Campbell Scientific CR9000X Measurement and Control System - Manual (Page 104)

Section 3. CR9000X Measurement Details
temperature range should be made by obtaining the actual temperatures
referenced to a junction within the reference temperature range and subtracting
one temperature from the other. Table 3.1.4-3 gives the reference temperature
ranges covered and the limits of error in the linearizations within these ranges.
Two sources of error arise when the reference temperature is out of range. The
most significant error is in the calculated compensation voltage, however error
is also created in the temperature difference calculated from the thermocouple
output.
For example, suppose the reference temperature for a measurement on a type
T thermocouple is 300 °C. The compensation voltage calculated by the
CR9000X corresponds to a temperature of 272.6 °C, a -27.4 °C error. The
type T thermocouple with the measuring junction at 290 °C and reference at
300 °C would output -578.7 µV; using the reference temperature of 272.6 °C,
the CR9000X calculates a temperature difference of -10.2 °C, a -0.2 °C error.
The temperature calculated by the CR9000X would be 262.4 °C, 27.6 °C low.
TABLE 3.1.4-4. Reference Temperature
Compensation Range and Polynomial
Error Relative to NIST Standards
Type Range o C Limits of Error o C
T -100 to 100
±
0.001
J -150 to 296 ± 0.005
E -150 to 206 ± 0.005
K - 50 to 100 ± 0.01
Error Summary
The magnitude of the errors described in the previous sections illustrate that
the greatest sources of error in a thermocouple temperature measurement with
the CR9000X are likely to be due to the limits of error on the thermocouple
wire and in the reference temperature determined with the CR9050 RTD.
Errors in the thermocouple and reference temperature linearizations are
extremely small, and error in the voltage measurement is negligible.
To illustrate the relative magnitude of these errors in the environmental range,
we will take a worst case situation where all errors are maximum and additive.
A temperature of 45
o C is measured with a type T (copper-constantan)
thermocouple, using the ± 50 mV range with reverse differential. As shown
earlier in this section, the voltage measurement error would be 0.166 o C. The
RTD is 25 o C but is indicating 25.1 o C, and the terminal that the thermocouple
is connected to is 0.05 o C cooler than the RTD, resulting in a reference
temperature error of 0.15 o C.
3-16