Depending on the gas flow rates you are working with will determine the best control valve solution. If the flow rate is small, pilot operated diaphragm valve is a good choice. If the flows are higher, valves like soft seated globe valves with high performance actuators are more appropriate...
We are testing valve for seat leakage, and rather than wait 10 minutes for a reasonable volume to accumulate, we would rather count the drips.
Is there a standard drip forming device?
Or its off to the kitchen sink for testing?
Butterfly valve can be configured for good control if some sensible installation considerations are made.
1. Match the Valve size to the particular conditions you will operate at most of the time.
2. If there are operating conditions's outside the valves control zone (20-70º open), add another...