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Pumping up pneumatic spearguns

popgun pete

Well-Known Member
Jul 30, 2008
3,035
590
153
Australia
I note that there have been comments about the difficulty in pressuring pneumatic spearguns. While it might appear obvious to some it is worthwhile reviewing why the hand pump supplied with each speargun works. The pump only transfers air into the gun when the pressure developed inside the pump exceeds the air pressure inside the gun. With the gun fully depressurised air flows through the inlet valve in the back of the gun as the pressure created in the pump forces the inlet valve open, the valve being closed again by a not very strong spring, so not much effort required at all to inject some air into the gun initially. At first this pumping procedure takes very little effort, but once the gun starts to develop some significant pressure inside it then the pressure created in the hand pump has to overcome the valve spring and the pressure level already in the gun to open that valve. Also as the pressure in the gun builds the piston on the end of the plunger in the hand pump has to go progressively further down the hand pump's bore before the pressure developed in the pump body cracks open the inlet valve and the air transfers. You can think of a hand pump having a compression ratio dictated by its swept volume (the distance between the air breather holes near the top of the pump and the bottom of the pump stroke) and the dead air space at the base of the pump when it is fitted to the gun. If that dead air space is large then the gun can only reach a certain final pressure because no matter how hard or how many times that you pump it the pump can only reach the pressure dictated by the swept volume and the final space that all the air in the pump bore is being squeezed down into, i.e. the dead space. Consequently most hand pumps are manufactured to have a very small dead space to ensure that they reach a sufficiently high final pressure, however the gun manufacturer can limit the pressure in their guns by manipulating the dead space to prevent a higher pressure being achieved. To exceed this limit one would have to reduce the dead space volume by adding some solid material to reduce the available volume, however this is not recommended! Some older pneumatic guns had small reservoir capacities and when pumped up to high pressure were nearly impossible to load, so the manufacturer built a limit into the hand pump by leaving more dead space at the bottom of the pump.

Another way to pressurise pneumatic spearguns is to use a 12 volt air compressor sold for inflating vehicle tyres. These usually plug into the cigarette lighter socket in the dashboard of your car or they have some clips which attach directly to the car battery terminals. Some of these portable compressors have a pressure gauge fitted and will pump to 250 or 300 psi depending on the model. If you make an adaptor then you can use these compressors to pump your gun up very quickly to 20 Bar or 300 psi. as their pumping ability is only governed by the dead space in their actual compressor unit, not how they are actually connected to your gun. Those wanting much higher pressure in their gun can then switch over to the hand pump and add more air as more than half of the hard work has already been done by the electric compressor, although when starting up at higher gun pressures the hand pump needs a good push to drive it down and you will hear a squeaking noise as the inlet valve cracks open each time and the air transfers into the gun, usually close to the bottom of the hand pump stroke. This squeaking noise is a good indication that you are really getting somewhere, if you do not hear it then air must be going elsewhere and not into the gun. Do not worry, the gun will not blow up at these pressures, but it is worth checking that you can still load the gun, otherwise you will need to bleed some air out through the inlet valve. Once you have carried out this procedure a few times then you will start to get an appreciation of how hard the pump is to push as an indicator of the air pressure in the gun. Also when you finish any pump stroke that compressed air still in the dead space (some will be there as not all the air transfers once the pressure on each side of the inlet valve equalises) will push the pump handle back up. This pump handle movement is a rough guide to the pressure in the gun because when you let go of the pump handle the inlet valve has closed, in fact it closed as soon as you stopped pushing on the handle, and only the air pressure trapped in the dead space will re-expand to push the pump handle back up. Think of this functioning as a sliding column type pressure gauge because the non-transferred pressurised air still trapped in the bottom of the hand pump, in that dead space, has expanded to match the surrounding air pressure. That is what a pressure gauge does, except that there is some friction in the hand pump bore, but the handle movement is only a relative measure in any case.
 
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MARMIR

New Member
May 1, 2007
82
5
0
...I agree with you popgun pete....just a couple of things:
1) I have some portable compressors for cars and I stopped trusting in their
accuracy long time ago...
2) A lot of people here have been using compressors recovered from old dismissed
refrigerators.....all gears cost few euros and you can pump up your gun in 20 seconds
(some of them can reach up to 50 bars) effortless....
 

tamoshee

me and the deep blue sea
Jan 1, 2004
89
14
0
England
homepage.ntlworld.com
Good posts.
I have been using pneumatics for years and have owned many. I read what Marmir said with great interest.

