MRR wrote:Keeping the pilot volume as small as possible is an practical issue for me.
I experienced that it is hard to make a really well fitting piston without professional equipment. If the piston doesn't fit properly or the equalization is to big a small pilot volume helps to reach the actuation point.
Your QDV uses a mechanical "trigger" and industrial QEV's are fabricated on a very high scale.
Another point is, if you try to go semi / auto, you fill from the chamber. A small pilot volume will make the piston seal faster because of the faster equalization. I think that's the main reason why I prefer small pilot volumes.
Good points, all of them. Well done.
One more to throw in; With an EQ port, the larger the pilot volume, the Hissssssssssssss is longer, the more chamber air is lost through the eq port into the pilot. A small pilot volume is desirable. I try to keep it about 1/3 to 1/2 the volume the piston displaces when it moves. Best of both worlds when adjusted. Too small is bad. Too big is bad. Best is somewhere in the 1/4 to 1-1/2 the displacement volume depending on the trip point pressure. Often the bumper space, ball valve, and pipe in between is enough to meet these requirements.
Done properly, the result is sssSSSSsssPOW.