Answer / carl_ellis

Reason: ground current added to loop current through a
ground created by moisture.

The analog input on the DCS references ground at some point.
The moisture in the junction box or remote terminal panel
makes a connection to a local ground at that point. The
difference in potentials between the grounds causes a ground
loop - current added to the modulated loop current. The
DCS's analog input 'sees' this current, even though the
transmitter is turned off, because the current is driven by
the potential of the ground loop, not the potential from the
power supply.

When the moisture (and hopefully, the dirt) is removed, the
local ground connection is broken and the loop is isolated,
except for the DCS's AI reference to ground. So the DCS AI
'sees' only the loop current modulated by the pressure
transmitter and reports the correct loop pressure.

Some handheld calibration instruments can detect current
leakage in loop powered loops. An example is the Practical
Instrument Electronics (PIE) 334 Plus calibrator.

http://www.piecal.com/products/evolution/334Plus_mA.php

Carl Ellis
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