If there is anyone here who uses a frigde compressor ... how about writing up a listing with photos and good technical explanation of what they have done to produce a device to pressurize a pneumatic gun.

I have noticed that many of my pneumatic pumps have failed. At the moment I have 5 seac sub asso guns. All the pumps are no longer effective. The rubber skirt at the end of the pump piston always seems to fail, I'm not sure why. Has anyone attempted to machine something up as a replacement that works better. I was thinking about a teflon piston with 2 O rings? If anyone has done this ... again photos and explanation would be great.

Thanks, David
 

MARMIR

New Member
May 1, 2007
82
5
0
Hi David,

....This has been going on in Italy for about a year I don't think compressor is used
somewhere else...anyway if you are not in a hurry I'll post some pics during week end....once you have a compressor you'll throw away all pumps....
 
OP
OP
popgun pete

popgun pete

Well-Known Member
Jul 30, 2008
3,035
590
153
Australia
Some pneumatic speargun hand pumps already use a brass piston with one "O" ring fitted near the bottom end of the piston in order to keep the "dead space" volume in the pump small. You could make the pump's piston out of Delrin, but the "O" ring retaining lip on the bottom edge is best keep small in width and something like brass stands up to the stress when material cross sections are necessarily small.

I only mentioned the 12 volt portable compressors as they are relatively inexpensive to buy and will do the bulk of the pumping work. They do tend to struggle as they near their upper pumping "design" pressure, but pumping time will be less than one minute, then you finish off with the hand pump if necessary. These compressors are basically designed to pressurise passenger car tyres up to 40 psi (large internal volume) or bicycle tyres up to 150 psi (small internal volume). A pneumatic speargun has a relatively small internal volume, so the 12 volt portable compressors attain high pressure in the gun very quickly. There was a sort of "arms race" going on some years ago as competing manufacturers uprated their compressors to unnecessarily high maximum pressures. Maybe a truck tyre needs over 200 psi, but using one of these to inflate a dead flat truck tyre would give you time to overhaul the engine while you were doing it! For pneumatic speargun operators the higher maximum pressures were ideal and represent one of the few uses these compressors have as they possess a low air flow rate, but that is fine for pressurising your gun as it gives you time to monitor the in-built pressure gauge and to switch the compressor off.

At times these small compressors can be purchased for $30 to $40 from discount stores, sometimes even for much less. They are virtually throw away quality, but if you have one just for your speargun then it will last for years. Sometimes you can strike a dud example that never reaches much over 120 psi, but generally they will get to 200 psi easily, it just depends on how well that particular compressor was assembled. Most are made in China or Taiwan and are virtually identical inside once you look under the plastic outer casings.
 
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tamoshee

me and the deep blue sea
Jan 1, 2004
89
14
0
England
homepage.ntlworld.com
Thanks Pete,
I had read your piece about dead space, and figured that the O ring needed to be near the head of the piston. I was thinking 2 O rings to make sure that the piston travel inside the pump cylinder is as true and parralel as possible, so that the wall of the cyylinder doesnt get scored. I am going to give this a go with a brass piston. I will test it out and post photos in a week or so.

David
 

Seahunter56

New Member
Aug 28, 2008
10
0
0
Marco Island, Florida
I love all the information. I have 2 phneumatic guns and have always wondered how much to pump them up. What about making the adapter for the 12 volt pump? I could use one of my hand pumps and work from that. Any instructions on how to make the rest? And has anyone ever converted the number of strokes with the hand pump (Bars?) to PSI? Would sure make llife easier. Thanks Everyone.
 
OP
OP
popgun pete

popgun pete

Well-Known Member
Jul 30, 2008
3,035
590
153
Australia
I made the adaptor for using a portable 12 Volt air compressor out of the end of a pneumatic speargun hand pump, the part that fits directly into the rear inlet port of the gun. On the Mares guns this part screws off the end of the hand pump body tube or you can buy the ends separately. My old Mares guns use a "quarter twist and lock" bayonet fitting on the end of the hand pump, later ones simply screw in. The front end of this adaptor is a standard rubber tubeless tyre valve insert with the valve core removed, you need the threaded brass end for the air compressor's inflator attachment to clip over it like when you blow up a passenger vehicle or bicycle tyre. The two end pieces in my adaptor are joined by an old spark plug body using only the metal part, but you can make it out of anything tubular provided you have access to some machining facilities. Basically you need something to screw into the gun, you could cut the end off an old pump or use the whole pump body, but you need to block off the air breather hole in the pump if you do. The assembly has to be air-tight so pumped air transfers into the gun without external losses.

