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Maths to check pressure?

Thread Status: Hello , There was no answer in this thread for more than 60 days.
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onefish

New Member
Feb 5, 2008
63
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Hey there, I have just got a new Cressi from the USA (in Oz) and had pressure let out prior to sending across. I tried to follow the Cressi manual and put in 200 pumps to reach 25 bar pressure. After 100 pumps I stopped counting because it was pretty obvious that I could not get 200 pumps into the gun. I'm guessing I only got another 20-30 pumps in and the pump was only puting air in at the bottom 3-4mm of the action. I figured that is about as much pressure as possible with the Cressi pump.

I tested the gun today and found it quite easy to load. I'm sure I could easily apply another 40lbs of loading pressure. I wanted to check the actual pressure and so tried to do it with a bathroom scale and mathmatics. I don't expect it is 100% accurate but should be close. I'm wondering if anybody else has tried a similar thing and can verify if its right.

OK this is what I did. Zero'd the scale with a roled up towel, gun, spear and loading handle on it. Then with the scale on the ground I placed the gun handle on the roled up towel and put pressure on the shaft until the piston began to move and measured the weight, 33kg.

Now for the maths. I changed all measurements to imperial just because I'm not used to working with bar. 33kg = 72.6lb. The piston of the Cressi gun is 13mm or .511811 inch. 25 bar = 354.8 psi.

Now for area of piston "Pi x radius squared" or 3.14 x .2559055 x .2559055 = .2056311

Now I just have to divide the pressure on the piston by the area of the piston and this should be the pressure inside the gun, right? Well here is:
72.6/.2056311 = 353.059 psi
max recommended pressure 25 bar = 354.8 psi

Is this sound logic? Is there anybody out there with a manometer that can verify this methodology by trying it to see how accurate it is compared to the manometer reading?

If it's close enough I won't bother buying a manometer. Now I can load much more pressure, will the gun handle any more? I suppose I would have to use a scuba tank to get the pressure much higher, but that's another issue.

Cheers,
TJ
 
Well i own a cressi and i done it the easy way and asked my Local Dive shop to pressurize it. It took all of 10sec to fill it up, so much easier.
But he told me the maximum i could have was 20 bar(300psi), but if your manual says 25bar im going back to grab some more psi LOL

In regards to your mathamatical equasion im sry i cant be anymore of help...
 
Hey Mongrel,

Unfortunately there's no dive shops within 800km of where I live. Ooops, your right, just checked the book again and it does say 20 bar. Not sure why I was thinking 25 bar. Well I may have already overpresurised the tank. It would be good to know if the method I used to measure is somewhat accurate. Might be way off.

Cheers,
TJ
 
Well im glad you know its overpressured and can rectify it before its too late

But on the other hand im spewing as i want more power LOL
 
The math is good but I'm not convinced about the method with the bathroom scales but hopefully it will put you +- 10%
 
Thanx Pastor, the bathroom scale the bit I'm not really certain about. It's not terribly accurate. Then there's friction of the piston in the barrel that would also have some effect on the pressure required to compress it........... A few variables there.

TJ
 
TJ
I think you are very close. It is very easy to design a pump so that it will only put out so much pressure and it looks like they've done that. 10-20% over won't make much difference except to get you closer to seal failure. Find an underwater cave with air in it and repeat the pumping, just don't bring the gun back to the surface when it's loaded.
 


Is it a bad thing to have the gun loaded above water?

I know its bad to fire out of water.
 
I think Bill might be refering to a way to overpressurise the gun that would be offset by the pressure at depth. The atmospheric pressure would decrease as the gun was brought up to sea level and hence increase the internal pressure.

It would work in theory but it's not really practical and especially not for a free diver.

Cheers,
TJ
 
Dave,

I have sent you a PM in relation to another thread I posted. Have you got it?

Cheers,
TJ
 
Reviving this thread for a quick question. While diving last weekend I had a couple fish get out of the way of my spear, frustrating! They were not to far away and the spear passed right where the fish was when I pulled the trigger but the fish wasn't there anymore. I'm getting a Tovarich kit which I'm sure will help with spead but in the mean time I pumped some more air into the gun.

I discovered that by blocking the breather hole on the pump when all the way out I could get an extra couple of CCs into the pump. The bathroom scale method revealed 50kg of pressure to begin to depress the piston. According to the calculations I posted earlier this is very close to 40 bar.

I can't load the gun with one hand anymore and I have to be careful not to bend the spear but I can still load it without too much trouble.

The question is, will the Cressi gun handle it? Dave mentioned a few posts ago that Cyranos are often run at 40 bar but are they better quality than Cressi so can handle it better? Anyone have any experience with Cressi guns at high pressure?

Cheers,
TJ
 
Getting risky i think mate...
All i would suggest is drop it some and wait for the Tovarich kit.
 

So much for the bathroom-scale-method rofl

There are not many people out there that will manage to load 13mm internal chamber/barrel airgun - like your Cressi SL - at 40 bar

It's a different story with Cyranos, Stealths/Sparks, or Mambas, because they feature a 11mm internal barrel

Cheers
 
Well, it takes two hands and a straight pull but I can do it easily enough (I'm a pretty big guy). The question is, will the gun handle the high pressure? I suppose if there ain't too many out there ever running Cressi's at that sort of pressure then the answer might not be known.

