I need to calculate the flow rate of air through a 3/4" orifice from the pressurized side of a 1000 PSI (~250 cu ft at pressure) tank to the open atmosphere and know the time it will take to release 20% - 35% - 50% of the volume of the pressure tank.

Tank size ~1.5 cu ft at atmospheric pressure, ~250 cu ft at max working pressure. (approx. 2500 PSI)

Basically I need to know how to calculate the the flow rate of a high pressure tank discharging to the atmosphere by knowing the following:

1. Tank Volume at pressure.

2. Tank pressure.

3. Tank orifice size.

4. Assume a discharge nozzle length of 1" the same size as the orifice.

5. Using an explosive burst disk to initiate the flow (time 300 ns)

The flow calculator at pipeflowcalculations.com/airflow are not doing the trick for me due mostly to my inability to completely understand their use.