Can flow be calculated in a pipe that has vacuum

[gsyd]

I am trying to calculate the flow in an 8" pipe, .25" wall thickness, that a gas with a density of .79 lb/cu ft flows through under vacuum. Through a port in the pipe, I measured .5 WC with a Dwyer gauge model 2050. Since the whole system operates in a vacuum, did I use the right gauge to begin with for the reading? It is what the people at the plant have used for years for all of their readings. I'm looking to buy a pitot tube and gauge from a distributor to see if I can get a differential pressure to apply to a formula to calculate flow. The rep isn't sure I can do this if the system is vacuum. Am I going about this the right way and can flow be calculated in a vacuum system? I may have not given enough information, so if anyone who can help needs more, ask and I'll get what I can.

[PipeFlowCalcs]

Yes, it can as you will use absolute pressures in calculation

[gsyd]

OK, that makes sense. A gauge distributor that our company works with recommeded a 5-0-5 magnehelic gauge. The units are in Inches of Water. I found a chart that converts Inches of Water to Pa which I can use in a flow calculation. I will use the velocity equation: V = (2P/d)^0.5 (V = velocity P = differential pressure d = air density @ STP). Then use the answer I get for velocity and apply it to the flow rate eq.: Q = VA. I know the area of the pipe so this should give me what I want. Am I forgetting anything? Thanks for your response.

[PipeFlowCalcs]

That is OK.
Here you can find diameter and flow calculator that can help you in calculations.