I have just ordered the Tovarich, trigger kit and manometer from Ramon. I'll check the actual pressure with the manometer when it arrives from Spain and post results on the accuracy of the bathroom scale method of measuring air pressure.
 
Last edited:
Hello Onefish.

I own an old Mares Mirage gun, a mares Ministen, and a Cressi SL too.
I personnally dismantle and service the guns, so I think I can tell you 2-3 things I hope will be of some help.

The Mirage was made for proper operation at a maximum pressure of 40 bar.
But I can tell you the air tank, the valve and the piston even the barrel, are exactly the same on the ministen. Ageing of the O-rings is a different story.. After 3-4 years at this pressure, their cross section is not circular anymore, and they need to be changed.

My Cressi (sl70) is not very different : same barrel thickness, same tank (seems it; could be made in the same factory), same kind of piston, and so on. So I think the major risk for your cressi at 40 bar is for the o-rings. EXCEPT if there is some heavy corrosion of the barrel. Then the gun would dismantle itself.

I would add, Cressi is a long-established firm, I think they have good security margins (in France, these margins are fixed by the law, for every pressurised tank. Probably the same in Italy, and, if not, European, American, etc.. export rules apply). If a Cyrano or a Stealth can be pushed to 40 bars, the Cressi can do it too.

One last thing : I found the site of a Greek diver, who modified some guns (essentially mares sten), with some kevlar and carbon added, he makes the gun work at 80 bars !
He cocks it with a winch mounted on his boat...

Ah, and onefish, maybe you are big, but you have some serious muscles too !

Regards.
 
I just found this thread and found it interesting. I like the maths, it's my sort of geeky thing.

Main point is the bathroom scale is a measure of the force required to start to load the gun. Not sure what length gun you have or what thickness spear but loading a pneumo with 70+lbs of force sounds a lot. No matter how strong you are unless the gun is short and the spear thick it is not easy to apply that much force, especially in the water.

Now you're talking 50kg (110lbs) then that's more force than i could generate to load my pneumo in the water and I'm a big guy too.

Without a dry barrel kit pneumatics tend to throw a slow spear. Especially true of short guns and heavy thick spears.

Just guessing here but I think you may have the wrong gun for the job and no amount of pumping up the pressure will make it do what it's not in its nature to do.

Dave
 
Hello Old Man Dave,

When you say, " Without a dry barrel kit pneumatics tend to throw a slow spear. Especially true of short guns and heavy thick spears.",
I think you are right, in my humble opinion, for the last part : if you shoot with a thick spear (say, 8 mm or worse) it will be slow.

But... I use, on a standard Ministen (58 cm - 2 Ft model) a 7 mm shaft, a bit long: 10 cm longer than the original one. I can tell you, at 30 bars, it is everything but slow ! Takes out easilly 4 turns of nylon. (Around 3 meters).
Short gun, but fast.
The question of the dry barrel is, I think, a bit different. The dry barrel systems give an opportunity to have the same speed with 25%/30% less pressure. I can give you the link to the tests made by an Italian engineer : they demonstrate that a pneumatic gun with 7 or 6.5 mm shafts, at 25 bars and 32 bars("wet" barrel) or at 25 bars ("dry" barrel), is faster than a bandgun.

Link : http://www.bluworld.com/Anglani/pneumatico.pdf

Please, don't be afraid if you don't understand the text in Italian. Go straight to page 9 and after : should not be too difficult.

If somebody needs it, I can translate a bit of the speech too. (An advantage to be from Corsica is, Italian is not too difficult to understand).

So, I must ask the question to Onefish : what kind of shaft does he use ?

Regards.
 
You fellas are correct, my gun is only SL55 currently with standard 8mm cressi spear. I have just ordered the Tovarich kit in 13 x 7.14mm (9/32) and I'm in the process of getting some custom spears made with line attachment as hydronamic as possible. SS spears in imperial measurements from the USA are much cheaper than metric ones in Australia.

I use the gun primarily for crayfish but still have to be able to nail a decent fish when I peak around a bombie and there's a big coral trout staring at me. It was a bit of a compromise, getting a gun short enough to reach around in holes and up under bombies, yet have enough grunt to nail a fish when required. The gun puts the spear straight through a trout out to about 2.5m but they have soft skin and small scales. The spear won't go through a parrotfish or bluebone at that range though. They have big tough scales. Mind you my tip is a bit blunt from shooting crays in the rocks.

I went for the SL55 because I figured that i could get it to be effective out to about 3m by using a dry barrel (am I wrong?). This is plenty of range for the type of hunting I do. The visibility is never too good anyway. If I want to catch big pelagics I troll a line out behind my boat. Likewise if I want to fill my big esky with reef fish I through in a handline.

But if I want crayfish...........................

The diving is also quite adictive. Last weekend my 8 year old daughter and I had a huge manta ray hang out with us for about 20 minutes. It got close enough to brush my daughter's leg. She was a little frightened until she found out it was nemo's teacher : )

I'm sure the Tovarich and smaller spear without slide rings will increase speed. I'll probably drop the pressure back to about 30-35bar after.

Cheers,
TJ
 
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