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Why is the length of cable layyed from RTD to temp. Transmitter should be minimum? Why does heating effect takes place in their?
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Answer Posted / carl_ellis
Why is the length of cable layyed from RTD to temp. Transmitter should be minimum? Why does
heating effect takes place in their?
An RTD develops a very low voltage signal, on the order of tenths of a volt. Low level signals on
that magnitude can be overwhelmed by industrial electrical noise. And voltage signals, unlike a
current loop signal, diminishes (decreases) over distance.
There is also a limitation on the maximum wire resistance (which is proportional to wire length
and gauge) that an RTD input on a temperature transmitter or controller analog input can handle.
These reasons are why RTD temperature transmitters are so popular; the transmitter connects
close to the RTD minimizing noise and producing a current loop signal that is relatively immune to
noise and that does not decay over distance. The loop power is delivered over the same to two
wires as the output signal uses.
The self-heating effect is at the RTD element itself, it is not in the extension wire/cable
Unless it's a cryogenic (low temperature) application, the self heating effect is usually ignored.
Look at how little heat is developed:
A 1ma (0.001A) excitation current through 120 ohms (approx 52 Deg C) drops 0.120V (E = IR)
0.120V * 0.001A = 0.00012W or 120uW or 0.12mW
That's not a lot of heat to dissipate.
Carl Ellis
Measure First
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