Pyromation Series 642 - Manual (Page 40)

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
---------
и
ж
1 ) T l
100
---------
и ш
ж ц
– + –
T
l
100
--------- 1 –
и ш
ж ц T l
100
---------
и ш
ж ц
3
-------------------------------------------------------------------------------------- =
R
T
R
0
R
0
a T d T
100
--------- 1 –
и ш
ж ц T
100
---------
и ш
ж ц
– b T
100
--------- 1 –
и ш
ж ц T
100
---------
и ш
ж ц 3
– + =
R
T
R
0
1 AT BT 2 100CT 3 – CT 4 + + + ( ) =
A a
a d ·
100
------------
и ш
ж ц
+ =
B
a d ·
100 2
------------ =
C
a b ·
100 4
------------ =