Campbell CR10X, Instruction Manual, Page: 270 / 363

CR10X Reference Manual
13-18
Error Summary
The magnitude of the errors described in the previous sections illustrate that the
greatest sources of error in a thermocouple temperature measurement are likely to
be the limits of error on the thermocouple wire and in the reference temperature
determined with the 10TCRT thermistor. Errors in the thermocouple and reference
temperature polynomials 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 ±2.5mV range. The nominal accuracy on this range is 2.5µV (0.1% of
2.5mV), which at 45
o
C changes the temperature by 0.06
o
C. The reference
temperature device (RTD) is at 25
o
C but is indicating 25.3
o
C, and the terminal
that the thermocouple is connected to is 0.3
o
C cooler than the RTD.
Table 13-11 Example of Errors in Thermocouple Temperature
Source Error
o
C % of Total Error
1
o
C 1% Slope
Error Error
Reference Temp. 0.6 36.1 69.6
TC Output
ANSI 1.0 60.1
0.01 x 20
o
C 0.2 23.2
Voltage
Measurement
0.06 3.6 7.0
Reference
Linearisation
0.001 0.1 0.1
Output
Linearisation
0.001 0.1 0.1
_________________________________________________________________
Total Error
With ANSI error 1.662 100
Assuming 1% 0.862 100
slope error
13.4.2 Use of External Reference Junction or Junction Box
An external junction box is often used to make connections simpler and to reduce
the expense of thermocouple wire when the temperature measurements are to be
made at a distance from the CR10X. In most situations, it is preferable to make the
box the reference junction, in which case its temperature is measured and used as
the reference for the thermocouples; copper wires are run from the box to the
CR10X. Alternatively, the junction box can be used to couple extension grade
thermocouple wire to the thermocouples being used for measurement, and the
CR10X panel used as the reference junction. Extension grade thermocouple wire
has a smaller temperature range than standard thermocouple wire, but meets the
same limits of error within that range. The only situation where it would be