handbook of modern sensors

16.1 Thermoresistive Sensors 463
Table 16.2. Temperature Differences Between IPTS-68 and ITS-90
t 90 ( ◦ C) −10 0 10 20 30 40
T 90 − t 68 ( ◦ C) 0.002 0.000 −0.002 −0.005 −0.007 −0.010
Source: Saunders, P. The International Temperature Scale of 1990, ITS-90. WOCE
Newsletter 10, 1990.
some materials. This scale designated Kelvin temperatures by the symbol T 68 and the
Celsius scale by t 68 . The International Committee for Weights and Measures adopted a
new International Temperature Scale (ITS-90) during its meetings in September 1989.
Its Celsius temperature designation is t 90 .The difference between the two scales may
be significant for some precision measurements (Table 16.2).
Equation (3.58) of Chapter 3 gives a best fit second-order approximation for
platinum. In industry, it is customary to use separate approximations for the cold
and hot temperatures. Callendar–van Dusen approximations represent the platinum
transfer functions:
For the range from −200 ◦ Cto0 ◦ C,
R t = R 0 [1 + At + Bt 2 + Ct 3 (t − 100)]. (16.10)
For the range from 0 ◦ Cto630 ◦ C, it becomes identical to Eq. (3.58) of Chapter 3:
R t = R 0 (1 + At + Bt 2 ). (16.11)
The constants A, B, and C are determined by the properties of platinum used in the
construction of the sensor. Alternatively, the Callendar–van Dusen approximation can
be written as
[ ( )( ) ( ) t t
t 3 ( )
R t = R 0
{1 ]} t
+ α t − δ
100 100 − 1 − β
100 100 − 1 , (16.12)
where t is the temperature in ◦ C and the coefficients are related to A, B, and C as
(
A = α 1 + δ )
, B=−αδ × 10 −4 , C=−αβ × 10 −8 . (16.13)
100
The value of δ is obtained by calibration at a high temperature, [e.g., at the freezing
point of zinc (419.58 ◦ C)] and β is obtained by calibration at a negative temperature.
To conform with ITS-90, the Callendar–van Dusen approximation must be corrected.
The correction is rather complex and the user should refer for details to ITS-90.
In different countries, some national specifications are applicable to RTDs. For instance,
in Europe, these are the following: BS 1904: 1984; DIN 43760–1980; IEC 751:
1983. In Japan, it is JIS C1604-1981. In the United States, different companies have
developed their own standards for α values. For example, SAMA Standard RC21-
4-1966 specifies α = 0.003923 ◦ C −1 , whereas in Europe, the DIN standard specifies
α = 0.003850 ◦ C −1 and the British Aircraft industry standard is α = 0.003900 ◦ C −1 .