A thermocouple may be directly connected to a control or monitor. Extension wires, if used, must be of the same materials as the thermocouple wires.

Thermocouples designed with their measuring junctions in contact with new surfaces are known as grounded junction thermocouples. These are the most common, generally have the fastest response times, and are the most economical. Ungrounded junction thermocouples offer the advantage of electrical isolation. Dwyer® manufactures both types of thermocouples. Thermocouples are generally more rugged and less expensive than other sensor types. All Dwyer® thermocouples are manufactured using industry standard alloys and meet astringent ANSI standards. This assures interchangeability with other standard thermocouples without special instrument recalibration.

Dwyer® thermocouples are used in all types of applications, can measure wide temperature ranges, and are offered in a large variety of standard configurations.

RTDs
RTDs are usually platinum wire wound on a glass or ceramic bobbin and sealed with a coating of ceramic or glass. They can also be made by depositing platinum as a film on a substitute and then encapsulating it. The electrical resistance of the RTD changes as a function of temperature. Circuitry similar to a Wheatstone bridge is built into controls designed for use with RTDs. Constant current into the bridge produces an output voltage that varies with temperature. Lead wire resistance can significantly affect the RTD measurement. This is typically corrected using a third (compensating) lead wire. See Figures 3 and 4.

Extension wires used with RTDs may be plain copper wire. RTDs are generally more accurate and more stable over time than thermocouples. Dwyer® RTDs are built to rigorous DIN (most common) or NIST standards and are offered in a wide variety of standard configurations.

The third wire in the 3-wire RTD compensates for the wire lead resistance and its temperature change.

Thermistors
Thermistors have a semiconductor material which changes its electrical resistance as a function of temperature. Extension wires used with thermistors can be plain copper wire.

Thermistors offer accuracy similar to RTDs within narrow temperature ranges near ambient temperature. They also generally offer faster response times. Since thermistor standards vary, care must be taken to match the instrumentation to the sensor.

Ordering Sensors:
Sensors are constructed with various types of protection/mounting hardware, extensions, and wire terminations. The sensor types and their temperature ranges are shown in the table below. See "Temperature Limits" below for maximum service temperatures applicable to the protection tube, mounting hardware, wire extensions, etc.

This section shows only a limited selection of the available sensors. The sensors are organized by hardware type. Most hardware can house any type thermocouple, RTD, or thermistor. Terminations are usually either lug type or standard plugs, but many other types are available. Various 'head enclosures' are also available. Dimensions can be custom designed to meet your specifications. The selections listed are the most popular configurations. Please ask your sales representative about other possible selections.

Temperature Limits:
Sensor selection depends on two separate temperatures: process temperature and connector temperature. Make sure the local temperature at each component does not exceed the maximum rated service temperature for that component. Note that the extension wire must withstand the process temperature. All Dwyer® thermocouple and RTD assemblies (including extension wire) shown in this bulletin are designed for process temperatures to at least 900°F. Please consult factory if higher service temperatures are needed.