Lecture 4: Level Measurement

1. Lecture 4:Level Measurement

2. Level Measurement There are many methods for the measurement of liquid level which can be categorized as follows: • Direct sensing, in which the actual level is monitored. • Indirect sensing, in which a property of the liquid, such as pressure, is sensed to determine the liquid level. • Single point measurement, in which it is only necessary to detect the presence or absence of a liquid at a specific level.

3. Direct Level Sensing Direct level sensing methods include: • Sight glass. • Float sensors. • Ultrasonic devices.

4. The Sight glass The Sight glass or gauge is the simplest method for direct visual reading.

5. The Sight glass • 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. • Glass gauges are cheap but easily broken, and should not be used with hazardous liquid.

6. 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. • The float sensor is accurate and have a linear output with level height. • However, if the surface of the material being monitored is turbulent, the float reading may vary excessively, some means of damping might be required, such as a stilling well. Stilling well arrangement

7. Ultrasonic level gauge • 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.

8. Ultrasonic level gauge • 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 are reliable, accurate, and cost-effective. However, vibration or high noise levels can affect the devices.

9. Indirect Level Sensing • In these methods, a property such as pressure, weight, capacitance, or resistance are measured first and from which the level is calculated. • These methods include: • Pressure level gauge. • Displacer gauge. • Capacitive level gauge. • Bubbler devices. • Resistive tapes. • Load cells.

10. Pressure level gauge • Pressure is often used as an indirect method of measuring liquid levels. As shown, a differential pressure sensor can replace the sight glass. • The hydrostatic pressure at the bottom of the container increases as the depth increases in a fluid. This pressure is given by: • where p is the pressure, is the density, g is the gravitational acceleration, and h is the depth. Hence, the level can be extrapolated from the pressure and the density of the liquid.

11. Displacer gauge • According to Archimides’sprinciple of Buoyancy, the buoyant force on the cylindrical float (displacer) is proportional to the volume immersed in the liquid. The higher the level of the liquid, the higher the buoyant force. • The apparent weight, as seen by the force sensor (e.g. a strain gauge), is equal to the weight of displacer minus the buoyant force. • 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. • The liquid must not rise above the top of the displacer, and the displacer must not touch the bottom of the container.

12. Capacitive probes • Capacitive probes can be used for continuous level measurement. • The capacitance is a function of the dielectric material between the two plates.

13. Capacitive probes • Materials placed between the plates of a capacitor increase the capacitance by a factor known as the dielectric constant of the material. For instance, air has a dielectric constant of 1, and water has a dielectric constant of 80. • In the portion of the probe that is out of the liquid, air serves as the dielectric between the rod and outer shell. • In the section of the probe immersed in the liquid, the dielectric is that of the liquid, which causes a large capacitive change. • The capacitance change is directly proportional to the level of the liquid and can be measured using an ac bridge. • The dielectric constant of the liquid must be known for this type of measurement.

14. Bubbler devices • Bubbler devices require a supply of clean air or inert gas to prevent interaction with the liquid. • 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 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, .

15. Resistive tapes • 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.

16. Load cell • The weight of a tank of liquid can be used as a measure of the height of liquid in the tank. Load cells are commonly used for such weight measurements. • Typically, a load cell consists of a strain gauged cylinder which is included in the supports for the tank of liquid. • When the level of the liquid changes, the weight or the load on the load cell changes and the resistances of the strain gauges change as well. • The resistance changes of the strain gauges are thus a measure of the level of the liquid. • Since the load cells are completely isolated from the liquid, the method is useful for corrosive liquids.

17. Single Point Sensing • Single point sensing is used in On/Off applications. • Two of the methods for single point sensing are: • conductive probes. • beam breaking probes.

18. Conductive probes • Conductive probes are used in liquids that areconductive and nonvolatile, since a spark can occur. • Two or more probes can be used to indicate set levels. 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. • Athird 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.

19. Beam breaking methods Beam breaking methods are sometimes used for pressurized containers.

20. Beam breaking methods • For single point measurement, only one transmitter and one detector are required. • The beams can be light, sonic or ultrasonic waves, or radiation. The devices are low-cost but can be affected by deposits. • If several single point levels are required, a detector will be required for each level measurement. • The disadvantages of radiation-based beam breaking systems are the cost and the need to handle radioactive material. However, this system can be used with corrosive or very hot liquids. High-pressure containers are used where conditions would be detrimental to the installation of other types of level sensors.