The fundamental problems with surface temperature measurement are that it is subject to large stem conduction errors, and that heat conducted from the surface of the hot plate causes a localized cold spot to occur, which means that the temperature indicated by the hot plate is not necessarily the temperature at the point of measurement.
An ideal system would not disturb the heat flux from the hot plate.
During 1993, such a system was described (ref. "Progress in Contact Thermometry" 1993 B. D. Foulis), and Isotech have the inventors' permission to make and market the device world-wide.
Principle of Operation
A fine wire type-N thermocouple is used as the surface temperature sensor, and a second junction, 2 to 3mm along the thermocouple, senses the temperature difference due to heat flux along the sensor.
A heater warms the thermocouple stem until the temperature gradient is zero, thus creating a measurement without stem conduction, or disturbance of the hot plate's surface.
The 944 can be used with the Isotech Model 983 Small Hot Plate. A traceable calibration certificate is available to order.