Page 170 - Mechanical Engineers' Handbook (Volume 2)

3 Resistance Temperature Detectors 159

For high-quality, laboratory-grade platinum resistance thermometers (used between 0
and 630 C), the Callendar equation describes the IPTS-68 temperature within 0.045 C,
38
according to Benedict. The difference between IPTS-68 and the Callendar equation varies
across the range of temperatures, as shown in Fig. 20.
The Callendar equation is
t 100 R R 0 t 1
t
t
R R 100 100 (7)
100 0
where R resistance at temperature t
t
R resistance at ice point
0
R resistance at 100 C
100
constant evaluated at zinc point (419.58)
As shown in Fig. 20, the tolerance of 0.045 C represents an agreed-upon difference
between the Callendar equation and the IPTS-68 equation.

Figure 20 The difference between the Callendar equation and IPTS-68. (From Ref. 38, with permis-
sion.)