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Platinum Resistance Thermometers (PRTs)

Typical PRT sensor configuration, with platinum wire arrayed in coils 
Typical PRT sensor configuration, with platinum wire arrayed in coils
Platinum Resistance Thermometers (PRTs) rely on the fact that platinum, like many other metals, exhibits increased electrical resistance as temperature rises. For example, a conventional PRT designed to provide 100 Ω at 0 °C has a resistance in the neighborhood of 80 Ω at -50 °C and 120 Ω at 50 °C, or a sensitivity of about 0.4 Ω per degree. PRTs constructed to particularly exacting specifications, termed Standard Platinum Resistance Thermometers (SPRT), are named as the defining measurement tools for interpolating temperatures under ITS-90. In general, PRTs can have high accuracy (0.01 °C), stability, and repeatability across a wide range of temperatures from -200 °C to 500 °C.

Typically the platinum element is formed into thick or thin films, or the platinum wire is arranged in two, three or four helical coils (see diagram, right) – the more coils, the higher the sensitivity. The film or wire is placed inside a glass or ceramic enclosure, and can be supported by loose or compacted MgO. Platinum-based leads connect the probe unit to the thermometer electronics, which convert the electrical signal to temperature.

PRTs are broadly divided into two groups: Industrial PRTs (IPRTs) and SPRTs, depending on sensitivity and robustness. ASTM and IEC define several classes of PRTs, each with a different set of specifications. An ASTM "Class A" unit, for example, has an out-of-the-box tolerance — maximum permissible error — that ranges from 0.47 °C at -200 °C to 0.13 °C at 0 °C to 0.98 °C at 500 °C.

Users considering an IPRT should consult ASTM E1137 for standards and specifications.

NIST calibrates these devices from -196 °C to 550 °C

Advantages

Wide temperature range

Resistance-temperature relationship is well characterized.

Rugged construction in IPRTs

Cost of an IPRT is less than an SPRT.

Available in different shapes and sizes – application specific

Can be used with a digital temperature read-out device.

Disadvantages

Mechanical shock and vibration will cause drift.

Deterioration at elevated temperatures (e.g., >500 °C)

2- and 3- wire devices need lead-wire compensation.

Non-hermetically sealed IPRTs will deteriorate in environments with excessive moisture.

Not as accurate as an SPRT

 

A variety of PRT probes, one of which has a visible wire-coil configuration 
A variety of PRT probes, one of which (magnified in inset) has a visible wire-coil configuration.
Image of a PRT sensor, with a penny next to it for size comparison
PRT sensors can be quite small.

 


Detail of a high-end PRT shows platinum wire spirals in the probe tip.

Detail of a Standard PRT shows platinum wire spirals in the probe tip.

Any mention or image of commercial products within NIST web pages is for information only; it does not imply recommendation or endorsement by NIST.

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