Page 171 - Mechanical Engineers' Handbook (Volume 2)

160 Temperature and Flow Transducers

For most measurement purposes, the Callendar equation can be considered accurate
within 0.045 C over the range 0–630 C.
Below 0 C, temperature according to the Callendar equation deviates significantly from
the IPTS-68 temperature. An additional term converts the Callendar equation into the
Callendar–Van Dusen equation, which is much closer to the IPTS-68 temperature and is
recommended for field service work between 190 C and 0 C.
The Callendar–Van Dusen equation is
t 100 R R 0 t 1 t 1 3
t
t
t
R 100 R 0 100 100 100 100 (8)
where R resistance at temperature t
t
R resistance at ice point
0
R 100 resistance at 100 C
constant determined at zinc point (419.58)
0 t 0
and
constant determined at oxygen point ( 182.962 C) t 0

According to Benedict, 38 the disagreement between the Callendar–Van Dusen equation
and IPTS-68 does not exceed 0.03 C from 190 to 0 C. The difference varies, as shown
in Fig. 21.

Figure 21 Difference between the Callendar–Van Dusen equation and IPTS-68. (From Ref. 38, with
permission).