Instrumentation Sensors Book

C H A P T E R 8

Level

8.1Introduction

This chapter discusses the measurement of the level of liquids and free-flowing solids. There are many widely varying methods for the measurement of liquid level. Level measurement is an important part of process control. Level sensing can be single point, continuous, direct, or indirect. Continuous level monitoring measures the level of the liquid on an uninterrupted basis. In this case, the level of the material will be constantly monitored, and hence the volume can be continuously monitored, if the cross-sectional area of the container is known.

Examples of direct and indirect level measurements are using a float technique, or measuring pressure and calculating the liquid level. Accurate level measurement techniques have been developed. New measurement techniques are constantly being introduced and old ones improved [1]. Level measuring devices should have easy access for inspection, maintenance, and replacement. Free-flowing solids include dry powders, crystals, rice, grain, and so forth.

8.2Level Measurement

Level sensing devices can be divided into four categories: (1) direct sensing, in which the actual level is monitored; (2) indirect sensing, in which a property of the liquid, such as pressure, is sensed to determine the liquid level; (3) single point measurement, in which it is only necessary to detect the presence or absence of a liquid at a specific level; and (4) free-flowing solid level sensing [2].

8.2.1Direct Level Sensing

A number of techniques are used for direct level sensing, such as direct visual indication using a sight glass or a float. Ultrasonic distance measuring devices also may be considered.

The Sight glass or gauge is the simplest method for direct visual reading. As shown in Figure 8.1, the sight glass is normally mounted vertically adjacent to the container. The liquid level then can be observed directly in the sight glass. The ends of the glass are connected to the top and bottom of the tank via shutoff valves, as would be used with a pressurized container (boiler) or a container with volatile, flammable, hazardous, or pure liquids. In cases where the tank contains inert liquids, such as water, and pressurization is not required, the tank and sight glass

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Shutoff valve

Sight glass

Figure 8.1 Sight glass for visual observation of liquid levels.

both can be open to the atmosphere. Glass gauges are cheap but easily broken, and should not be used with hazardous liquid. For safety reasons, they should not be longer than 4 ft. Cold liquids also can cause condensation on the gauge. Gauges should have shutoff valves in case of breakage (sometimes automatic safety shutoff valves are used) and to facilitate replacement. In the case of high pressure or hazardous liquids, a nonmagnetic material can be used for the sight glass with a magnetic float that can rotate a graduated disk, or can be monitored with a magnetic sensor, such as a Hall Effect device [3].

Float sensors are shown in Figure 8.2. There are two types of floats shown: the angular arm and the pulley. 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 Figure 8.2(a), a 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 help level the solids. The advantages of the float sensor are that they are almost independent of the density of the liquid or solid being monitored, are accurate and robust,

Angular position scale E

1/4

1/2

3/4

F

Arm

Linear scale

Liquid

Figure 8.2 Methods of measuring liquid levels, using (a) a simple float with level indicator on the outside of the tank, and (b) an angular arm float.

and have a linear output with level height. However, accuracy can be affected by material accumulation on the float, corrosion, chemical reactions, and friction in the pulleys. If the surface of the material being monitored is turbulent, causing the float reading to vary excessively, some means of damping might be required, such as a stilling well. In Figure 8.2(b), a ball float is attached to an arm, and the angle of the arm is measured to indicate the level of the material. A spherical float shape is used to provide maximum buoyancy, and it should be one-half submerged for maximum sensitivity, and to have the same float profile independent of angle. An example of this type of sensor is the fuel level monitor in the tank of an automobile. Due to lack of headroom in this application, the angle of the float arm can go only from approximately 0° to 90°. The fuel gauge shows the output voltage from a potentiometer driven by the float. Although very simple and cheap to manufacture, the angular float sensor has the disadvantage of nonlinearity, as shown by the line-of-sight scale in Figure 8.2(b).

Figure 8.3(a) shows a pulley-type float sensor with a linear radial scale that can be replaced with a potentiometer to obtain an electrical signal. Figure 8.3(b) shows an angular arm float. The travel of the arm on this float is ±30°, giving a scale that is more linear than in the automotive application, and which can be linearized for industrial use. The scale can be replaced by a potentiometer to obtain an electrical signal.

Ultrasonic or sonic devices can be used for single point or continuous level measurement of a liquid or a solid. A setup for continuous measurement is shown in Figure 8.4. A pulse of sonic waves (approximately 10 kHz) or ultrasonic waves (more than 20 kHz) from the transmitter is reflected from the surface of the liquid to the receiver, and the time for the echo 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 (approximately 340 m/s). Since there is no contact with the liquid, this method can be used for solids, and corrosive and volatile liquids. Sonic and ultrasonic devices

E

F

Weight

Figure 8.3 Float level sensors with radial scales or potentiometers: (a) pulley type, and (b) angular arm type with ±30° angle.

