Choosing spear for dry barrel gun

[Diving Gecko]

will not work as in air, without vacuum and pressure of water! It will be in place all the time except in the last moment when water left in a muzzle will shoot it out.

If this is true, then all of a sudden there is not much advantage to Evo-Air after all... Well, maybe it helps water escape more easily at the very end and reduce the risk of water being squeezed behind the piston? But it does explain why some people have said they can not tell a difference in firing power if they have forgotten to loosen the muzzle before shooting (at least I think I have read that...).
What if the shooting took place in shallow water, let's say at 3 meters. Would that pressure and the vacuum in the barrel be enough to hold the o-ring back in its compressed seat you think? I guess the only thing pushing it out again is friction (and reducing vaccum as spear travels), right? And that also explains why you say, that for Evo-Air to work as intended, it should not be greased, right?

Interesting...

 

[tromic]

I think you got it!
It was not so difficult, was it?

 

[Diving Gecko]

I think I did - thanks for walking me through it. I am not an engineer, but I find these things interesting. I can't do the math nor do I know the right formulas to use, but I trust you on that
And actually, very stylish of you not to point it out too clearly but let me reach my own conclusions. Otherwise, it could look like you were potentially out to criticize another product.

Thanks!
D.

 

[tromic]

I think I did - thanks for walking me through it. I am not an engineer, but I find these things interesting. I can't do the math nor do I know the right formulas to use, but I trust you on that
And actually, very stylish of you not to point it out too clearly but let me reach my own conclusions. Otherwise, it could look like you were potentially out to criticize another product.

Thanks!
D.

Actually I was very exact and direct, but did not want to overload you with a lot of informations.
You took a measure of previous Evo-Air. Maybe the latest with silent damper is different? I asked Sub Fireman about some details but he did not say anything exact. I told him I can not see any point of hiding details. Everything will be revealed sooner or later.

 

[Diving Gecko]

I don't think the two models are different at the front. I lent my Seac to a friend who took it on holiday, but unfortunately, he screwed the front element too far off and dropped it where he couldn't get it again.
Since I have the one sent to me by mistake (for a Cressi gun) I asked Seatec if I could just use the tip from that one. I also explained that the one I have on my gun is the newer version. They told me, that the front is the same and only the thread and absorber is different between the models.
Actually, I just read their email again - I think the back part of the muzzle is also the same because they asked me if I wanted them to send me the new absorber.
I think what they just did on the redesign of the absorber is to turn it inside out, so to speak. Earlier, the elastomer sits around a piece of delrin with a flange that takes the hit from the piston and transfers it to the elastomer. In the new version the piston hits the elastomer part directly.

 

[tromic]

I don't think the two models are different at the front. I lent my Seac to a friend who took it on holiday, but unfortunately, he screwed the front element too far off and dropped it where he couldn't get it again.
Since I have the one sent to me by mistake (for a Cressi gun) I asked Seac if I could just use the tip from that one. I also explained that the one I have on my gun is the newer version. They told me, that the front is the same and only the thread and absorber is different between the models.

In that case I would use it only with 7 mm shaft. You can undersand why. You know how to use Eriks O-ring calculator. There is the answer.

 

[Diving Gecko]

Here are pics of the new one from their website and the old one from my table

 

[Diving Gecko]

In that case I would use it only with 7 mm shaft. You can undersand why. You know how to use Eriks O-ring calculator. There is the answer.

Well, I have them already, the two calculations. But I guess with the 6.75mm shaft 9% compression is a little bit too little, but worse is that the compression turns negative with excentrical position of the rod meaning a leak if I bend the spear when loading. With a 7mm spear there is less gap between the spear and the bore, so less room for bending and potential leaking when loading. Is that correctly understood?
At least it shows that I have to be very careful about loading...

 

[Diving Gecko]

I now also see why Marko (or you, can't recall) told me, that you could only really make the Tomba for one size of shaft to make it ideal. I now understand that it's not only about the right o-ring and compression of that o-ring with different shafts, it is also very much about having a small clearance between the bore and the shaft to avoid bending. Until now, I didn't think of the delrin bore parts as anything other than guides for making sure the spear was flying centered when shooting. But even more important is to avoid bending on loading. I guess with a design which allows as little bending as possible, one can use less static compression on the o-ring and then have less friction...?

 

[Diving Gecko]

In that case I would use it only with 7 mm shaft. You can undersand why. You know how to use Eriks O-ring calculator. There is the answer.

