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Pneumatic Speargun Compression Ratio

Diving Gecko

shooter & shooter
Jun 24, 2008
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Hello Pete,
Sorry to be slow on this but I have read the thread many time over the years and still have to grasp it fully. It might come some day.
Not to be lazy, because I have really tried to figure it out myself, but could I possibly ask a few questions from a lowly position in a more practical manner?

Thing is, I may have a go at making a custom, cuttlefish-shaped outer tank but I would also like a tad more buoyancy. I could still get there by making some of the volume of the new custom tank "blocked off" from the reservoir - so that it would still count towards the total volume/buoyancy of the gun but the resoir volume would stay closer to the original.

But say I keep all of the new and larger volume acting as a reservoir what would happen in very practical terms in the way the gun was acting?

Eg. I add 20% volume to my gun.
I reckon, since the CR decreases, I will have an easier time loading if I maintain the same pressure as with the original tank, right? But the decrease in loading effort would also mean a decrease in force exerted onto the piston as I shot it, I guess? So, I would end up with a weaker shot?
To get to the same power in my shot in the enlarged tank as in the original one, I guess I would then have to up the pressure?
What would the co-relation between the increase in volume and the increase in pressure be if I was looking to maintain the same level of power? It's not as simple as adding 20% more pressure since it was a 20% increase in volume, right?

Again, sorry for being thick, but if you could possibly help me out by explaining it in very practical terms, I'd be grateful!
 

Jegwan

Well-Known Member
Oct 1, 2012
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Hello Pete,
Sorry to be slow on this but I have read the thread many time over the years and still have to grasp it fully. It might come some day.
Not to be lazy, because I have really tried to figure it out myself, but could I possibly ask a few questions from a lowly position in a more practical manner?

Thing is, I may have a go at making a custom, cuttlefish-shaped outer tank but I would also like a tad more buoyancy. I could still get there by making some of the volume of the new custom tank "blocked off" from the reservoir - so that it would still count towards the total volume/buoyancy of the gun but the resoir volume would stay closer to the original.

But say I keep all of the new and larger volume acting as a reservoir what would happen in very practical terms in the way the gun was acting?

Eg. I add 20% volume to my gun.
I reckon, since the CR decreases, I will have an easier time loading if I maintain the same pressure as with the original tank, right? But the decrease in loading effort would also mean a decrease in force exerted onto the piston as I shot it, I guess? So, I would end up with a weaker shot?
To get to the same power in my shot in the enlarged tank as in the original one, I guess I would then have to up the pressure?
What would the co-relation between the increase in volume and the increase in pressure be if I was looking to maintain the same level of power? It's not as simple as adding 20% more pressure since it was a 20% increase in volume, right?

Again, sorry for being thick, but if you could possibly help me out by explaining it in very practical terms, I'd be grateful!

Hi David :)

The total air volume in modern guns (like the sten types and so on) is quite big compared to the reduced volume from the piston travel. Air is compressible. So the total pressure in these does not increase that much when it's loaded.
You know how to handle a speargun safe - even when loaded - so; put your manometer in the valve of the gun in question and look at it while you push the piston carefully back and forth...

Jégwan
 
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popgun pete

popgun pete

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If you increase the tank volume by 20% then Vr (the reservoir volume) becomes Vr x 1.2 which can be represented as 1.2Vr, but Vb (the inner barrel swept volume) stays the same. The formula shown at the start of this thread for the compression ratio CR of a pneumatic speargun is:

CR = initial volume divided by final volume or (Vb + Vr)/Vr = Vb/Vr + 1

which now becomes (Vb + 1.2Vr)/1.2Vr = Vb/1.2Vr + 1 = (Vb/Vr) x 0.833 + 1. The inverse of 1.2 is 0.83333 repeating, but 0.833 will do for our purposes.

