Campbell Scientific 43347 RTD Temperature Probe and 43502 Aspirated Radiation Shield - Manual (Page 35)

Appendix D. Measure Two 43347-IX
Probes Using One Current Excitation
Channel
One current excitation channel can excite multiple 43347 probes if the “Current
Return” wire of the first probe is connected to the “Current Excitation” wire of
the second probe.
In theory, a single Ix channel can excite up to 25 of the 43347-IX probes with
170 µA if all probes are at a temperature less than or equal to 45°C (see
Section 6, 43347-IX Measurement using Current Excitation). At 45°C, the
43347 has a resistance of ~1175 ohms. The resistance increases as more
probes are connected in series. The increase of resistance requires the Ix
channel to raise the driving voltage to maintain the same current. The
maximum voltage the Ix channel can drive is ±5 Vdc. Therefore, the maximum
number of 43347 probes is:
Max. voltage/(current * resistance per probe at 45°C)
5 volts/(0.00017 amps * 1175 ohms) = 25
The CR3000’s differential channel count limits the number of probes to 14
without a multiplexer.
One disadvantage to driving multiple probes with a single Ix channel is that if
one probe shorts or opens then the measurements of all the probes on that Ix
channel will be bad. If, for example, there are two probes at each of three
levels, it might be best to drive one probe from each level on one Ix and then
drive the remaining probes on a second Ix. This creates separate A and B
systems, which allow maintenance to be done on one system while the other
system continues to make good measurements.
D-1