Endress+Hauser iTEMP HART TMT142 Manuel De Mise En Service page 87

TMT142
Endress+Hauser
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
only α and δ (7):
β
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):
R = R + R
T 0
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):
R
T
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.
, and the temperature that would result from employing
l
⎛
RT – R
δ
0
l
-------------------- -
T – +
⎝
• α
l
R
0
--------------------------------------------------------------------------------------
=
⎛ ⎞ T
T
l
-------- - 1
–
⎝ ⎠
100
⎛ ⎞ ⎛
T
α T δ
-------- -
-------- - 1
– –
⎝ ⎠ ⎝
0
100 100
(
2
= R 1 + AT + BT –
0
α δ
⎛
α
----------- -
A = +
⎝
100
α δ •
----------- -
B =
100
α β •
------------
C =
100
⎛ ⎞
T T
1 )
l l
-------- - -------- -
–
⎝ ⎠
100 100
⎛ ⎞
3
l
-------- -
⎝ ⎠
100
⎞ ⎛ ⎞ ⎛
T T T
β
-------- -
-------- - 1
– –
⎠ ⎝ ⎠ ⎝
100 100
)
3
4
100CT + CT
•
⎞
⎠
2
4
Appendix
⎞
3
⎠
87