|Typical PRT sensor configuration, with platinum wire arrayed in coils
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
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.
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
Detail of a Standard PRT shows platinum wire spirals in the probe tip.
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