chamber seal versus barrel seal

[Anatine Duo]

I've been playing around with various sprinkler valves (Hunter, Rainbird, Toro). HUnter and Toro are chamber seals. I converted one Hunter to barrel seal because I like the idea of equal pressure on both sides of the flexible part of the diaphragm. In chamber seal configuration a perforated collar supports this flexible part and might impede flow during firing.

Anyway, playing with GGDT I selected barrel seal and lost significant performance (594fps down to 511) with my configuration. Even more interesting I ran my higher pressure, faster flowing, smaller caliber pneumatic numbers and there was no difference at all!

Also what is dead volume? I assumed it was the volume between projectile and valve seat, and assumed less is better, but increasing it in GGDT increased performance with light projectiles (reduced with heavy).

Can anyone explain this? The more I learn the less I "know"

Also I love GGDT... way more precise than the formula I was using before.

[jackssmirkingrevenge]

GGDT is great however it sometimes goes a little odd at the extremes, so one is best advised to use it as a guide but not treat it as gospel.

Also what is dead volume? I assumed it was the volume between projectile and valve seat, and assumed less is better, but increasing it in GGDT increased performance with light projectiles (reduced with heavy).

That is what dead volume is, and yes on the face of it less is better. There is however the theory that the air in this dead space can be suddenly compressed and therefore heated by the rush of air from the chamber, in the same way that air is compressed by a spring piston in a conventional air rifle. This means that a counter-intuitive increase in performance is possible.

[Anatine Duo]

It's good to know I am at the extremes

The Hunter valve that I converted to barrel seal might give slightly better performance but the main advantage is how fast it seals when using a bike pump. The chamber seal version takes 3 pumps to seal. The barrel seal holds air instantly.