Resistance Elements and RTD - Heanjia Super-metals Co., Ltd.

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Resistance Elements and RTD

Resistance Elements and RTD

Resistance elements come in many types conforming to different standards, capable of different temperature ranges, with various sizes and accuracies available. But they all function in the same manner: each has a pre-specified resistance value at a known temperature which changes in a predictable fashion. In this way, by measuring the resistance of the element, the temperature of the element can be determined from tables, calculations or instrumentation. These resistance elements are the heart of the RTD (Resistance Temperature Detector). Generally, a bare resistance element is too fragile and sensitive to be used in its raw form, so it must be protected by incorporating it into an RTD.

Resistance Temperature Detector is a general term for any device that senses temperature by measuring the change in resistance of a material. RTD come in many forms, but usually appear in sheathed form. An RTD probe is an assembly composed of a resistance element, a sheath, lead wire and a termination or connection. The sheath, a closed end tube, immobilizes the element, protecting it against moisture and the environment to be measured. The sheath also provides protection and stability to the transition lead wires from the fragile element wires.

Some RTD probes can be combined with thermowells for additional protection. In this type of application, the thermowell may not only add protection to the RTD, but will also seal whatever system the RTD is to measure (a tank or boiler for instance) from actual contact with the RTD. This becomes a great aid in replacing the RTD without draining the vessel or system.

Thermocouples are the old tried and true method of electrical temperature measurement. They function very differently from RTD but generally appear in the same configuration: often sheathed and possibly in a thermowell. Basically, they operate on the Seebeck effect, which results in a change in thermoelectric emf induced by a change in temperature. Many applications lend themselves to either RTD or thermocouples. Thermocouples tend to be more rugged, free of self-heating errors and they command a large assortment of instrumentation. However, RTD, especially platinum RTD, are more stable and accurate.

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