design already

available in the literature. The proposed sensor design is reported in Section II. The simulation results and discussions are mentioned in Section IV. Finally, concluding remarks are provided in Section V.

II. LITERATURE REVIEW ON CONVENTIONAL DESIGNS To monitor water levels, different types of water-level gauges are already reported. These designs include pressure type, encode mechanical type, ultrasonic type and radar type, and so on. These designs find their applications in different situations, which basically depend upon the measurement requirements and desired performance characteristics [13]–

[16], [17]–[21], [22]. Table I summarizes the comparative study of various water-level gauges available in literature.

Apart form these, optical fiber-based sensors for liquid level measurement are also avilable. For example, Nathet al.

[23] presented a simple intensity-modulated fiber optic sensor, which is based on frustrated total internal reflection effect caused by refractive index change of a medium surrounding an optical fiber. Khaliqet al. [24] proposed and demonstrated a liquid-level sensor based on the refractive-index sensitivity of long-period fiber-optic gratings. Shenget al. [25] developed a temperature-independent differential pressure sensor based on two FBGs. These studies state that the sensor are capable of providing simultaneous measurements of both the temperature and the differential pressure, and are suitable for applications involving liquid level, liquid density or specific gravity measurement. However, in the sensors proposed above, the liquid level can only be obtained if the specific gravity of the liquid is known in advance. In practice, this specific gravity is not always available, and thus the proposed sensors have only limited applicability.

III. THE PROPOSED SENSOR TECHNIQUE

The propsed sensor design utilizes a light emitting diode (LED) torch as a source (S) and light depending resistor (LDR) as detector (D) to measure the water level without direct contact with water. The microcontroller is used as a data processor and controller to other electronic components.

Microcontroller based automated water level sensing systems

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196 have been already developed for monitoring of liquid levels in

reservoirs, silos, and dams etc. Usually, these kind of systems provides visual multilevel as well as continuous level indication. Audio visual alarm at desired levels and automatic control of dam outlets can be included in these systems. The proposed design overcomes several issues of these system.

The main components of the presented design are as follows:

A. Sensing Principle and Sensor Modeling

The advantages of LDR include its simplicity, reliability and low cost. In order to measure the water level, we propose a concept of sensor as shown in Fig. 1. Float is placed at the water surface. The pipe prevents it from floating off out of the sensor’s detection range. The bottom of the pipe is partially closed. Holes are made in the pipe, so that maximum amount of water can enter. The length of the pipe depends on sensor’s capacity.

The output obtained from sensor undergoes through signal conditioner. Signal conditioning is required in order to make the output linear. Analog-to-Digital Converter (ADC) becomes necessary to interface microcontroller in the system.

Auto-calibration is achieved through software. Alarm in the system activates to indicate dam gates opening. Liquid Crystal Display is interfaced for level display.

B. Design of the Sensor

Five series connected Light Dependent Registers are soldered in the general purpose Printed Circuit board. A low resistance of value 100 Ω is soldered in series with the Light

Dependent Registers. 5V Power supply is given in the circuit.

The circuit arrangement is shown in fig.2. A Light Emitting Diode based torch light is incident over the Light Dependent Registers. A reflecting surface is kept opposite to the soldered Printed Circuit Board. The reflected ray is exposed to the Light Depending Registers. Output voltage across low resistance of value 100 Ω is measured through multimeter.

Change in voltage is obtained with change in distance between reflector and Light Dependent Registers.

IV. LABORATORY CHARACTERIZATION AND TESTING Required experiments have been performed in the laboratory for the testing of the sensor. The following sections illustrates these laboratory work:

A. Experimental Procedures in Laboratory

We marked many sensing points on a 225 cm long table. A multimeter was responsible for the sensor characterization.

Sensor was characterized in the terms of voltage as a function of distance. Fig. 3 shows the experimental set up.

B. Testing

Table II shows the different readings obtained from the sensor both in the case of increasing and decreasing levels.

The error in the measured values have been also tabulated in the table to clearly illustrate the difference in the measured value occurred due to hysteresis.

TABLE I.