Yeah, now it says that I risk ejecting the o-rings because the distance between the shaft and bore is too great if I use a 6.75mm shaft...
That warning didn't come up on the first calculations because I had disabled the tolerances - see, that proves I am not an engineer...:duh
Anyways, I have two lovely spears in 6.75mm and I am going to stick with them. I might see if there is another o-ring that would solve the problem or at least give better compression. But that's what these online calculators are missing - there are not good at proposing which rings to use.

 

[tromic]

Yeah, now it says that I risk ejecting the o-rings because the distance between the shaft and bore is too great if I use a 6.75mm shaft...
That warning didn't come up on the first calculations because I had disabled the tolerances - see, that proves I am not an engineer...:duh
Anyways, I have two lovely spears in 6.75mm and I am going to stick with them. I might see if there is another o-ring that would solve the problem or at least give better compression. But that's what these online calculators are missing - there are not good at proposing which rings to use.

According to Eriks you could use O-ring 6.4 x 3.3 for 6.75 mm shaft.
That would be far better than 7 x 3, althought the gap with 7.2 mm boring is too large for 6.75 mm shaft.
There is maybe other possibilities too.

 

[Diving Gecko]

How did you find that one?
Did you know that there is a 3.3 series and you looked that up or did you find a way to make Eriks suggest it? I can't get it to do that...
But yes, compression is much better with that ring and no negative compression possible with excentrical positioning of shaft.
Thanks!
D.

 

[tromic]

How did you find that one?
Did you know that there is a 3.3 series and you looked that up or did you find a way to make Eriks suggest it? I can't get it to do that...
But yes, compression is much better with that ring and no negative compression possible with excentrical positioning of shaft.
Thanks!
D.

Eriks brought me close to desired value. Than you choose from standard values. You'll find the right way...

 

[tromic]

David, here is about testing if O-ring is movable in Evo-Air like design:

forum.pescasubacquea.net • Leggi argomento - evo air

http://img571.imageshack.us/img571/4923/immagine6xz.png

Here is a video:

[ame="http://youtu.be/FaDt0CAAbEI"]Video004 - YouTube[/ame]

But nevertheless, movable or not, EvoAir is a great kit for 7 mm free shaft.

 

[Diving Gecko]

Very interesting. And it looks like the front reservoir was replicating the pressure of diving to "just" 10 meters and the o-ring still did not move. So, in designing a dry barrel muzzle it is better to focus on getting the o-ring compression right than trying to figure out a smart way to let it move because it wont, not at depth at least.
As you said, better to slightly oil the spear then as friction wont be enough to eject the o-ring in the first place, it seems.
(Of course, there might be a few factors that can explain why the o-ring stays put at such low external pressure. Maybe he did indeed oil the spear and/or the compression is very slight in his design. But I still trust your point).

Thanks as always,
David

 

[Diving Gecko]

A bit of an update on the saga of the Evo-Air and my Seac Hunter.
So, I brought my Seac 90 to Denmark to hunt a bit but did a full disassembly first.
I put it together a bit more tight this time, as last time when I just changed the original muzzle for the Evo-Air muzzle, there was a tiny gap between the muzzle and the outer barrel. That's not really an issue as the front is sealed with an o-ring which doesn't sit too close to that gap.
Anyways, this time, I screwed everything in tight. And... now the spear is actually binding a bit just from going through the elastomer bumper in the muzzle. I know this because when I took out all the air (for traveling by plane) and "loaded" the gun I could certainly feel the binding. I even cocked the piston in the trigger to make sure it was not the piston friction I was feeling and the pulled the spear out of the piston, so it was only the spear and the muzzle interacting.
I am pretty sure it's because the bumper is a bit too long from Seac's side and now it gets compressed from the tighter assembly and binds on the spear. Also, of course it doesn't help much that the bumper's hole for the spear is not drilled completely in the center.
All in all, I am left with the impression that they did a somewhat sloppy job. I like tinkering, and it does take a bit of tinkering to change your muzzle but still, this could/should have been better.
If I knew how to post in Italian, I would put it on the forum there. And I am thinking of emailing Seac my findings as well. Perhaps they'll listen and fewer people will face this issue in the future.

On another note, they did actually send me a complete and new handle to deal with the leaking trigger pin.

I just feel a tiny bit bad about posting about these things as it might deter others from buying this gun - which I actually still feel is worthwhile. I do hope, I have just been a bit unlucky and that if I have lived near the dealer, he could have solved these issues in a heartbeat the first time around.

D.