Air Draft Chamber

>what is the requirement of air draft chamber?

I don't know. I can tell you that furnace draft range measurements are made to determine if the pressure in the furnace is being maintained correctly.

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

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

If the furnace is to have a positive pressure with respect to the atmosphere, then the high side is connected to the furnace, the low side 'sees' atmosphere.

Good practice puts a filter, like a scintered pneumatics filter on the low side, so that dirt doesn't get into the low side cavity and so that air movement (wind, passing fork lift trucks) is buffered and damped to prevent the forces of air movement from affecting the draft measurement on the low side.

With positive pressure in the furnace, if the impulse line were connected to the low side, the DP would be negative because a DP subtracts the low side pressure from the high side pressure (atmosphere minus some positive pressure = negative pressure).

If the furnace has a negative pressure with respect to that atmosphere (slight vacuum), then connecting the impulse tube to the negative side will give a positive value.

The 4-20mA output of a pressure transmitter is arbitrarily configured, where the range and units assigned to the 4-20mA signal depends solely on setup/parameterization/configuration.

That means that a negative pressure value (slight vacuum) might be less than 4mA, if the LRV is 0.0 units gauge pressure (with respect to the atmosphere).

If the pressure is a slight vacuum and the connection is to the negative side, it could be that it was done that way to get a positive output value, even though the pressure is les than atmospheric.