Campbell Scientific CR7 Measurement and Control System - Manual (Page 71)

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-3
The temperature of the 107 Probe is stored in
Input Location 1 and the thermocouple
temperatures in Locations 2-11.
PROGRAM
01: P11 Temp 107 Probe
01: 1 Rep
02: 1 IN Card
03: 21 IN Chan
04: 1 EX Card
05: 1 EX Chan
06: 1 Loc [:Ref. Temp]
07: 1 Mult
08: 0 Offset
02: P14 Thermocouple Temp (DIFF)
01: 10 Reps
02: 3 15 mV slow Range
03: 1 IN Card
04: 1 IN Chan
05: 1 Type T (Copper-Constantan)
06: 1 Ref Temp Loc Ref. Temp
07: 2 Loc [:TC temp#1]
08: 1 Mult
09: 0 Offset
7.5 THERMOCOUPLES FOR
DIFFERENTIAL TEMPERATURE
MEASUREMENT
When configured correctly, thermocouples are
capable of measuring small temperature
gradients very accurately (Section 13.4). In this
example, the CR7 is used to make five
differential temperature measurements with
chromel-constantan thermocouples. The
connections are shown in Figure 7.5-1 where
the voltage measured between the chromel
leads is proportional to the temperature
difference between junctions R and D.
FIGURE 7.5-1. Connection for
Thermocouple Differential Temperature
Measurement
When the temperatures are within the reference
junction compensation range (Table 13.4-3),
three instructions are required in the
measurement sequence:
1. A CR7 Panel Temperature Measurement
(#17) used as a reference temperature for
the measurement at R.
2. A single ended TC measurement (#13) of R
temperatures to be used as reference
temperatures for the measurement D.
3. A differential TC measurement of D
temperatures where the reference
temperature at R are subtracted from the
results as specified in Parameter 5.
The connection shown in Figure 7.4-1 yields the
conventional polarity (sign) for the temperature
difference, i.e., D>R=+T, D<R=-T. Using R as
the reference temperature maintains this
convention whereas using D reverses the sign
of the output.
Prefixing a 2 onto the TC type in Parameter 5 of
Instruction 13 causes the CR7 to skip every
other single ended channel. Keying C before
entering Parameter 6 in Instruction 14 causes
the reference temperature location to be
incremented each rep.
The ±5mV range used in Instruction 13 allows
measurement of temperatures at R within a
range of approximately ±80 oC of the I/O
Module temperature. The ±1.5mV range used in
Instruction 14 allows the temperature difference
between D and R to approach a range of ±24
oC, for temperatures around 25 oC (output = 61
uV/oC). The resolution of the differential
temperature measurement is approximately
0.0008 oC (50 nV/61µV/oC).
The panel temperature is stored in Input
Location 1, the temperatures of the R junctions
in locations 2-6 and the temperature differences
(D-R) in locations 7-11. If it is not necessary to
retain the temperatures of the R junctions, the
temperature differences could be stored in
locations 2-6 by changing Parameter #7 in
Instruction 14 to 2.