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23.08.2018  |  4852x
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How does the capacitive measuring principle work?

- the function of a moisture sensors explained easily!

There are plenty of different measuring methods to determine the residual moisture of material such as bulk solids. Now we are focusing on the capacitive functional principle. Inter alia it finds its use in in-line moisture measurement (directly integrated within a working process).

It can be classified into two methods of moisture measurement, direct and indirect.

Direct measuring procedures are e.g. the Karl-Fischer-Tritation or drying out material (loss of water by using a drying cabinet, probably the most often used method).

Moisture sensors and systems belong to the indirect moisture measuring techniques as they allow conclusion concerning the water content by using for example the capacitive moisture measuring principle. Further possible measurements work with microwave, infrared or conductance methods. The main criteria is that these procedures do not detect the residual moisture of the bulk solids directly. Rather they use their physical characteristics and how they change after the material has been dehydrated or moisturized. That is the reason why it is necessary to calibrate the sensor to the individual material.

The capacitive measuring principle is based on the working principle of a capacitor. A capacitor creates an electric field between two conductive plates when the voltage is applied.

Capacitance (C) names the ability of a capacitor to accumulate electric charging (depending on the voltage). It is the effect of the combination of the area of overlap (A) on the two plates (in square meters), “d” the separation between the plates (in meters) and “ɛ” the relative permittivity (a.k.a. the dielectric constant) of the material.

Capacitance is proportional to the area of overlap but inversely proportional to the separation between the conducting sheets. The bigger the area of overlap is and the closer the sheets are to each other, the greater is the capacitance.

The dielectric constant depends on the insulation characteristics of the dielectric medium (bulk solids) within an electric field. It indicates how permeable the material is for electrical fields.

The distance of the conducting sheets (d) and the size of the plates themselves are always kept constant to a capacitive sensor. So the changing of the relative permittivity of the material allows a conclusion relating to the water content because it is the only parameter that is changing.

As you can see on the image below water has a dielectric constant of 80 (based on standard ambient conditions). Many carrier materials have a relative permittivity between 2 and 10. This great difference allows this capacitive moisture measuring technology to get a high response regarding the water content of the measured materials (bulk solids).

Vacuum: εr = 1

Measuring medium: εr = 2 to 10

Water: εr = 80

The capacitive moisture measuring method is very adaptable and can be used for many applications. So the water content of various material like sawdust, wood chips, cocoa beans, gypsum and many others can be determined throughout the complete moisture range.

On the front of a capacitive moisture sensor there is a stray field capacitor mainly protected by a robust ceramic plate (or rubber/teflon for rough areas) to generate an electrical field. The material that should be measured has to pass constantly this electrical field (e.g. sand on a conveyor belt). That affects the stray field capacitor. After the sensor has been calibrated to the material the measuring signal can be transmitted via analogue norm signal (4…20mA/0…10V) or by Profibus DP to a PLC or another control system. The reference value for calibration use is determined by dehydration process with a moisture laboratory device (moisture lab analyzer).
 

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