Opening time is the length time it takes for any given valve to move from the fully closed position to the position allowing the maximum possible volume of gas to flow through. This time period is typically measured in milliseconds, and as expected is most important when the barrel dwell time (the length of time it takes for the projectile to transit the barrel) is low. The approximate opening time of some valves:
- Burst disks Practically instant... most likely under a millisecond.
- Pilot operated valves (diaphragm valves, piston valves): 3-5ms.
- Ball valves: about 100 ms hand-actuated, maybe 20-30 ms spring-actuated.
GGDT will give you a graph of your valve opening. (valve position vs. time or projectile position)
You can estimate the needed dwell time for a particular gun if you have an estimate of the barrel dwell time. As a rough estimate, assume the average speed in the barrel is one half the desired muzzle velocity. The time it takes for the projectile to transit the barrel is the barrel length divided by this half muzzle velocity. For example, assume you hope the gun will fire at 600 FPS and it has a three foot barrel, the barrel dwell time is about;
(3 feet)/[(600 FPS)(0.5)] = 10 milliseconds
To get the most out of your gun you would like the valve opening time to be significantly less than the barrel dwell time. In this case, a valve opening time of less than perhaps 3 ms would be a good target.