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| Resistance Temperature Detectors (RTDs) |
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Resistance Temperature Detectors
(RTDs) rely on the predictable and repeatable phenomena of the
electrical resistance of metals changing with temperature. The
temperature coefficient for all pure metals is of the same order
- 0.003 to 0.007 ohms/ohm/°C. The most common metals used for
temperature sensing are platinum, nickel, copper and molybdenum.
While the resistance - temperature characteristics of certain
semiconductor and ceramic materials are used for temperature sensing
(see sections on Semiconductor and Thermistor ), such sensors
are generally not classified as RTDs.
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How are RTD constructed?
OORTDs are manufactured in two ways:
using wire or film. Wire RTDs are a stretched coil of fine wire
placed in a ceramic tube that supports and protects the wire.
The wire may be bonded to the ceramic using a glaze. The wire
types are generally the more accurate, due to the tighter control
over metal purity and less strain related errors. They are also
more expensive.
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*
Film RTDs consist of a thin metal film that
is silk-screened or vacuum spluttered onto a ceramic or glassy
substrate. A laser trimmer then trims the RTD to its correct resistance
value.
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