COMPARISON OF DIFFERENT TYPES OF WATER GAUGES

Type Pressure Encoders/Floats Ultrasonic Radar Technical

Parameter

Precision:+0.1~0.3%FS;

Range: < 100 m

Precision:+0.2~0.3%FS;

Range: < 40 m

Precision:+0.15%FS;

Range: < 0.25 ~12 m

Precision:+3~10mm Range: < 90 m Advantage High measuring speed;

Convenient for installing

Wide Adaptability to water quality; Low power consumption

Non-contact measurement; High measuring accuracy

Non-contact measurement; High measuring accuracy;

wide range Drawback The precision is affected

by the density of water

Big cumulative measurement errors needs re-calibrated frequently

The ultrasound speed is affected by temperature and humidity of the transmission medium;

Small measured range

Expensive; Measured procedure is affected by raindrop and snow- flake

Absolute Error

< 2cm @ measured range < 20m;

>2cm @ measured range

> 20m

>2cm @ measured range

> 10m

>2cm @ measured range > 14m

+3mm~10mm

Cumulative Error

>2cm monthly >2cm daily

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197 It is observed from the Table 2 that the output voltage got diminished to a small value. The reason behind it is that the light got dispersed at very long distance. An instrumentation amplifier of gain 2.5 has been designed to set the output voltage

within 0-5 V.

For achievi ng gain 2.5 resistan ces values are set as follows : R1=1.5 KΩ, Rg

= 2

KΩ, R2

= 10 KΩ and R3 = 10 K Ω.

Further, this work

also utilizes a signal conditioner as instrumentation amplifier as shown in Fig. 4. A logarithmic amplifier is used to make the sensor response linear. Logarithmic amplifier circuit is

demonstrated in Fig. 5. Fig. 2 Circuit diagram of sensor

Fig. 3. Experimental Set up for Sensor Ou Fig. 1. Block diagram of the

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198 TABLE II:SENSOR OUTPUT VOLTAGE

Distance, cm

Increasing Level voltage, mV

Decreasing Level voltage, mV

Error

5 1989 1979 10

10 1971 1971 00

15 1897 1895 02

20 1810 1850 -40

25 1751 1713 38

30 1652 1625 27

35 1524 1547 -23

40 1375 1379 -04

45 1248 1240 08

50 1109 1121 -12

55 1049 1073 -42

60 903 903 00

65 798 792 06

70 681 691 -10

75 575 575 00

80 533 532 01

85 490 499 -09

90 477 481 -04

95 460 458 02

100 438 443 -05

105 418 408 10

110 384 383 01

115 332 331 01

120 285 290 -05

125 278 272 06

130 242 240 02

135 230 232 -02

140 217 212 05

145 206 208 -02

150 199 201 -02

155 182 188 -06

160 174 176 -02

165 159 160 -01

170 147 146 01

175 122 126 -04

180 114 111 03

185 104 101 03

190 91 97 -06

195 86 85 01

200 76 82 -06

205 72 69 03

210 66 65 01

215 58 54 04

220 48 44 04

225 42 40 02

V. R^ESULTSAND DISCUSSION

Sensor output voltage is measured across 100 Ω resistance connected in series with the Light Dependent Registers. This measurement is achieved with the aid of multimeter. Light intensity decreases at large distance which causes very low output voltage. Fig.6 illustrates the testing result of laboratory

Fig.4 Instrumentation Amplifier

Fig.5 Log Amplifier

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199 characterization. It depicts the sensor response as a graph between distance (cm) and output voltage (mV). The output voltage is taken for both increasing and decreasing levels.

Hysteresis is seen almost negligible in the sensor response.

VI. CONCLUSION

In this paper we present a water level sensor. The proposed sensor has a particular design suitable to be used in dams, tanks, and reservoir. The sensor uses a Light Emitting Diode based torch as source and Light Dependent Register as detector. The microcontroller used in the system facilitates auto-calibration of the sensor. Alarm feature allows indication for opening of dam gate outlets. And Liquid Crystal Display has been included in the system for measured water level display. It can measure the water level up to 225 cm or more, choosing the appropriate source. The paper described development of the sensor and the results of the laboratory test. More studies have to be done to improve the accuracy of this sensor technology.

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