Thus the compression ratio CR’ with the 20% larger air tank is lower than the original CR, but there is not a lot in it. If the original CR was 1.1 then Vb/Vr was 0.1 which means that (Vb/Vr) x 0.833 is now equal to 0.1 x 0.833 = 0.0833. Therefore CR’ equals 1.0833. If the air tank was increased by 40% in internal volume then the new compression ratio CR’’ would be 1.0714, so this is the reason why air tanks on modern pneumatic spearguns have been kept to the sizes that they are as the reduction in compression ratio is not worth making the gun more bulky than it already is.

If your start pressure was 20 Bar then the cocked pressure would have been 20 x 1.1 which is 22 Bar. With the 20% larger air tank and the consequently lower CR’ the cocked pressure will be 21.666 Bar, hence you have less power as the energy stored in the gun depends on the difference between the start and cocked pressures, however to return to a cocked pressure of 22 Bar you only have to increase the start pressure to 20.3083 Bar as 20.3083 x 1.0833 = 21.9999 Bar.

Highly elevated air pressure in a deformed metal wall tank will always be trying to turn that tank cross-section back into a cylinder, so the strength of the tank material has to resist this without the tank “oil canning” with any internal pressure changes and that includes temperature changes. Aluminium alloys work harden very quickly, any significant flexing cycles and the wall material will fail as it fractures, so be very wary of how you manufacture a non-cylindrical tank if you are using this material. The major pneumatic speargun manufacturers have done the necessary testing and selected both the appropriate alloy and how it is processed to create the hydroformed tanks we see today, but it is not a home workshop proposition unless you thoroughly test the results. A home-made pneumatic speargun tank coming apart in a public space as it suddenly blows to pieces will not be a good ambassador for the sport, need I say more. Personally I don’t like hydroformed tanks as the gun becomes a paddle in one direction or another and the cylindrical tank has stood the test of time, it will be interesting to see if the hydroformed tanks are similarly long-lived.
 
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Diving Gecko

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Jun 24, 2008
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Thanks guys, just what I needed.
I guess, I should have surmised it myself as Jegwan points out - the barrel volume is already so small om modern guns in relation to the reservoir (which I did know), that a change in reservoir size wouldn't have that much of an impact.

Thanks Pete so much for taking your time on both the techie and hands-on explanation. I think I even got the tech part, this time;-)

In reg. to your paddle observation, for the type of hunting I have done so far, I have realized one simple thing - I "pan" way more than I "tilt" the gun. Fish tend to swim sideways more than up and down and though I did lose one grouper once because the oval barrel of my One Air 120 could not tilt downwards fast enough - and when it did it started wobbling from side to side - I am more than happy that it tracks sideways better.

Honestly, I think the reason we don't see more cuttlefish shaped tanks for oleos is one of cost and lack of demand. As you know, it costs a lot to make molds and you would ideally need one for each size of gun in your product line.
Until recently, I think it is safe to say that the oleo market has been pretty stagnant and the development likewise for 20-30 years?. Customers haven't really had much to choose from. Well, they had but within reason, it was pretty much the same independent of brands.
Why spend the money and R&D when customers didn't cry out for anything out of the ordinary? And perhaps, they wouldn't have paid for it.

To add the safety aspect to this, which I fully agree with, the Omer Airbalete has been out there for a while. Seac made a hydroformed tank for their Caccia(?), too, some years ago and now, we have the Mares Evo HF. With the Med spearos shooting these guns, I think we would have heard of issues by now if there were any? On the other hand even with frequent use, metal fatigue can take a while setting in.

It shouldn't come as a surprise that I intend to make the tank in composites. What will help with the strength is that I am not out on a weight-saving mission at all. Quite the contrary. So, the walls can be made quite thick. I could make an internal webbing/brace structure, too. But depending on such a structure is risky if the bonding to the walls fail. Better to have all the structural strength needed in the walls alone.

I think the biggest problem with a cuttlefish shape in an oleo is that they become nose heavy and many customers really don't like that. One way to solve that would be to add counter weight, or rather, negative buoyancy, aft of the belly (aft of the center of buoyancy) of such a gun to balance out the nose. But then you would need to increase the total volume to keep the gun buoyant.
And right there, in this last paragraph, I summed up what it is I am trying to do myself...;-)

Thanks again, gentlemen. This is the 2016 project:)
 
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Jegwan

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Oct 1, 2012
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Thanks Peter.

