Capacitive transducer circuits for liquid level measurement

Sheroz Khan, AHM Zahirul Alam, Kyaw Kyaw Htike @ Muhammad Ali, Mohd Rafiqul Islam and Othman O Khalifa, “Capacitive transducer circuits for liquid level measurement”, International Journal of Computer Sciences and Engineering Systems, Vol. 2, no. 3, July 2008, pp. 195-197. [Journal]

Abstract:

This work is making use of the capacitive element that is made of two electrodes – a column of liquid the level of which is to be measured, and a metallic tube that is pulled and fitted over a PVC tube of accordingly smaller dimensions. Both of the electrodes are provided with contact terminals for easy connection within a bridge circuit, which is powered by a sinusoidal source of given amplitude and known frequency. Precautionary measures are taken for improved linearity and minimal interference of the power supply on experimental data obtained as a result of measurement. The experimental data shows good consistency with the theoretical details and derivations.

Key words: Capacitive Transducers, Bridge Circuits, Liquid Level Measurement

Conclusions:

The results show good linearity and an acceptable level of accuracy in all of the measurements, however, a few critical points need to be mentioned:

  • The results follow an approximately horizontal straight line until the liquid level reaches around 6 cm where the metal casing is connected to plastic water tank, a point where from a good linearity is obtained.
  • Even when there is no liquid inside the tank, there is still some small output voltage, which means some residual capacitance is present.
  • The second circuit appears to give more accurate and consistent data than the old op-amp circuit.