Most pneumatic guns come with a pumping table for the number of pump strokes from empty to reach a certain pressure in the gun expressed in Bar or Atm or kg/cm2. As the volumetric capacity of both the hand pump and the gun's internal air reservoir (including the volume of the inner barrel) dictate the pressure level achieved with a certain number of pump strokes there is no "universal" pumping table. My advice is pressurise the gun to the maximum and see if you can still load it. If not then bleed some air out carefully until you can. A good idea is to have the power regulator set to "low power" when doing this, it limits the air that can escape if you overdo it, only a momentary push on the valve stem lets more air out than you would think. After letting any air out flip the power regulator lever back to "full power" and then try the gun for loading effort again as this action allows air pressure in all parts of the gun to equalise.
 

Seahunter56

New Member
Aug 28, 2008
10
0
0
Marco Island, Florida
Thanks Popgun, The addapter sounds easy to make. I have a small pump on one of my battery jump packs, and it has a guage on it. I also have a commercial air compressor. I am a little nervous about over inflating it. I do have the table for the hand pump, but it would be much easier with the 12 volt compressor if I knew what the psi say 25 bars was. Do you or anyone else here know that? Thanks!
 

Alphamic

New Member
May 13, 2012
9
1
0
Newcastle, Australia
Hi everyone.
Only new to the forum but have been reading extensively for many weeks now trying to figure out ways to increase the power on my Seac Sub Asso 90. Or maybe just gettng it to work correctly.
I have pumped the gun 400 times with the hand pump and can still load it with not too much pressure. How muchis too much?
I know this is a very old post but I have a few questions. Probably for Pete. Where in Australia can I get a gauge for my Seac Sub? Do you have a picture of the 12 volt tyre inflator adaptor you made?
Any help would be greatly appreciated.
 
OP
OP
popgun pete

popgun pete

Well-Known Member
Jul 30, 2008
3,035
590
153
Australia
Hi everyone.
Only new to the forum but have been reading extensively for many weeks now trying to figure out ways to increase the power on my Seac Sub Asso 90. Or maybe just gettng it to work correctly.
I have pumped the gun 400 times with the hand pump and can still load it with not too much pressure. How muchis too much?
I know this is a very old post but I have a few questions. Probably for Pete. Where in Australia can I get a gauge for my Seac Sub? Do you have a picture of the 12 volt tyre inflator adaptor you made?
Any help would be greatly appreciated.
300 pump strokes gives you 20 Bars according to the pumping table. Double that will not give you 40 Bars as the pump loses more air from the dead space the higher the pumping pressure is. I suggest you add more air until the pressure is what you can load against without going overboard in terms of effort required to latch it. Could be the pump and connection to the gun is not absolutely air-tight, so counting pump strokes is just a guide, you may need more strokes to achieve the pressure you want.
 

Alphamic

New Member
May 13, 2012
9
1
0
Newcastle, Australia
Thanks Pete

I have been researching all previous threads regarding pneumatic spearguns. What a steep learning curve. In fact its almost becoming a technical obsession. I can see why once you start tinkering around with one you cant top. I have learnt so much between your threads and Tromich threads. If I could speak Italian or Spanish I wouldnt get any sleep looking at all those wonderfull web pages.

The truth is we here in Australia are so far behind in the practical uses of pneumatics, especially on the East Coast. Where I spear in the shallows for estuary fish the water is most times cloudy. The benefit of a shorter barrel to swing around is vital, especially if it has the power of a longer band gun. I can only imagine the power in those new Omer and Cressi guns.

After further looking into the problem I found that the hand pump seals are shot. I went to the local dive shop and pumped the gun there. It feels much better. I can really feel the power in it when trying to load. Hopefully I can test it out in the next few days.

Does anyone in Australia distribute/sell dry barrel kits? I have sent emails to various companies in Europe with no replies. In your opinion what is the best dry barrel kit for a Seac Sub Asso 90?

Once again thanks for the info.