My mistake, I was talking about a QDV in the size of the launcher this topic is about.
You're right, with such a big QDV the air pushes the piston, even without barrel.
Even reminds me that I made a video 1year ago with a 3/4 one which shoots pretty well without barrel:
www.youtube.com/watch?v ...

You got the large lines, but this is not how it really works.
Firts, the QDV used is a "differential" one.
It is put in motion by the user, then a larger seat takes all the pressure and pulls the piston really fast.

And there's a space between the chamber and the hole in the barrel, to let the ...

I'd say it's between blue and grey, so between 300°C and 427°C.
More 300 than 400 I think, because I only heat the tip of the steel tube.

Really interesting scale btw

No sorry, I don't have any idea.
All I can say is that I do this with a little torch, one of those who work with butane cartridges.
The "indicator" I use to know if it's enough, is the color of the steel.
But it's not accurate at all, it's really a matter of feeling, and tries.

Indeed, high ...

Basically, here's all the process in one picture:

http://i23.servimg.com/u/f23/16/53/66/10/510.jpg

I heat the tip of a little steel tube, which is gonna communicate the heat from the torch to the o-ring, without melting it.
The o-ring internal diameter is smaller than the steel tube external ...

This valve was from my imagination at first.
I found the principe by myself, started to give it a name on the french forum... and Al-xg told me that it was already used on spudfiles, and called "QDV" (damn !), so I adopted the name :)
I was so proud of my invention and... nope ! Already done ! :lol ...

Negative pressure could be interesting to trigger it, but I'm not used to this kind of thing ^^

Btw, I recommand to leave the bigger seal without and o-ring.
Why ?
Because if the smaller o-ring before it is not perfectly airtight, there's a major risk of auto-triggering.
A metal/metal sealing works ...

Ah I see were's the problem, you're thinking in automatic.
The project I'm talking about will be manual, instead of the one in the video.
It was just a little try, I prefer manual airguns.

So I'm going to use a differential QDV, because of the opening speed, and I'm going to trigger it with a ...

Well, why not, I always tell myself that I should come here more often :P
But I don't have really much to show for now...

Sure, there are many ways.
The possibilities are almost limitless, but I've choosen to use only QDVs and QDV derivatives on my launchers !
I really like this firing system ...

It works really well with 8mm bb's, that makes a nice sound and the bb go kinda far.
But you're right, it won't work right with high pressures.
Except if you use a spring powered hammer, which is what I intend to use to trigger the 50bar version !

But this is definitely not the best way to do semi ...

It is a semi-auto, the pressure inlet was supposed to be a super thin hole, which would have let me the time to release the trigger, whithout spending all the gas while the piston is open.

The trigger is attached to the grey part you see moving first in the schematic, and there's a spring in the ...

Hi guys, nice to see that my project got your attention ^^
I was actually working on a new project using the same QDV, but made in steel, working 40-50bar.
There's a good amount of work remaining (I was studying for my finals), but it's on a good way.
The main problem is the machining of the steel ...

No, I don't know exactly the volume of my air chamber.

But I can show you:


What is the green thing in your scheme ?
An industrial QEV ?
Don't you want to try a QDV ?

The piston is not perfectly adjusted, there's a very little space between the tube and it, so the air can flow.
It may look little, but it's totally enough to delete the vacuum effect !

In fact, the piston is centered by the O-ring.

It's totally normal that the valve leake, because of the very little surface of the piston. You cannot blow strongh enough to shut the valve as the spring will.

Don't worry, I made the same constatation on mine, but with only 1bar,
it's perfectly airtight ;)
You can add some oil too, it helps ...