The Capacitive Principle

HISTORY In the late 1940`s a British firm pioneered the detection of CAPACITANCE between two electrodes. Capacitance, being influenced by the Dielectric Constant of the material being measured rather than the conductivity, indicated that the concept could be used in virtually any material. A new industry was born.

HB Products has 20+ years of experience in the development of electronic sensors, originally for the refrigeration industry, for use with Ammonia (R717), and today we evolved our sensor technology to work with all types of refrigerants even CO2 . A capacitor is formed when a level-sensing electrode is installed in a vessel. The metal rod/electrode acts as one plate of the capacitor and the reference tube acts as the other plate. As the level rises, the air or gas normally surrounding the electrode is displaced by liquid having a different/higher dielectric constant. Capacitance instruments detect this change and convert it into a proportional output signal, 0 to 100% level. The capacitance relationship is illustrated with the following equation

C = K ( A/D )

Where: C = Capacitance in picofarads “pF”
K = Dielectric constant of the media
A = Area of the inner electrodes
D = Distance between the electrodes

The measured capacitance (pF valve) is converted by the HB sensor electronics into either an analog 4-20 mA output signal or a digital ON/OFF signal. All electronics are factory calibrated to specific fluids.

The dielectric constant (relative permittivity) is a numerical value, which relates to the ability of the dielectric (material between the electrodes) to store an electrostatic charge. The dielectric constant of a material is determined in an actual test cell. Values for many materials are published. In actual practice, a capacitance change is produced in different ways, depending on the material being measured and the level electrode selection. However, the basic principle always applies. If a higher dielectric material replaces a lower one, the total capacitance output of the system will increase. If the electrode is made larger (effectively increasing the surface area) the capacitance output increases. Level measurement can be organized into two basic categories: the measurement of nonconductive materials and conductive materials.

Non-Conductive Liquids/Materials
(Nonconductor/insulations as glass, paper, Plastic and Oil) If the dielectric constant is lower than 10, then the material acts as NonConductive. ( All HFC/Freon types and CO2 is Non-Conductive)

Conductive Liquids/Materials
(Transfer/conduct electric current) If the dielectric constant is higher than 10, then the liquid acts as conductive with a conductivity value at minium 100 µS/cm (tap water has a value from 500 to 1000 µS/cm). (Water, brine and ammonia are conductive)

Generally it is not necessary to calculate the actual capacitance, but it is extremely important to understand the principle and how it works. When we design a new capacitive sensor we always base it on practical experience, measurements and tests.

It is possible to calibrate a level sensor measuring Non-Conductive liquids in water, if you know the exact difference between the dielectric constants.

Special Considerations
The most devastating effect on the accuracy of capacitive measurements is caused by the buildup of conductive material on the electrode surface. Non-conductive build-up is not as serious since it only represents a small part of the total capacitance. Oil is nonconductive, fine metal powders are examples of materials that are conductive.

Chemistry effect on the insulating material
The accuracy of the capacitive measurements can be affected by the absorption/swelling of refrigerant (Freon and CO2 ) into the insulating material (PTFE). For the greatest accuracy, the sensor should be recalibrated after the system has operated for a time, when the refrigerant chemistry and level sensor have reached equilibrium. Measuring errors caused by absorption will result in an off set.

HBLC-CO2 and HFO / HFC liquid level sensors and switches are not affected by chemistry and are therefore long-term stable and do not normally require re-calibration.