Campbell CR10 PROM, Operator'S Manual

Page: 195 / 253

SECTION 13. CR10 MEASUREMENTS
13-16
indicating 25.3
°
C, and the terminal that the
thermocouple is connected to is 0.3
°
C cooler
than the RTD.
TABLE 13.4-4. Example of Errors in
Thermocouple Temperature
Source Error
°
C % of Total Error
1
°
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
Linearization
0.001 0.1 0.1
Output
Linearization
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 facilitate
connections and to reduce the expense of
thermocouple wire when the temperature
measurements are to be made at a distance from
the CR10. 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 CR10 (Section 3.4).
Alternatively, the junction box can be used to
couple extension grade thermocouple wire to the
thermocouples being used for measurement, and
the CR10 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
necessary to use extension grade wire instead of
an external measuring junction is where the
junction box temperature is outside the range of
reference junction compensation provided by the
CR10. This is only a factor when using type K
thermocouples, where the upper limit of the
reference compensation linearization is 100
°
C and
the upper limit of the extension grade wire is
200
°
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 13.4-4 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 CR10. This thermoelectric offset voltage
is a factor whether or not the junction box is used
for the reference. It 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.
An external reference junction box must be
constructed so that the entire terminal area is
very close to the same temperature. This is
necessary so that a valid reference temperature
can be measured, and to avoid a thermoelectric
offset voltage which will be induced if the
terminals at which the thermocouple leads are
connected (points A and B in Figure 13.4-4) are
at different temperatures. The box should
contain elements of high thermal conductivity,
which will act to rapidly remove any thermal
gradients to which the box is subjected. It is
not necessary to design a constant temperature
box; it is desirable that the box respond slowly
to external temperature fluctuations.