Velocity of pressurized air through a vent?

Suppose I have a tank of air pressurized to, say, 90psi. Now suppose I open a valve on the tank that is 1-inch (0.083 feet) in diameter. How would I calculate the velocity at which air is escaping through that valve? Presumably the velocity would decrease as the loss of air causes a drop in pressure. How would I... show more Suppose I have a tank of air pressurized to, say, 90psi. Now suppose I open a valve on the tank that is 1-inch (0.083 feet) in diameter. How would I calculate the velocity at which air is escaping through that valve? Presumably the velocity would decrease as the loss of air causes a drop in pressure. How would I calculate the velocity for any given momentary pressure? The CFM that would result is unknown, so that cannot be part of the equasion. Thanks.
Update: With regard to Bomba's answer: Suppose that, for reasons I won't explain, my tank's pressure wil never drop below 60psi. Does your answer mean that I can basically just assume a velocity of 1117ft/sec (sonic velocity) for the duration of the drain from 90psi to 60psi, because the differential will not... show more With regard to Bomba's answer: Suppose that, for reasons I won't explain, my tank's pressure wil never drop below 60psi. Does your answer mean that I can basically just assume a velocity of 1117ft/sec (sonic velocity) for the duration of the drain from 90psi to 60psi, because the differential will not be reduced sufficiently to require use of the other formula you mentioned?
Update 2: Paducahbill: Thanks for the answer, but I can't use any measuring instruments. I'm trying to write a computer simulation for something which I do not have first-hand access to. The simulation doesn't have to be perfect, but I do have to rely on known formulas rather than first-hand measurements.
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