Furnace draft transmitter range is -50 to +50 mmwc. why the impulse line connec

Furnace draft transmitter range is -50 to +50 mmwc. why the
impulse line connected in hp side?

Question Posted / vigneswaran

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Furnace draft transmitter range is -50 to +50 mmwc. why the impulse line connected in hp side?..

Answer / carl_ellis

A draft range measurement is typically a gauge pressure
measurement, referenced to atmosphere.

Draft range is very, very low pressure, so DP transmitters
are typically used because they're readily available in
a 'draft' range with the large diaphragms necessary for low
pressure measurements.

The high side is connected to the process with an impulse
tube. The low side is exposed to atmospheric pressure.
Good practice puts a filter, like a scintered pneumatics
filter on the low side, so that dirt doesn't fill the low
side cavity and air movement (wind, passing vehicles) is
buffered and damped to prevent the forces of air movement
on the low side from affecting the draft measurement.

If the impulse line were connected to the low side, the DP
would be negative, a DP subtracts the low side pressure
from the high side pressure (atmosphere minus some positive
pressure = negative pressure). Convention says that draft
range is positive so the impulse connection is to the high
side, to get a positive value.

Tell 'em Carl sent you.