Page 163 - Mechanical Engineers' Handbook (Volume 2)

152 Temperature and Flow Transducers

3 RESISTANCE TEMPERATURE DETECTORS
The terms resistance temperature detector and resistance thermometer are used interchange-
ably to describe temperature sensors containing either a fine-wire or a thin-film metallic
element, whose resistance increases with temperature. A small current (ac or dc) is passed
through the element, and its resistance is measured. The temperature of the element is then
deduced from the measured resistance using a calibration equation or a table.

3.1 Types and Ranges
Resistance temperature detectors can be designed for standards and calibration laboratories
or for field service, although the probe designs are vastly different. Field service probes are
generally encased in stainless steel protective tubes, with the wire or film elements bonded
to sturdy support structures. They are made to handle considerable physical abuse. Labora-
tory standards probes are often enclosed in quartz tubes, with the resistance wire mounted
in a strain-free manner on a delicate mandrel. High-precision temperature measurement re-
quires that the element be strain free, so the electrical resistance is a function only of tem-
perature, not of strain.
Resistance temperature detectors are well suited for single-point measurements in
steady-state service at temperatures below 1000 C where long-time stability and traceable
accuracy are required and where reasonably good heat transfer conditions exist between the
probe and its environment.
These detectors are not recommended for use in still air or in low-conductivity envi-
ronments unless the self-heating effect can be accounted for appropriately. They are not
recommended for averaging service unless computer data acquisition and software averaging
are contemplated. They are not recommended for transient service or dynamic temperature
measurements unless specifically designed for such characteristics, since the usual probe is
not amenable to simple time constant compensation.
Resistance temperature detector probes tend to be larger than some of the alternative
sensors and to require more peripheral equipment (i.e., bridges and linearizers).

3.2 Physical Characteristics of Typical Probes
Probes intended for field service concentrate on ruggedness and repeatability. Their calibra-
tions may not agree with the standard expectation values of laboratory-grade probes but
should be repeatable. Drift tolerances on field service probes may be stated in terms of
percent drift per 100 hours.
Probes for laboratory service are designed to ensure freedom from mechanical strain
either due to fabrication or thermal expansion during service.
The bare sensing elements of resistance temperature detectors range in size from wafers
0.5 1.0 2.0 mm, with pig-tail leads 0.25 mm in diameter and 2.5 cm in length, to wire-
wrapped mandrels, again with pig-tail leads 4 mm in diameter and 2 cm in length. With
protective tubes in place, typical probes range from 1.0 to 5.0 mm in diameter.
Some typical sensors and probes for field service are shown in Fig. 16. Stainless steel
is used for the protection tubes on most such units. The sensing element can be either a wire
wound on a mandrel or a thin film deposited on an insulating substrate. Wire resistance
elements may also be bonded to their support structures and encapsulated in glass or ceramic,
but strain-free, steel-jacketed probes are also available. Thin-film elements are usually formed
directly on the substrate by sputtering or vapor deposition.
Some, but not all, laboratory-grade probes use quartz for the protection tubes. For high-
est accuracy and best long-term stability, the laboratory-grade probes use strain-free rigging