Answer Posted / carl_ellis

-100mm wc is a vacuum with respect to atmospheric pressure
as a reference.

Given the range, the pressure transmitter is likely a
differential pressure transmitter.

-100mm wc pressure can be produced with a vacuum source,
like a hand pump connected to the high side, with the low
side open to atmosphere.

Or 100mm wc positive pressure can be applied to the low
side, with the high side open to atmosphere:
DP = high side minus low side
DP = 0 minus 100 = -100mm wc

The URV, 300mm wc, is a calibrated with a pressure source
connected to the high side, with the low side open to the
atmosphere.

Carl Ellis
Measure First

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