Capacitance MeasurementLevel sensors for R744, R717, R134a, R22, R410a & R507

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 = 0.225 K ( A/D )

where:

C = Capacitance in picoFarads “pF”

K = Dielectric constant of material (relative permittivity)

A = Area of the inner electrodes in square inches

D = Distance between the electrodes in inches

 

The dielectric constant (relative permittivity) is a numerical value on a scale of 1 to 100 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 non-conductive materials and conductive materials.

 

Non-Conductive Liquids/Materials
(Nonconductor/insulations as glass, paper, Plastic and Oil) If a the dielectric constant is lower than 10, then the material act as Non-Conductive. ( 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 act as conductive with conductivity value at minium100 µS/cm (tap water has a value from 500 to 1000 µS/cm). (Water, brine and Ammonia is 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 based it on practical experience, measuring and test.

 

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

Dielectric Constant: Temperature 20°C
Water/brine 80 (0°C is 88)
Ammonia 17 (-40°C is 22)
CO2 1.5 (-40°C is 2.0)
Oil type PAO, PEO
Oil type PAG
2.2 Mineral and synthetic types
3.5 Synthetic types
R134a 9.24
R22 6.35
R410A  7.78
R507 6.97
R1234ze 7.7
Air 1.0
Ice 3.2

 

Special Considerations
Material Buildup—The most devastating effect on the accuracy of capacitive measurements is caused by the buildup of conductive material on the electrode surface. Non-conductive buildup is not as serious since it only represents a small part of the total capacitance. Oil is non-conductive, 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 absorbtion/swelling of Refrigerant (Freon and CO2) into the insulating material(PTFE). For the greatest accurracy, the sensor should be recalibrated after the system has operated for a time, when the refrigerant chemistry, and level sensor have reached equilibrium. Measuring error caused by absorbtion will effect a small offset.

The new HBDX Refrigerant gas Quality sensor is based on the same principle, where change in dryness/ saturated phase makes the capacitance change/measurement in pF.

 

HBDX-sensor-design