Maths to check pressure?

[onefish]

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

[Mongrel454]

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...

[onefish]

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

[Mongrel454]

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

[Pastor]

The math is good but I'm not convinced about the method with the bathroom scales but hopefully it will put you +- 10%

[onefish]

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

[Bill]

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.

[Mongrel454]

Originally Posted by Bill

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.

[onefish]

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]

you cannot overpressurise an airgun with a hand pump, the burst pressure is way, way over what can be pumped by hand. (it is quite common to run Cyranos at 40bar)
Exact pressure really does not matter, loading effort is the only gauge you need
cheers
dave
Spearguns by Spearo uk ltd finest supplier of speargun, monofins, speargun and freediving equipment

[onefish]

Dave,

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

Cheers,
TJ

[onefish]

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

[Mongrel454]

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

[ulysses]

Originally Posted by onefish

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

So much for the bathroom-scale-method

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

[onefish]

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.