Campbell CR3000 Micrologger, Руководство оператора

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Section 7. Installation
294
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 CR3000. 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.