Considering the pressure rise from loading (in a given gun with large internal volume), I believe that you as a thumb rule only need to increase the applied pressure in a gun with lover CR (due to higher volume) with half of the difference in pressure rise from compression between the two.

Peters remarks about structural strength are very relevant - also when using CF. Actually I don't know any way to strictly calculate the strength of CF constructions - I suppose CF manufacturers have some empiric knowledge to do so.
As you have mentioned yourself elsewhere, you could solve both problems if you exclude the additional "cuttelfish" volume from the pressurised volume - i.e. using the floater principle for balancing the gun instead of the internal pressurised volume.

The pressurised barrel could be "Skorpion-like" added what so ever shape on the outside...

Looking forward to follow your project :)

Jégwan
 
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Diving Gecko

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Jun 24, 2008
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Thanks Peter.

Considering the pressure rise from loading (in a given gun with large internal volume), I believe that you as a thumb rule only need to increase the applied pressure in a gun with lover CR (due to higher volume) with half of the difference in pressure rise from compression between the two.

Peters remarks about structural strength are very relevant - also when using CF. Actually I don't know any way to strictly calculate the strength of CF constructions - I suppose CF manufacturers have some empiric knowledge to do so.
As you have mentioned yourself elsewhere, you could solve both problems if you exclude the additional "cuttelfish" volume from the pressurised volume - i.e. using the floater principle for balancing the gun instead of the internal pressurised volume.

The pressurised barrel could be "Skorpio-like" added what so ever shape on the outside...

Looking forward to follow your project :)

Jégwan
Oops, this, once again, ended up too long. So, will headline the different chapters;-)

About Composites and Calculations
The high end composites manufacturers and engineers have data models for this. But it is not something that mere mortals can get their hands on. They make test laminates, then test the laminates' properties in those machines that break things and then feed that into their software... Yeah, I'm def not an engineer, but I am pretty sure, that's how they do it:).
It depends a lot on the type of fiber and resin used, the fiber orientation, the fiber to resin ration and curing schemes, so this approach makes sense for high end use.
There might be some more generic plug-ins for Solidworks though. I am not sure.

My Basic Thoughts on Build and Safety
I will basically over-build and then try to blow up the tube when cured. I know this does not take into account repetitive use and pressure cycling nor the elements effect on the laminate, like sun and salt water, or gasoline slushing around in the bilge of a boat.
To cope with the latter, the best I can do is to have it properly coated with 2 component PU topcoat at an auto body repair shop.

The thing I am the most afraid of is "blunt trauma' in ER speak... A fully loaded gun, having sat in the sun for hours gets dropped on the railing... Not a nice scenario. For that reason alone, it might make sense to go with a mix of CF and Kevlar. CF is more brittle, though stronger than Kevlar.

Again, I have one distinct advantage on this build - I have no need nor wish to build light. I no longer need to save weight as with all my other oleo modifications. The increase in volume will allow me to "over-build". Think of it as a C4-inspired oleo;-).

But Back To The Compression Ratio
Let's try to boil it down further.
So, a very basic question - is there any downside to going for a lower CR?
Let's say, I push reservoir volume by as much as 40%.
As a result, I will have a more "bulky" gun as Pete points out but that is actually something I need and want - I need more volume to carry a heavier 8mm spear and a tad more mass (volume) to help with recoil would be nice. I will need to go up in pressure a bit as CR goes down, but is that it in very practical terms?

I can understand that a gun with lower CR will have less pressure difference - less difference in force - acting on the piston between the muzzle end and sear end, than one with higher CR.
In terms of practically loading a gun,let's say the force on the sear is a set value as a representation of what I can load - then with the lower CR, the force on the muzzle will be slightly higher than in a comparable tradition ole0 with the same force at the sear.

But I would think that what governs what I can load is the pressure/force at the sear end, more than the total energy expenditure of moving the piston from the muzzle to the sear, right? The total energy expenditure is a bit higher on the lower CR gun than on the traditional one if they both have the same pressure at the sear, right? But that should be an advantage as that gives me a more energized shot with a flatter power delivery curve.
Right....?;-)
 
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Jegwan

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Oct 1, 2012
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Yes, correct.

