What is the principle of operation?
The Capacitec Amplifier produces an analog output voltage proportional to the distance between the probe face and an electrically conductive surface, connected to the Amplifier signal ground. The principle of this conversion is based on the fact that capacitive reactance (Xc) is proportional to the spacing of a parallel-plate capacitor, which is formed by the Probe's Sensor & Guard and the target surface.?
Isn't capacitance also related to parallel plate area?
Yes it is. In most cases the grounded target area is larger than the Probe's measuring surface, making that surface the controlling parameter. In cases where the target area is smaller than the Probe surface (such as edges or small diameters), extra care must be taken to define the measurement area.
What about target material?
The signal is independent of (ferrous and non-ferrous) material properties that affect other non-contacting systems. Many "semi-conductive" materials can also be measured. To date this has included silicon semi-conductor, carbon and graphite composites, sand, soil, and water surfaces. This property of a capacitive system is extremely useful because it eliminates the need for recalibration when measuring targets of different metals.
What about temperature effects?
Temperature errors are limited to instrument stability (which is excellent), thermal expansion offsets and the change in the dielectric of air which is generally the dielectric medium. Fortunately, the dielectric constant of air changes only 4% from room temperature to 1400F (760C). High temperature performance of capacitance probes in air is generally limited by the electrical ionization of air at higher temperatures.