Page 168 - Mechanical Engineers' Handbook (Volume 2)

3 Resistance Temperature Detectors 157

In some cases, the linearizing amplifier is provided as an integral part of the probe,
which may also contain its own power supply. The output of such a probe is linearly pro-
portional to the probe temperature.

3.6 The Standard Relationships for Temperature versus Resistance
Platinum resistance thermometers are sold in the United States under two different calibra-
tions (U.S. and European) and are subject to tolerances, which can also be described by
either of two standards.
The two calibrations differ in their expected values of : the average temperature co-
efficient of resistivity ( / C) over the interval 0–100 C. The European standard [Deutsches
¨
Institute fur Normung (DIN) 43670] specifies to be 0.003850 while the U.S. standard
specifies 0.003925/ C. The IPTS-68 specified 0.003925 for probes acceptable as standards.
The value of increases with increasing purity, and values as high as 0.003927 have been
observed, according to Norton. 37 Norton also mentions that thin films of platinum do not
follow either of the two standard calibration curves just mentioned for bulk platinum but
tend to approach the European standard value.
Table 4 illustrates the differences in resistance of U.S. and European calibrations.
From the user’s standpoint, the important issue is, ‘‘How much different are the tem-
peratures deduced from a measured resistance?’’ Figure 19 shows the temperature difference
(European–U.S.) as a function of temperature level.
The equations proposed for data interpretation (i.e., the Callendar equation or the
Callendar–Van Dusen equation) are attempts at fitting the tabular data—they are not the
sources of the tables.
Just as there are different standards for the resistance–temperature relationship, so there
are different standards for interchangeability among commercial probes. The hot resistance
of a probe at a particular temperature depends on its cold resistance and its average value
of up to the operating temperature. Standards for interchangeability must acknowledge
both sources of difference. Interchangeability is typically discussed in terms of ‘‘percent of
reading’’ (in degree Celsius), with values from 0.1 to 0.6% being available in commercial
probes for field service.

Table 4 Resistances of a Platinum Resistance Thermometer
European
Temperature, U.S. Curve, Curve,
C 0.003925 0.003850
200 17.14 18.53
100 59.57 60.20
0 100.00 100.00
100 139.16 138.50
200 177.13 175.84
300 213.93 212.03
400 249.56 247.08
500 284.02 280.93
600 317.28 313.65
Source: Reference 37.