Campbell Scientific CR1000 Measurement and Control System - Manual (Page 254)

Section 7. Installation
254
non-standard types. Measured temperatures are compared against the ITS-90
scale, a temperature instrumentation-calibration standard.
PRTCalc() follows the principles and equations given in the US ASTM E1137-04
standard for conversion of resistance to temperature. For temperature range 0 to
650 °C, a direct solution to the CVD equation results in errors < ±0.0005°C
(caused by rounding errors in CR1000 math). For the range of -200 to 0°C, a
fourth-order polynomial is used to convert resistance to temperature resulting in
errors of < ±0.003°C.
These errors are only the errors in approximating the relationships between
temperature and resistance given in the relevant standards. The CVD equations
and the tables published from them are only an approximation to the true linearity
of an RTD, but are deemed adequate for industrial use. Errors in that
approximation can be several hundredths of a degree Celsius at different points in
the temperature range and vary from sensor to sensor. In addition, individual
sensors have errors relative to the standard, which can be up to ±0.3°C at 0°C with
increasing errors away from 0°C, depending on the grade of sensor. Highest
accuracy is usually achieved by calibrating individual sensors over the range of
use and applying corrections to the R
S
/R
O
value input to the PRTCalc()
instruction (by using the calibrated value of R
O
) and the multiplier and offset
parameters.
Refer to CRBasic Editor Help for specific PRTCalc() parameter entries. The
following information is presented as detail beyond what is available in CRBasic
Editor Help.
The general form of the Callendar-Van Dusen (CVD) equation is:
R/R
0
<1: T = g * K^4 + h * K^3 + I * K^2 + j * K, where K =
R/R
0
– 1
R/R
0
>=1: T = (SQRT(d * (R/R
0
) + e) -a) / f
Depending on the code entered for parameter Type , which specifies the platinum-
resistance sensor type, coefficients are assigned values according to the following
tables.
Note Coefficients are rounded to the seventh significant digit to match the
CR1000 math resolution.
Note Alpha is defined as (R
100
/R
0
-1)/100, where R
100
and R
0
are the resistances of
the PRT at 100°C and 0°C, respectively.
Table 45. PRTCalc() Type-Code-1 Sensor
IEC 60751:2008 (IEC 751), alpha = 0.00385. Now internationally
adopted and written into standards ASTM E1137-04, JIS 1604:1997,
EN 60751 and others. This type code is also used with probes
compliant with older standards DIN43760, BS1904, and others.
(Reference: IEC 60751. ASTM E1137)
Constant Coefficient
a 3.9083000E-03
d -2.3100000E-06