Campbell Scientific CR23X Micrologger - Manual (Page 219)

SECTION 13. CR23X MEASUREMENTS
13-19
maximum and additive. A temperature of 45°C
is measured with a type T (copper-constantan)
thermocouple, using the ±5 mV range. The
nominal accuracy on this range is 2.5 µV
(0.05% of 5 mV), which at 45°C changes the
temperature by 0.06°C. The RTD is 25°C but is
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
o
C 1% Slope
Error Error
Reference Temp. 0.6 36.1 69.6
TC Output
ANSI 1.0 60.1
0.01 x 20°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 CR23X. 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 CR23X (Section 7.4). Alternatively,
the junction box can be used to couple
extension grade thermocouple wire to the
thermocouples being used for measurement,
and the CR23X 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 a external measuring junction is
where the junction box temperature is outside
the range of reference junction compensation
provided by the CR23X. 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 13.4-3 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 CR23X. 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.