Answer / carl_ellis

Atmospherice pressure equals zero pressure (gauge).

Vacuum pressures are less than atmospheric so are negative with respect
to atmospheric pressure. -100mm would be a vacuum pressure.

Pressures above atmospheric are positive with respect to atmospheric
pressure.

Carl Ellis
Measure First

Answer / 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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