A high CR will give an easy start and hard end when loading it. A low CR will mean that the loading effort will be more similar from start to end.
So again; Factors like the length of the gun, loading technique and the persons physic are also factors that influence how easy it is to load a gun. I personally feel that I'm strongest when I'm in the first half way loading..

As mentioned by Peter a high internal volume will make the gun more bulky.

Yes, another problem that can arise from low volume and high CR is the force applied so the locking mechanism and hence the necessary trigger pull force.

I have an extreme example in my collection, the RPO-2 green stick (sorry about the picture quality David):

RPO-2 with shaftjpg.jpg


Tracking is excellent, but it's low volume, high CR and the front locking mechanism makes it very hard to load and shoot even with a relatively low initial barrel pressure.
I have yet to experience to shot a fish other than flatties with this gun. :)

Jégwan
 
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Diving Gecko

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Jun 24, 2008
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Thanks for posting your green popsicle!
It's fun to look at those old, skinny guns.
What's the tube for underneath the upper barrel? It's part of the reservoir?
 

Jegwan

Well-Known Member
Oct 1, 2012
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Sealand, Denmark
Thanks for posting your green popsicle!
It's fun to look at those old, skinny guns.
What's the tube for underneath the upper barrel? It's part of the reservoir?
No, it's a half tube cover for the pulling rod and spring for the front sear and trigger mechanism. It's a really simple gun. It's actually an USSR copy of an ancient Nemrod that is similar but blue with black handle. The Russians has nicknamed their version "long green stick" or "Green shit". LOL.

Jégwan
 
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popgun pete

popgun pete

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The critical aspect with any pneumatic speargun is that you can cock it by overcoming the final resistance as the piston reaches the latch position. That will always be the limiting factor, if you cannot make the latch position then you need to let some air pressure out of the gun as otherwise you cannot successfully load it in the water. Generally the power of any pneumatic speargun can be increased by upping the start pressure, but for the gun to be useful you then need to be able to push against that pressure when multiplied by the CR for the gun, hence “modern guns” (which have been that way for nearly 50 years) have opted for low compression ratios that make “high pressure” guns practical in terms of using a high start pressure. Otherwise the start pressure needs to be low if say you were using a mono-tube gun (like that Torpedero/RPO-2) with a compression ratio of around 2.0 where, if your pushing limit on that piston size in terms of its cross-section was 25 Bar, then your gun’s start pressure would have to be 12.5 Bar or less. This mono-tube gun would be less powerful than a gun of the same bore size and piston travel with a CR of 1.1 using a start pressure of 22.72 Bar which would also require an effort to overcome 25 Bar at the latching position. You don’t get any more out of a pneumatic gun than you have put into it with your muzzle loading effort, in fact with inefficiencies you usually get less, but lower CR guns are a way of producing powerful guns that can be muzzle loaded repeatedly by keeping the maximum pushing effort to the minimum for the start pressure being used in the gun. That is why the full length tank, concentric reservoir pneumatic speargun has become the standard layout as it maximises the tank capacity for any particular gun length and the necessary tubing has been mass produced for it which keeps manufacturing costs down.
 

Diving Gecko

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Jun 24, 2008
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Thanks Pete,
It all makes sense.
For years, I kinda assumed that our guns had the best possible CR - one that was, without much doubt, the most power efficient. Given to spearos by the grace of God or the law of physics and a change in it would certainly lead to a less powerful gun;-).

I now think that this widely adopted CR of a traditional design, as you also touched upon, has a lot to do with not only power and loading effort but also very much the final size of the gun.
It is, as most other things, a compromise. Here, a compromise between power, handling and buoyancy. As you, yourself, often mentions one of the things that really sets these guns apart from the earlier ones, is that ours float.

Also, I agree that adding volume to lower the CR in a traditional cylindrical design is not desirable. I would not want a gun to be more than Ø40mm at the tip, it is bulky enough as it is.

