Direct level sensing

Sight glass(open end/differential) or gauge is the simplest method for direct visual reading. As shown in Fig. 6.2 the sight glass is normally mounted verti- cally adjacent to the container. The liquid level can then be observed directly in the sight glass. The container in Fig. 6.2ais closed. In this case the ends of the glass are connected to the top and bottom of the tank, as would be used with a pressurized container (boiler) or a container with volatile, flammable, haz- ardous, or pure liquids. In cases where the tank contains inert liquids such as water and pressurization is not required, the tank and sight glass can both be open to the atmosphere as shown in Fig. 6.2b. The top of the sight glass must have the same pressure conditions as the top of the liquid or the liquid levels in the tank and sight glass will be different. In cases where the sight glass is excessively long, a second inert liquid with higher density than the liquid in the container can be used in the sight glass (see Fig. 6.2c). Allowance must be made for the difference in the density of the liquids. If the glass is stained or reacts with the containerized liquid the same approach can be taken or a different material can be used for the sight glass. Magnetic floats can also be used in the sight glass so that the liquid level can be monitored with a magnetic sensor such as a Hall effect device.

Floats (angular arm or pulley) are shown in Fig. 6.3. The figure shows two types of simple float sensors. The float material is less dense than the density of the liquid and floats up and down on top of the material being measured. In Fig. 6.3aa float with a pulley is used; this method can be used with either liquids or free flowing solids. With free flowing solids, agitation is sometimes used to level the solids. An advantage of the float sensor is that it is almost independent of the density of the liquid or solid being monitored. If the sur- face of the material being monitored is turbulent, causing the float reading to vary excessively, some means of damping might be used in the system. In Fig. 6.3b a ball float is attached to an arm; the angle of the arm is measured to indi- cate the level of the material (an example of the use of this type of sensor is the monitoring of the fuel level in the tank of an automobile). Although very simple and cheap to manufacture, the disadvantage of this type of float is its

Figure 6.2 Various configurations of a sight glass to observe liquid levels (a) pressurized or closed container, (b) open container, and (c) higher density sight glass liquid.

nonlinearity as shown by the line of sight scale in Fig. 6.4a. The scale can be replaced with a potentiometer to obtain an electrical signal that can be lin- earized for industrial use.

Figure 6.4bshows an alternative method of using pulleys to obtain a direct visual scale that can be replaced by a potentiometer to obtain a linear electri- cal output with level.

A displacer with force sensing is shown in Fig. 6.5a. This device uses the change in the buoyant force on an object to measure the changes in liquid level. The displacers must have a higher specific weight than that of the liquid level being measured and have to be calibrated for the specific weight of the liquid. A force or strain gauge measures the excess weight of the displacer. There is only a small movement in this type of sensor compared to a float sensor.

Figure 6.3 Methods of measuring liquid levels using (a) a simple float with level indica- tor on the outside of the tank and (b) an angular arm float.

Figure 6.4 Scales used with float level sensors (a) nonlinear scale with angular arm float and (b) linear scale with a pulley type of float.

The buoyant force on a cylindrical displacer shown in Fig. 6.5b using Eq. (6.2) is given by

(6.8)

whereg=specific weight of the liquid d=float diameter

L=length of the displacer submerged in the liquid The weight as seen by the force sensor is given by

Weight on force sensor =weight of displacer – F (6.9) It should be noted that the units must be in the same measurement system and the liquid must not rise above the top of the displacer or the displacer must not touch the bottom of the container.

Example 6.1 A displacer with a diameter of 8 in is used to measure changes in water level. If the water level changes by 1 ft what is the change in force sensed by the force sensor?

From Eq. (6.9)

Change in force =(weight of dispenser −F1) – (weight of dispenser −F2) =F2– F1 From Eq. (6.8) F2 F1 3 2 2 62 4 8 12 4 21 8 − = . lb /ft ×π( ft) × = . lb F= γ πd L 2 4

Figure 6.5 Displacer with a force sensor for measuring liquid level by (a) observing the loss of weight of the displacer due to the buoyancy forces of the displaced liquid and (b) dispenser dimensions.

Example 6.2 A 3.5-cm diameter displacer is used to measure acetone levels. What is the change in force sensed if the liquid level changes by 52 cm?

Probesfor measuring liquid levels fall into three categories, i.e., conductive, capacitive, and ultrasonic.

Conductive probesare used for single-point measurements in liquids that are conductive and nonvolatile as a spark can occur. Conductive probes are shown in Fig. 6.6a. Two or more probes as shown can be used to indicate set levels. If the liquid is in a metal container, the container can be used as the common probe. When the liquid is in contact with two probes the voltage between the probes causes a current to flow indicating that a set level has been reached. Thus, probes can be used to indicate when the liquid level is low and to operate a pump to fill the container. Another or a third probe can be used to indicate when the tank is full and to turn off the filling pump.

Capacitive probesare used in liquids that are nonconductive and have a high mand can be used for continuous level monitoring. The capacitive probe shown in Fig. 6.6bconsists of an inner rod with an outer shell; the capacitance is meas- ured between the two using a capacitance bridge. In the portion out of the liquid, air serves as the dielectric between the rod and outer shell. In the immersed section, the dielectric is that of the liquid that causes a large capac- itive change, if the tank is made of metal it can serve as the outer shell. The capacitance change is directly proportional to the level of the liquid. The dielec- tric constant of the liquid must be known for this type of measurement. The dielectric constant can vary with temperature so that temperature correction may be required. F2 F1 2 2 6 7 74 3 5 52 4 10 3 − = × × × = . . . kN/m cm cm cm /m 3 _π 8 87 N(395 g)

Figure 6.6 Methods of measuring liquid levels (a) using conductive probes for detecting set levels and (b) a capacitive probe for continuous monitoring.

