Specifications

Section 3. CR3000 Measurement Details
TABLE 3.4-5. 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
:11.7% 0.15
o
:31.1% 0.15
o
:12.4% 0.15
o
:36.4%
TC Output 1.0
o
:78% 0.2
o
:41.5% 1.0
o
:82.5% 0.2
o
:48.6%
Voltage
Measurement
0.10
o
:7.8% 0.10
o
:20.8% 0.05
o
:4.1% 0.05
o
:12.1%
Noise 0.03
o
:2.3% 0.03
o
:6.2% 0.01
o
:0.8% 0.01
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 1.282
o
:100% 0.482
o
:100% 1.212
o
:100% 0.412
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 CR3000. 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 CR3000. Alternatively, the junction box can be used to couple
extension grade thermocouple wire to the thermocouples, and the CR3000
panel temperature used as the reference. 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 CR3000. 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 3.4-4 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
CR3000. 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.
3-16