Owner manual

Appendix Series 642
40 Pyromation, Inc.
Calculation of β
At negative temperatures (6) will still give a small deviation. Van Dusen therefore introduced a term
of the fourth order, β , which is only applicable for T < 0 °C. The calculation of β is based on the
disparity between the actual temperature, t
l
, and the temperature that would result from employing
only α and δ (7):
With the introduction of both Callendar's and van Dusen's constant, the resistance value can be
calculated correctly for the entire temperature range, as long as one remembers to set β = 0 for
T > 0 °C (8):
Conversion to A, B and C
Equation (8) is the necessary tool for accurate temperature determination. However, seeing that the
IEC 751 coefficients A, B and C are more widely used, it would be natural to convert to these
coefficients.
Equation (1) can be expanded to (9):
and by simple coefficient comparison with equation (8) the following can be determined (10):
(11)
(12)
The device accepts the coefficients to be specified as α, β, δ and A, B, C.
Information on the coefficients can be requested from the sensor manufacturers in question.
b
T
l
RT
l
R
0
–
R
0
a·
---------------------
d
T
l
100
-------- -
è
æ
1 )
T
l
100
-------- -
èø
æö
–+–
T
l
100
-------- -
1–
èø
æö
T
l
100
-------- -
èø
æö
3
--------------------------------------------------------------------------------------
=
R
T
R
0
R
0
a T d
T
100
-------- -
1–
èø
æö
T
100
-------- -
èø
æö
– b
T
100
-------- -
1–
èø
æö
T
100
-------- -
èø
æö
3
–+=
R
T
R
0
1 AT BT
2
100CT
3
– CT
4
++ +()=
A a
ad·
100
------------
èø
æö
+=
B
ad·
100
2
------------
=
C
ab·
100
4
------------
=