Page 27 - Instrumentation and Measurement

2.4.4 Capacitive Level Indicator

A common form of capacitive level gauge consists of two concentric conducting cylinders, or a
circular rod inside a cylinder, acting as capacitor plates with the liquid between them acting as the
dielectric of a capacitor (Figure 2.21). If the liquid is an electrical insulator then the capacitor plates
can be bare metal, if the liquid is conducting then they are metal coated with an insulator, e.g. Teflon.
The arrangement consists essentially of two capacitors in parallel, one formed between the plates
inside the liquid and the other from that part of the plates in the air above the liquid. A change in the
liquid level changes the total capacitance of the arrangement.

FIGURE 2.21 Capacitive gauge.

2.4.5 Ultrasonic Level Gauge

In one version of an ultrasonic level indicator, an ultrasonic transmitter/receiver is placed above the
surface of the liquid (Figure 2.22). Ultrasonic pulses are produced, travel down to the liquid surface
and are then reflected back to the receiver. The time taken from emission to reception of the pulses
can be used as a measure of the position of the liquid surface. Because the receiver/transmitter can
be mounted outside the liquid, it is particularly useful for corrosive liquids. Errors are produced by
temperature changes since they affect the speed of the sound wave. Such errors are typically about
0.18% per ͦ C.

FIGURE 2.22 Ultrasonic gauge.

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