Campbell Scientific CR9000X Measurement and Control System - Manual (Page 315)

Section 8. Processing and Math Instructions
Parameter
& Data Type Enter PRT PARAMETERS
Dest
Var. or Array
The variable in which to store the temperature in degrees C.
Reps
Constant
The number of repetitions for the measurement or instruction.
Source
Var. or Array
The name of the Variable that is the input for the instruction. Must be the ratio R
S
/R
0
, where R
S
is the
RTD resistance and R
0
the resistance of the RTD at 0 ° C.
Mult, Offset
Constant, Var.,
Array, or
Expression
A multiplier and offset by which to scale the raw results of the measurement. See the measurement
description for the units of the raw result; a multiplier of one and an offset of 0 are necessary to output in
the raw units. For example, the TCDiff instruction measures a thermocouple and outputs temperature in
degrees C. A multiplier of 1.8 and an offset of 32 will convert the temperature to degrees F.
PRTCalc (Dest, Reps, Source, PRTType, Mult, Offset)
The PRTCalc instruction is used to calculate temperature from the resistance of an
RTD.
A number of different types of RTDs are supported.
Syntax
PRTCalc(Dest, Reps, Source, PRTType, Mult, Offset)
Remarks
This instruction uses the result of a previous RTD bridge measurement to
calculate the temperature in degrees Celsius.
The input (Source) must be the ratio
R
S
/R
0
, where R
S
is the RTD resistance and R
0
the resistance of the RTD at 0 ° C.
A number of different sensor types are supported. The correct PRT type should
be entered into the PRTType parameter to match the standard to which the
sensor is said to conform and/or the alpha value for the sensor. The alpha value
is the fundamental measure of the change of resistance for a given temperature
change.
For industrial grade RTDs the relationship between temperature and resistance
are characterized by a formula called the Callendar-Van Dusen (CVD)
equation. The parameters for different sensor types are given in the standards or
by the manufacturers for non-standard types. Temperature is now referenced to
the ITS-90 temperature scale. PRTCalc follows the principles given in the US
ASTM E1137-04 standard for conversion back from resistance to temperature.
For the temperature range of 0 to +850 degrees Celsius a direct solution to the
CVD equation is used resulting in errors <+/-0.0005 Celsius (caused by
rounding errors in the datalogger math). For the range of -200 to 0 Celsius a 4th
order polynomial is used to convert from resistance to temperature resulting in
errors of <+/-0.003 Celsius.
Note 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 in reality 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 degrees Celsius at different points
in the temperature range and will vary from sensor to sensor. In addition
individual sensors have errors relative to the standard, which can be up to +/-0.3
Celsius at 0 Celsius with increasing error as the temperature moves away from 0
Celsius, depending on the grade of sensor.
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