An awareness of the principles involved in contact thermometry, knowledge of the various practical technologies available, and an understanding of their current relative cost and performance parameters are important to building or maintaining a competitive edge.

Irwin Bluestein, RdF Corporation

ngineers need to maintain knowledge of all components used in the products that they design. While semiconductors have been evolving at blazing speed for more than 30 years with no letup in view, the evolution of sensor technology has been grinding out improvements at a comparative snail’s pace. An awareness of the principles involved in contact thermometry, knowledge of the various practical technologies available, and an understanding of their current relative cost and performance parameters are important to building or maintaining a competitive edge. Some of the following may seem a bit obvious, but discussions with many engineers have convinced me that these fundamentals are often overlooked or misunderstood.

	The contact thermometer measures only its own temperature. It is brought into contact with or proximity to the substance whose temperature is to be measured, and that substance is supposed to bring the thermometer to the same temperature.
	To bring the thermometer to the temperature of the substance being measured requires the transfer of heat, a phenomenon that may involve several other substances. For example, if the sensing element is enclosed in a protective metal sheath, the heat must transfer from the process substance to the sheath, to the internal supporting structure, and then to the sensor itself.
	The amount of heat required to alter the temperature of the sensor may alter the temperature of the process.
	While the sensor is absorbing heat from the process, it is also yielding heat to its own environment.
	The measurement instrumentation measures the electrical effect that is related to the temperature of the thermometer.

Earlier, I said “and the substance is supposed to bring the thermometer to the same temperature”. Lets examine the process qualitatively to improve our understanding of the factors involved in obtaining accurate data.