The pressure can be measured by any of the methods given in the section on pressure. In Figure 8.1, a differential pressure sensor can replace the sight glass. These devices are affected by liquid density and are susceptible to dirt. It is sometimes necessary to mount the pressure sensor above or below the zero liquid level, as shown in Figure 8.5, in which case an adjustment to the zero point is required. When the sensor is below the zero level, as shown in Figure 8.5(a), “zero suppression” is required to allow for H; the distance of the measuring instrument above or below the bottom of the container when it is above the zero level, as shown in Figure 8.5(b), “zero elevation” is required to allow for H. Shutoff valves should be used for maintenance and replacement, and cleanout plugs to remove solids. The dial on the pressure gauge can be calibrated directly in liquid depth.

A displacer with force sensing is shown in Figure 8.6. This device uses the change in 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 whose level is being measured, and has 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 the movement in a float sensor. Displacers are simple, reliable, and accurate, but are affected by the (tempera- ture-dependent) specific weight of the liquid, and buildup on the dispenser of coatings and depositions from the liquid. A still well may be required where turbulence is present in the liquid [5].

The buoyant force on the cylindrical displacer shown in Figure 8.6 is given by:

where = specific weight of the liquid, d is float diameter, and L is the length of the displacer submerged in the liquid.

Figure 8.5 Pressure sensors positioned (a) below tank bottom, and (b) above tank bottom.

Figure 8.7 Methods of measuring liquid levels using a capacitive probe for continuous monitoring in (a) nonconducting liquid, and (b) conducting liquid.

The dielectric constant is now that of the insulator, and the liquid level sets the area of the capacitor plate.

Bubbler devices require a supply of clean air or inert gas to prevent interaction with the liquid, as shown in Figure 8.8. Gas from a pressure regulator is forced through a tube via a flow regulator, and the open end of the tube is close to the bottom of the tank. The specific weight of the gas is negligible compared to the specific weight of 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 (requiring temperature correction). This method can be used with corrosive liquids, since the material of the tube can be chosen to be corrosion-resistant. Electrical power is not required, and variations in specific weight will affect the readout [8].

Example 8.5

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 263 kPa?

From (8.1):

Figure 8.8 Bubbler device for measuring liquid level.

Resistive tapes can be used to measure liquid levels, as shown in Figure 8.9. A resistive element is placed in close proximity to a conductive strip in an easily compressible nonconductive sheath. The pressure of the liquid pushes the conductive strip against the resistive element, shorting out a length of the resistive element that is proportional to the depth of the liquid. The sensor can be used in corrosive liquids or slurries. It is cheap, but is not rugged or accurate. It is prone to humidity problems, measurement accuracy is dependent on material density, and is not recommended for use with explosive or flammable liquids.

Load cells can be used to measure the weight of a tank and its contents. The weight of the container is subtracted from the total 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. The level (depth) depends on the density of the material [9].

The weight of a container can be used to calculate the level of the material in the container. From Figure 8.10, the volume (V) of the material in the container is given by:

Figure 8.9 Resistive tape level sensor.

r

Area = πr2

Load

d (Liquid level)

Cells

Figure 8.10 A liquid container mounted on force sensors.

where r is the radius of the container, and d is the depth of the material. The weight of material (W) in a container is given by:

Example 8.6

What is the depth of the liquid in a container if the specific weight of the liquid is 56 lb/ft3, the container weighs 33 lb, and has a diameter of 63 in? A load cell measures the total weight to be 746 lb.

Using (8.6) and (8.7), we get the following:

Weight of liquid = 746 − 33 = 713 lb

Depth (d ) = 1273. × 576 ft = 0588.ft = 7in 12470

8.2.3Single Point Sensing

In On/Off applications, single point sensing can be used with conductive probes, thermal probes, and beam breaking probes.

Conductive probes are used for single point measurements in liquids that are conductive and nonvolatile, since a spark can occur. Conductive probes are shown in Figure 8.11. Two or more probes can be used to indicate set levels. If the liquid is in a metal container, then 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 when to operate a pump to fill the container. A third probe can be used to indicate when the tank is full, and to turn off the filling pump. The use of ac voltages is normally preferred to the use of dc voltages, to prevent electrolysis of the probes.

Figure 8.11 Probes for single point sensing in conductive liquids.