Example 6.3 A capacitive probe 30-in long has a capacitance of 22 pF in air. When partially immersed in water with a dielectric constant of 80 the capacitance is 1.1 nF. What is the length of the probe immersed in water?

From Eq. (6.6)

Ultrasonicscan be used for single point or continuous level measurement of a liquid or a solid. A single ultrasonic transmitter and receiver can be arranged with a gap as shown in Fig. 6.7ato give single-point measurement. As soon as liquid fills the gap, ultrasonic waves from the transmitter reach the receiver. A setup for continuous measurement is shown in Fig. 6.7b. Ultrasonic waves from the transmitter are reflected by the surface of the liquid to the receiver; the time for the waves to reach the receiver is measured. The time delay gives the distance from the transmitter and receiver to the surface of the liquid, from which the liquid level can be calculated knowing the velocity of ultrasonic waves. As there is no contact with the liquid, this method can be used for solids and corrosive and volatile liquids. In a liquid the transmitter and receiver can also be placed on the bottom of the container and the time measured for a signal to be reflected from the surface of the liquid to the receiver to measure the depth of the liquid. 6.3.2 Indirect level sensing

The most commonly used method of indirectly measuring a liquid level is to meas- ure the hydrostatic pressure at the bottom of the container. The depth can then be extrapolated from the pressure and the specific weight of the liquid can be cal- culated using Eq. (6.1). The pressure can be measured by any of the methods given in the section on pressure. The dial on the pressure gauge can be calibrated directly in liquid depth. The depth of liquid can also be measured using bubblers, radiation, resistive tapes, and by weight measurements.

d= × − × = ( . ) . 1 1 10 22 30 80 22 18 4 3 _{pf pf in} pf in

Figure 6.7 Use of ultrasonics for (a) single-point liquid level measurement and (b) contin- uous liquid level measurements made by timing reflections from the surface of the liquid.

Example 6.4 A pressure gauge located at the base of an open tank containing a liquid with a specific weight of 54.5 lb/ft3registers 11.7 psi. What is the depth of the fluid in the tank?

From Eq. (6.1)

Bubbler devicesrequire a supply of clean air or inert gas. The setup is shown in Fig. 6.8a. Gas is forced through a tube whose open end is close to the bottom of the tank. The specific weight of the gas is negligible compared to the liquid and can be ignored. The pressure required to force the liquid out of the tube is equal to the pressure at the end of the tube due to the liquid, which is the depth of the liquid multiplied by the specific weight of the liquid. This method can be used with cor- rosive liquids as the material of the tube can be chosen to be corrosion resistant. Example 6.5 How far below the surface of the water is the end of a bubbler tube, if bubbles start to emerge from the end of the tube when the air pressure in the bubbler is 148 kPa?

From Eq. (6.1)

Radiation methods are sometimes used in cases where the liquid is corrosive, very hot, or detrimental to installing sensors. For single-point measurement only one transmitter and a detector are required. If several single-point levels are required, a detector will be required for each level measurement as shown in Fig. 6.8b. The disadvantages of this system are the cost and the need to handle radioactive material. h= p= × = − γ 148 10 1 14 8 4 3 kPa gm/cm . cm h= p= × = γ 11 7 144 54 5 3 30 9 . . . psi lb/ft ft

Figure 6.8 Liquid level measurements can be made (a) using a bubbler technique or (b) using a radi- ation technique.

Resistive tapescan be used to measure liquid levels (see Fig. 6.9). A resistive element is placed in close proximity to a conductive strip in an easily com- pressible nonconductive sheath; the pressure of the liquid pushes the resistive element against the conductive strip, shorting out a length of the resistive ele- ment proportional to the depth of the liquid. The sensor can be used in liquids or slurries, it is cheap but is not rugged or accurate, it is prone to humidity prob- lems, and measurement accuracy depends on material density.

Load cellscan be used to measure the weight of a tank and its contents. The weight of the container is subtracted from the reading, leaving the weight of the contents of the container. Knowing the cross-sectional area of the tank and the specific weight of the material, the volume and/or depth of the contents can be calculated. This method is well suited for continuous measurement and the material being weighed does not come into contact with the sensor. Figure 6.10 shows two elements that can be used in load sensors for measuring force. Figure 6.10ashows a cantilever beam used as a force or weight sensor. The beam is rigidly attached at one end and a force is applied to the other end, a strain gauge attached to the beam is used to measure the strain in the beam, a second

Figure 6.9 Demonstrating a resis- tive tape level sensor.

Figure 6.10 Force sensors can be used for measuring weight using (a) strain gauge tech- nique or (b) a piezoelectric technique.

strain gauge is used for temperature compensation. Figure 6.10bshows a piezo- electric sensor used to measure force or weight. The sensor gives an output voltage proportional to the force applied.

Example 6.6 What is the depth of the liquid in a container, if the specific weight of the liquid is 82 lb/ft3; the container weights 45 lb and is 21 in in diameter? A load cell measures a total weight of 385 lb.

Using Eq. (6.4) and (6.5) we get the following: Weight of liquid =385 – 45 =340 lb

Paddle wheelsdriven by electric motors can be used for sensing the level of solids in the form of power, grains, or granules. When the material reaches and covers the paddle wheel, the torque needed to turn the motor greatly increases. The torque can be an indication of the depth of the material. Such a set up is shown in Fig. 6.11a. Some agitation may be required to level the solid particles.