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Figure 7. Cutaway view of thermocouple or resistance temperature detector probe for temperature measurement. (Courtesy of the Foxboro Co., Foxboro, Mass.)

Resistance temperature detectors, RTD's, are more sensitive than thermocouples especially when measuring small temperature ranges. As a result, they are preferred for accurate and precise measurements. The principle behind these devices is based on the use of materials, such as platinum or nickel, whose resistance to current flow changes with temperature. These materials are used as one leg in a wheatstone bridge circuit with the other legs being known precision resistors. A voltage is applied across the bridge and the voltage drop midway through each path of the circuit is compared. The potential difference at the midway point is directly related to the ratio of each set of resistances in series. Since three ofthese are known, the resistance of the RTD can be calculated and the temperature inferred. If the RTD is remote from the bridge circuit, the resistance of lead wires can affect the measurement. Therefore, for highly precise measurements, compensating circuits are included which require increasing the wiring for this measuring device from two to as many as four leads.

Thermistors are a special class of RTD's and are constructed from semiconductor material. Their primary advantage is their greater sensitivity to changes in temperature, therefore making them a more precise measuring method. Their disadvantage is their nonlinear response to temperature changes. This form of RTD is gaining popularity for narrow range applications, particularly in laboratory environments.

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