The answer is not simple and it depends on the fluid (gas, liquid and on the type of gas). It depends on the  initial pressure and temperature too. At high and very high pressures the gas is no longer behave as an ideal gas so you will need to check the gas specific volume at every time step and apply it to calculate the mass flow/leak. I assume you want to make a leakage test.

It is a leakage test.  The fluid is air at 80 psi and will hopefully not leak to far down from there.  The temperature will be room temperature, probably 72.

If your test set up is charging an air tank, say 10 gallons (V1) at 94 psia (Pi).  then letting it blow through the mainfold for 1 minute, stopped, and the new tank pressure is 84 psia (Pf).  

Assume the tank expansion was slow enough there was not a change in the temperature, the mass in the tank before and after can be calculated and the differance is the mass through the manifold PV=mRT.

I think (Pi X V1) - (Pf X V1) = Volume discharged, thus
94 X 10 - 84 X10 = 100 gallons (~13.4 CF)

If the tank changes temp, then include the Ti and Tf

Correction to my above post.  I forgot to include the constant to correct units.  For 72F and P in PSIA, the P*V products need to be multiplied by 0.068

Perfect, thank you.  I actually came up with the same theory, but wanted to see if anyone else had.  I am going to use a load cell to calculate mass before and after and the pressure change to find my flow rate.

this method is used to determine the gas leakage from pressurized electric generators.  Hope I got the constant correct, I normal plug P,T, t, & V into spreadsheet.