What is the pressure on the exhaust side of a fan open to atmosphere? The suction side is a closed system, and at the exhaust side, the pipework leads to a flue stack open to atmosphere. The fan is circa 1.2m diameter and running on a direct on line stop/start 2kW motor. It is sized to provide >20mb pressure.

If I have a pressure differential switch across this fan, what will the pressure transmitter read out on the discharge side? I'm thinking around 1bar as its open to atmosphere or will the velocity of the gas that is travelling out of the stack influence the pressure reading?