Error summary – Campbell Scientific CR5000 Measurement and Control Module User Manual
Page 69
Section 3. CR5000 Measurement Details
3-15
TABLE 3.4-4. Reference Temperature Compensation Range and
Polynomial Error Relative to NIST Standards
TC
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 CR5000 are likely to be due to the limits of error on the thermocouple wire
and in the reference temperature. 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
±
20 mV range. The nominal accuracy on this range is
1 µV (0.01% of 10 mV) which at 45
o
C changes the temperature by 0.012
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.
TABLE 3.4-5. Example of Errors in Thermocouple Temperature
Source
Error:
o
C : % of Total Error
Single Differential
No Integration
Reversing Differential
250 µs Integration
ANSI TC Error
(1
o
C)
TC Error 1%
Slope
ANSI TC
Error (1
o
C)
TC Error 1%
Slope
Reference Temp.
0.15
o
:11.5%
0.15
o
:29.8%
0.15
o
:12.6%
0.15
o
:38.4%
TC Output
1.0
o
:76.7%
0.2
o
:39.8%
1.0
o
:84%
0.2
o
:51.2%
Voltage
Measurement
0.081
o
:6.2%
0.081
o
:16.1%
0.024
o
:2%
0.024
o
:6.1%
Noise
0.07
o
:5.4%
0.07
o
:13.9%
0.015
o
:1.2%
0.015
o
:3.8%
Reference
Linearization
0.001
o
:0.1%
0.001
o
:0.2%
0.001
o
:0.1%
0.001
o
:0.25%
Output
Linearization
0.001
o
:0.1%
0.001
o
:0.2%
0.001
o
:0.1%
0.001
o
:0.25%
Total Error
1.303
o
:100%
0.503
o
:100%
1.191
o
:100%
0.391
o
:100%
3.4.2 Use of External Reference Junction or Junction Box
An external junction box is often used to facilitate connections and to reduce
the expense of thermocouple wire when the temperature measurements are to
be made at a distance from the CR5000. 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 and copper wires are run from