Capacitive devices are widely used for measuring the level of both liquids and solids in powdered or granular form. They perform well in many applications, but become inaccurate if the measured substance is prone to contamination by agents that change the dielectric constant. Ingress of moisture into powders is one such example of this. They are also suitable for use in extreme conditions measuring liquid metals (high temperatures), liquid gases (low temperatures), corrosive liquids (acids, etc.) and highpressure processes. Two versions are used according to whether the measured substance is conducting or not. For non-conducting substances (less than 0.1 µmho/cm3 ), two bare-metal capacitor plates in the form of concentric cylinders are immersed in the substance, as shown in Figure 17.3. The substance behaves as a dielectric between the plates according to the depth of the substance. For concentric cylinder plates of radius a and b (b>a), and total height L, the depth of the substance h is related to the measured capacitance C by:
where ε is the relative permittivity of the measured substance and ε0 is the permittivity of free space. In the case of conducting substances, exactly the same measurement techniques are applied, but the capacitor plates are encapsulated in an insulating mate rial. The relationship between C and h in equation then has to be modified to allow for the dielectric effect of the insulator. Measurement uncertainty is typically 1–2%.