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.
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