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Capacitance type level measurement
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- 4.11 Radiation type level measurement
- 4.12 Ultrasonic type level measurement
| 4.10 Capacitance type level measurement The amount of capacitance depends on the distance between the plates, the area of the plates, and the height of the dielectric between the plates. The equation is, CK (AD) Where, C = capacitance, K = dielectric constant, A = area of plate,D = distance between plates. In this method a probe is inserted in a tank and capacitance is measured between probe and tank. Capacitance varies with respect to tank level. This phenomenon is due to the difference between dielectric constant of air and liquid in tank. This method is normally used for non conductive liquids. 90 Field Instrumentation GEN Rev) In applications where liquid conduct, the electrode is encased in an insulating material. The liquid acts as the capacitor’s ground electrode, and the insulated conductor serves as the other electrode. 4.11 Radiation type level measurement In this method a radioactive source is kept on one side of tank and detector on other side. As radiation passes through the tank, its intensity varies with amount of material in tank and can be related to level. Its advantage is that nothing comes in contact with liquids. It is very costly and difficult to handle. 4.12 Ultrasonic type level measurement Ultrasonic sound waves with frequencies of 1 to 5 MHz can be used to detect liquid or solid levels. Ultrasonic are sound waves but at higher frequencies than 20 KHz (detected by human ear. It consists of an ultrasonic transducer (piezoelectric crystal. When voltage is applied to plates, the piezoelectric crystal expands or contracts. The crystal 91 Field Instrumentation GEN Rev) vibrates, and these vibrations can be transferred to a diaphragm to produce ultrasonic sound waves. The liquid surface acts as a reflector, and the transducer receives the reflection of its transmitted pulses. The transmitter and receiver are both connected to an echo timer, which measures the amount of time between the emission of sound wave and the reception of the echo. Time required by sound wave to travel to the liquid and back to receiver is carefully measured and this time is related to level. In case transmitter could not be installed in tank, a noninvasive (not in contact with liquid) sensor transmits an ultrasonic signal through the walls of a vessel. When the vessel is filled with liquid, the signal travels through the liquid and the opposite wall to a receiver transducer, where it is converted to and electrical signal. It has good accuracy. It is costly. Download 3.37 Mb. Share with your friends:
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