Specifications

Section 4. Sensor Support
4-28
4.4.1.7 Error Summary
The magnitude of the errors described in Section 4.4.1 illustrate that the
greatest sources of error in a thermocouple temperature measurement are likely
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.
TABLE 4.4-5 illustrates the relative magnitude of these errors in the
envi
ronmental range. It shows 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 ±2.5 mV range. The reference
thermistor measures 25.1
o
C, The terminal the thermocouple is connected to is
0.05
o
C cooler than the reference thermistor (0.15 °C error).
TABLE 4.4-5. Example of Errors in Thermocouple Temperature
Source
Error:
o
C : % of Total Error
Single Differential
250 µs Integration
Reversing Differential
50/60 Hz Rejection 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
:29.9% 0.15
o
:12.2% 0.15
o
:34.7%
TC Output
1.0
o
:76.8% 0.2
o
:39.8% 1.0
o
:81.1% 0.2
o
:46.3%
Voltage
Measurement
0.12
o
:9.2% 0.12
o
:23.9% 0.07
o
:5.7% 0.07
o
:16.2%
Noise
0.03
o
:2.3% 0.03
o
:6.2% 0.01
o
:0.8% 0.01
o
:2.3%
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
1.302
o
:100% 0.502
o
:100% 1.232
o
:100% 0.432
o
:100%
4.4.1.8 Use of External Reference Junction
An external junction in an insulated box is often used to facilitate thermocouple
connections. It can reduce the expense of thermocouple wire when
measurements are made long distances from the CR1000. Making the external
junction the reference junction, which is preferable in most applications, is
accomplished by running copper wire from the junction to the CR1000.
Alternatively, the junction box can be used to couple extension grade
thermocouple wire to the thermocouples, with the PanelTemp() instruction
used to determine the reference junction temperature.
Extension grade thermocouple wire has a smaller temperature range than
standard thermocouple wire, but meets the same limits of error within that
range. One situation in which thermocouple extension wire is advantageous is
when the junction box temperature is outside the range of reference junction
compensation provided by the CR1000. This is only a factor when using type