This can be either because the technique is not stable at temperature
extremes or because the device's electronics cannot be compensated for the
particular application or have not been reliably tested and performance
verified under the specific conditions.
Non-contact capacitive sensing can be very suitable for high temperature
sensing of non-metallic targets especially if electronics can be suitably
compensated or located remote from the hot spot.
Separating sensing elements into "remote sensing heads" and separate
electronics allows electronics to be protected from temperature extremes while
the sensing element(s) alone remain in the hot spot. The sensing element that
consists only of electrodes made out of metal and insulating ceramics can
operate at great extremes of temperature for example in industrial drying.
Typically these sensors are custom made by Sensatech.
See Also: Solder Wave Height Sensor