ASL INTERCOM CTR5000, Руководство пользователя

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CTR5000 Operating Instructions
112 ASL operating instructions, model CTR5000
15. Appendix 3 – Callendar van Dusen
15.1 Callendar van Dusen equation (CvD)
Platinum Resistance Thermometers (PRTs) are one of the most linear temperature sensors that
we have. Even so, the relationship between resistance and temperature is not entirely linear
and so it is necessary for the conversion from resistance to temperature to allow for this.
The IEC751 standard provides an equation that can be used to convert resistance to
temperature taking into account the small non-linearities of the PRT –
 
 
3 2
100 1 t t C Bt At R R
o t
    
Where ‘ R
t
‘ is the resistance at temperature ‘
t
’ and ’ R
0
‘ is the resistance at 0  C. The coefficients
A , B and C ( C = 0, if
t
> 0  C ) are defined in IEC751 for standard PRTs. However, the
coefficients must be measured individually (by regression) for a particular PRT when greater
accuracy is required.
An alternative (and easier) method exists via the Callendar van Dusen (CvD) equation, based
on measurement of four accurately known temperatures. The CTR5000 allows these
coefficients to be entered for each probe used. The resistance to temperature conversion
calculation is then carried out automatically within the instrument. However, this does rely on
the user knowing the coefficients A, B and C accurately and entering these into the CTR5000.
It is worth noting that the coefficients vary between standards. Typical values (PRT dependent)
are –
Standard  A B C
DIN 43760 0.003850 3.9080 x 10
-3
-5.8019 x 10
-7
-4.2735 x 10
-12
ASTM 0.003911 3.9692 x 10
-3
-5.8495 x 10
-7
-4.2325 x 10
-12
ITS-90 0.003926 3.9848 x 10
-3
-5.870 x 10
-7
-4.0000 x 10
-12
C= 0 if t > 0  C