Campbell Scientific CR9000X Measurement and Control System - Manual (Page 105)

Section 3. CR9000X Measurement Details
3-17
TABLE 3.1.4-5. Example of Errors in Thermocouple Temperature
Source Error: o C : % of Total Error
Single-Ended or single
Differential
Reversing Differential
w:500 µs Integration
ANSI TC
Error (1
o C)
TC Error 1%
Slope ANSI TC Error
(1
o C)
TC Error 1%
Slope
Reference
Temp. 0.150 o : 10.6% 0.150 o : 24.3% 0.150 o : 12.3% 0.150 o : 36.2%
TC Output 1.000 o : 70.5% 0.200 o : 32.3% 1.000 o : 82.4% 0.200 o : 48.3%
Voltage
Measurement 0.166 o : 11.7% 0.166 o : 26.8% 0.052 o : 4.3% 0.052 o : 12.6%
Noise 0.100 o : 7% 0.100 o : 16.2% 0.010 o : 0.8% 0.010 o : 2.4%
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 1.418 o : 100% 0.618 o : 100% 1.214 o : 100% 0.414 o : 100%
3.1.4.2 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 CR9000X. 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
the box to the CR9000X. Alternatively, the junction box can be used to couple
extension grade thermocouple wire to the thermocouples being used for
measurement, and the CR9000X I/O Module 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 necessary to use extension grade
wire instead of a external measuring junction is where the junction box
temperature is outside the range of reference junction compensation provided
by the CR9000X. This is only a factor when using type K thermocouples,
where the upper limit of the reference compensation linearization is 100
o C and
the upper limit of the extension grade wire is 200 o C. With the other types of
thermocouples the reference compensation range equals or is greater than the
extension wire range. In any case, errors can arise if temperature gradients
exist within the junction box.
Figure 3.1.4-2 illustrates a typical junction box. Terminal strips will be a
different metal than the thermocouple wire. Thus, if a temperature gradient
exists between A and A' or B and B', the junction box will act as another
thermocouple in series, creating an error in the voltage measured by the
CR9000X. This thermoelectric offset voltage is a factor whether or not the
junction box is used for the reference. This offset can be minimized by making
the thermal conduction between the two points large and the distance small.
The best solution in the case where extension grade wire is being connected to
thermocouple wire would be to use connectors which clamped the two wires in
contact with each other.