Campbell Scientific CR5000 Measurement and Control Module - Manual (Page 63)

Section 3. CR5000 Measurement Details
3-9
0
0.05
0.1
0.15
0.2
0.25
0.3
-60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100
Temperature ºC
Thermistor Tolerance ºC
FIGURE 3.4-1. Thermistor Tolerance
The error in the reference temperature measurement is a combination of the
error in the thermistor temperature and the difference in temperature between
the panel thermistor and the terminals the thermocouple is connected to. The
terminal strip cover should always be used when making thermocouple
measurements. It insulates the terminals from drafts and rapid fluctuations in
temperature as well as conducting heat to reduce temperature gradients. In a
typical installation where the CR5000 is in a weather proof enclosure not
subject to violent swings in temperature or lopsided solar radiation loading, the
temperature difference between the terminals and the thermistor is likely to be
less than 0.2 °C.
With an external driving gradient, the temperature gradients on the input panel
can be much worse. For example, the CR5000 with the rechargeable battery
back was placed in a controlled temperature chamber. Thermocouples in
channels at the ends and middle of each analog terminal strip measured the
temperature of an insulated aluminum bar outside the chamber. The
temperature of this bar was also measured by another datalogger. Differences
between the temperature measured by one of the thermocouples and the actual
temperature of the bar are due to the temperature difference between the
terminals the thermocouple is connected to and the thermistor reference (the
figures have been corrected for thermistor errors). Figure 3.4-2 shows the
errors when the chamber was changed from -55 to 80 °C in approximately 15
minutes. Figure 3.4-3 shows the results when going from 80 to 25 °C in
approximately 45 minutes, a less dramatic change but still greater than would
be seen in most natural circumstances. During these rapid changes in
temperature, the temperature of panel thermistor will tend to lag behind the
terminals because it is buried a bit deeper in the CR5000 and is closer to the