Thanks again. I am very happy to learn that we can mess with the "mysterious" CR and the Oleo Universe should still stand.

BTW, now I am wondering why the Sporasub One Air, with it's much larger tank (and thus buoyancy/mass) than traditionally has been partitioned so not all if it is reservoir volume? I can't recall the exact measurements, but I know they are in some pics on some older thread, but my gut feeling is that the CR is slightly higher than a traditional gun.
Maybe it could have something to do with keeping pressure in it under a certain limit. Though again, a substantially increase in volume doesn't not mean pressure needs to rise that much.
 
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popgun pete

popgun pete

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Thanks Pete,
It all makes sense.
For years, I kinda assumed that our guns had the best possible CR - one that was, without much doubt, the most power efficient. Given to spearos by the grace of God or the law of physics and a change in it would certainly lead to a less powerful gun;-).

I now think that this widely adopted CR of a traditional design, as you also touched upon, has a lot to do with not only power and loading effort but also very much the final size of the gun.
It is, as most other things, a compromise. Here, a compromise between power, handling and buoyancy. As you, yourself, often mentions one of the things that really sets these guns apart from the earlier ones, is that ours float.

Also, I agree that adding volume to lower the CR in a traditional cylindrical design is not desirable. I would not want a gun to be more than Ø40mm at the tip, it is bulky enough as it is.

Thanks again. I am very happy to learn that we can mess with the "mysterious" CR and the Oleo Universe should still stand.

BTW, now I am wondering why the Sporasub One Air, with it's much larger tank (and thus buoyancy/mass) than traditionally has been partitioned so not all if it is reservoir volume? I can't recall the exact measurements, but I know they are in some pics on some older thread, but my gut feeling is that the CR is slightly higher than a traditional gun.
Maybe it could have something to do with keeping pressure in it under a certain limit. Though again, a substantially increase in volume doesn't not mean pressure needs to rise that much.
The central bore diameter of the Sporasub “One Air” body or outer barrel tube is 37 mm, which is close to the 38 mm bore diameter of a standard 40 mm tank tube, given that the wall of that tubing is 1 mm thick. Hence its compression ratio will be about the same as that of a standard gun as it has no power regulator block taking up internal space. As an alloy extrusion the three cavity gun bodies can be cut off to the required length which makes the guns cheaper to produce compared with hydroforming a cylindrical tube for each “Airbalete” which shares many of the same parts.

Once pneumatic guns became less likely to lose pressure it was realized that larger volume air tanks, which would give better performance, were a possibility as the investment required in pumping them up was not going to be lost, however that was not true for very early pneumatic spearguns. There the priority was the ability to reach operating pressure quickly by requiring fewer pump strokes which meant that the air reservoir capacity needed to be kept small. Some guns even had a built-in hand pump which could be used to top up the gun pressure before each dive to replace what had been lost since the gun was last used. The earliest guns were prone to leak pressurized air because their narrow front barrels were only a pressed in or shrink fit into the handle castings and the guns leaked along those long overlapped metal to metal joins, however they were very slow leaks, just a small bubble every now and then, but enough to rob the gun of its power after several days. Being directly inspired by the previous spring gun layout, the early pneumatic guns had a long narrow barrel at the front end, a cast alloy mid-handle and a small diameter tank at the rear with the built-in pump handle creating a counter balance to the barrel. The spear when cocked only occupied the front barrel, the trigger mechanism and sear tooth being installed inside the cast alloy handle. Thus the compression ratio achieved was more a consequence of other design decisions in order to create a useful gun despite the guns being somewhat less than perfect in holding air pressure.

