4 thermocouple measurements – Campbell Scientific CR9000X Measurement and Control System User Manual
Page 98
Section 3. CR9000X Measurement Details
Each trace in Figure 3.1-1, Settling Time for Pressure Transducer, contains all
20 PT() values for a given record number, along with an averaged value
showing the measurements as percent of final reading. The reading has settled
to 99.5% of the final value by the fourteenth measurement, PT(14). This is a
suitable accuracy for the application, so a settling time of 1400
μs is
determined to be adequate.
Settling Time
0.036
0.037
0.038
0.039
0.04
0.041
0.042
0.043
0.044
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Time (x100 us)
mV
/V
olt
85
87
89
91
93
95
97
99
% of
Fina
l V
a
lue
Series1
Series2
Series3
Series4
Series5
Average %
FIGURE 3.1.3-1. Settling time for pressure transducer
3.1.4 Thermocouple Measurements
A thermocouple consists of two wires, each of a different metal or alloy, which
are joined together at each end. If the two junctions are at different
temperatures, a voltage proportional to the difference in temperatures is
induced in the wires. When a thermocouple is used for temperature
measurement, the wires are soldered or welded together at the measuring
junction. The second junction, which becomes the reference junction, is
formed where the other ends of the wires are connected to the measuring
device. (With the connectors at the same temperature, the chemical
dissimilarity between the thermocouple wire and the connector does not induce
any voltage.) When the temperature of the reference junction is known, the
temperature of the measuring junction can be determined by measuring the
thermocouple voltage and adding the corresponding temperature difference to
the reference temperature.
The CR9000X determines thermocouple temperatures using the following
sequence. First the temperature of the reference junction is measured. If the
reference junction is the CR9000X Analog Input Module, the temperature is
measured with the PRT in the CR9050 Analog Input Module (ModuleTemp
instruction). The reference junction temperature in
o
C is stored and then
referenced by the thermocouple measurement instruction (TCDiff or TCSE).
The CR9000X calculates the voltage that a thermocouple of the type specified
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