Pressure Differential Flow Meter
 

I am trying to simulate the use of a pressure differential flowmeter with the gas air. The flowmeter has an upstream pressure tapping, a variable orifice and another pressure tapping downstream.
The flow goes off to atmosphere. This is not a pressurised flow. Tests to back up my simulation show that the downstream pressure tapping shows virtually zero, particularly at low flows. Therefore it would appear that its possible to characterise the flow simply by the upstream pressure gradient. Is it possible to remove the downstream pressure tappng in this situation? What are the implications of doing this?

Thanks in advance for any advice.