Section 7. Installation
347
Measuring Multiple PRTs (PT100 Resistance() Basic-Circuit Series)
If you connect only one PRT to a [
U
] [
Ix
] terminal configured for current
excitation, the previous procedure serves well. However, if multiple PRTs are
measured from a single excitation terminal, the sum of voltages output from all
PRTs connected to one excitation terminal must not exceed 5000 mV, which is
the input limit (InLim) for the CR6. This limitation also serves to limit the
current load on the excitation terminal.
Calculate the maximum number of PRTs for one excitation terminal. Do this by
calculating IX
max
at several PRT numbers:
IX
max
= ((InLim / (n • RS
min
)) • 0.99
where,
InLim = Input Limits = 5000 mV
n = number of PT100 sensors
RS
min
= minimum resistance in the temperature range = 84270 mΩ
so,
IX
max
= 0.029 mA (29 µA) when n = 2
Other results are tabulated in the following table.
Number of PT100
PT100 Number, (n)
Max Excite Current, (IX
max
)
1
0.059 mA
2
0.029 mA
3
0.020 mA
4
0.015 mA
5
0.012 mA
6
0.010 mA
When using a current excitation greater than 1 mA, self-heating of the PRT
element becomes a problem. Having your temperature sensor heat itself is far
from desirable! You can reduce self-heating by minimizing the
SettlingTime
and
Integ
parameters in the
Resistance()
instruction, and by minimizing measurement
frequency, perhaps by increasing the
Scan()
interval.
Summary of Contents for CR6 Series
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Page 76: ...Section 5 Overview 76 FIGURE 20 Half Bridge Wiring Example Wind Vane Potentiometer ...
Page 80: ...Section 5 Overview 80 FIGURE 23 Pulse Input Wiring Example Anemometer ...
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Page 454: ...Section 8 Operation 454 FIGURE 104 Narrow Sweep High Noise ...
Page 459: ...Section 8 Operation 459 FIGURE 106 Vibrating Wire Sensor Calibration Report ...
Page 535: ...Section 8 Operation 535 8 11 2 Data Display FIGURE 121 CR1000KD Displaying Data ...
Page 537: ...Section 8 Operation 537 FIGURE 123 CR1000KD Real Time Custom ...
Page 538: ...Section 8 Operation 538 8 11 2 3 Final Storage Data FIGURE 124 CR1000KD Final Storage Data ...
Page 539: ...Section 8 Operation 539 8 11 3 Run Stop Program FIGURE 125 CR1000KD Run Stop Program ...
Page 541: ...Section 8 Operation 541 FIGURE 127 CR1000KD File Edit ...
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