Campbell Scientific ASPTC Aspirated Thermocouple - Manual (Page 9)

Model ASPTC Aspirated Shield with Fine Wire Thermocouple
EXAMPLE 2. Sample CR10(X) Instructions for measuring Absolute
Temperature with ASPTC
1: Temp (107) (P11)*
1: 1
Reps
2: 1 SE Channel
3: 3 Excite all reps w/E3
4: 1** Loc [ CR10TCR_C ]
5: 1 Mult
6: 0 Offset
CAUTION: The CR10TCR reference temperature must be in degrees Celsius when used in
Instruction 14.
2: Thermocouple Temp (DIFF) (P14)
1: 1
Reps
2: 21† ± 2.5 mV 60 Hz Rejection Range
3: 6*** DIFF Channel
4: 2 Type E (Chromel-Constantan)
5: 1** Ref Temp Loc [ CR10TCR_C ]
6: 2** Loc [ ASPTC_C ]
7: 1‡ Mult
8: 0‡ Offset
* Instruction 17 is used to find the
panel temperature on a 21X.
** Proper entries will vary depending on the program.
*** Differential input channel 4 on the 21X.
†
On the 21X the 5 mV slow input range is used.
‡ For degrees Fahrenheit use a Mult of 1.8 and Offset of 32.
3.2 Delta Temperature
Delta temperature can be measured with two ASPTCs. Wire them as shown in
Figure 3-2. A voltage is induced between the lower and upper ASPTC and is
directly related to the difference in temperature at those two levels. There is no
inherent sensor offset error. The delta temperature is then measured directly
with the TCDiff or Instruction 14. The temperature of the lower ASPTC is
used as the reference for the differential thermocouple measurement.
If the accuracy of the absolute temperature at both the lower and upper ASPTC
is a concern, measure both ASPTCs with TCDiff or Instruction 14 (differential
voltage thermocouple measurement) as outlined in Section 3.1. To find the
delta temperature, simply subtract the upper temperature from the lower one.
If the 21X is used to power the fans (or any other 12 VDC sensor) the current
drawn by the fans may cause a difference in ground potential between the 21X
ground terminals and the reference ground point in the datalogger. This
ground potential difference results in an offset on single ended measurements.
In thermocouple measurements this offset can translate to as much as ±1°C.
This offset does not affect differential measurements, thus, the delta
temperature is not affected.
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