One way to eliminate any leaky body joins was to use a mono-tube design, but that required the gun to be of the forward latching type which was not ideal as the spear required a notch for the sear tooth to engage at its front end just rearwards of the spear tip. The resultant gun had a high compression ratio, but could be pressurized rapidly with relatively few pump strokes. When “O” rings become more widely available, which would replace the formerly used assortment of interference fits, rubber packing pieces and gaskets, then the pneumatic gun could finally evolve to the form we know today which is pressure tight over months and even years thanks to the dynamic nature of the seals which can accommodate small movements (expansion and contraction of adjacent parts) while still retaining pressure. Hence big tank volume, low compression ratio spearguns became a realistic proposition for mass production without owners complaining about the need to pump their guns up before every dive. As some long guns need over 1,000 pump strokes to fully pressurize them I think that manufacturers figured that the tanks were big enough at the 40 mm diameter which had become the standard for the 13 mm inner barrels.
 
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Diving Gecko

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Ah, the pumping effort. Yes, I forgot that in my "equation of compromises".
I really dislike pumping these guns as I travel with them or service them. Best thing I ever made in terms of spearing was the scuba tank adapter. No more hand pumping:)
 
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popgun pete

popgun pete

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The low start pressure, high compression ratio pneumatic gun probably endured as an acceptable underwater weapon for a number of reasons, one of which was it was quick to pump up to operating pressure and the loading characteristics as you pushed the spear in were somewhat similar to the spring gun which had inspired it. That is an initial relatively easy start to getting the spear moving in the barrel at the muzzle end which progressively became harder to push as the latching position was reached by the sliding piston. A similar experience today to that of loading a dual power speargun after a shot on low power with the selector lever (or knob) left on the “low power” setting, which is how those guns are meant to be used, but such weapons were decades away from being invented. Hence the early form of pneumatic speargun was to emulate the spring gun, the long metal coil spring being replaced with a compressed air “spring” that was noiseless as there were no metal coils to buckle against the interior of the barrel tube and the “spring” never needed greasing or cleaning because it was inside a sealed chamber. Spring gun manufacturers tried to match the pneumatic speargun by creating dry spring guns such as the models from Hurricane, a French company, but the guns were heavy and were not truly dry as water could leak in and the metal spring was always going to create inefficiencies due to coil friction acting against the barrel bore.

Once the pneumatic gun became completely reliable by eliminating air leaks the spring gun was finished, especially when it was realised that a pneumatic gun could have a preloading by increasing the spring pressure, that is the start pressure, and yet still be able to successfully load it as the force required ramped up at a much slower rate as the spear was inserted. This of course is the characteristic of a low compression ratio pneumatic speargun, something which was not possible with a coil spring powered gun and that preloading is what provides the rectangle sitting under the triangle on those work-energy graphs. While a metal coil spring can be compressed slightly in guns like the old Cressi-Sub “Saetta” where the coil spring had a piston on the front end that pressed on a muzzle anvil rather than a free coil end poking out of a non-restricted muzzle opening as most spring guns do, the coil spring's movement is limited when it becomes coil bound and it always requires a certain amount of space inside the gun.

Eliminating that heavy metal coil spring (some guns had two springs, one inside the other) and increasing the tank volume to produce a low compression ratio paved the way for guns that floated after the shot once aluminium alloy components that provided handles and end bulkheads were replaced with moulded plastic items. Hence the compression ratio, which at first seems a rather esoteric concept, is the key to transforming what had been a “spring gun without a spring”, such as the Nemrod “Torpedero”, into the powerful and very versatile weapon we use today.
 
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Diving Gecko

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Thanks again, Pete.
Always a privilege having you put these things into context and historical perspective.
Much appreciated!
 
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popgun pete

popgun pete

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I found this Mares high/low power diagram on my old tower computer which I have finally been able to recover the files from after it conked out nearly a decade ago. This diagram shows how a partitioned reservoir system in a pneumatic speargun operates in most cases. Here the “pre-chamber” is being referred to as the “rear chamber”. The power regulator control rod operates a downstream valve; in Salvimar guns an upstream valve is used which combines the "one-way" non-return valve function in the same valve.
mares_1979 schematic.jpg
 
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popgun pete

popgun pete

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Just as the Arbalete Pneumatico Santino is being discussed it may be worth highlighting this thread.
Santino rhs and line release.jpg

This pneumatic gun is a long slim barrel and grip handle tank that form a sub structure that inserts and locks into the timber bodywork with a few motions and can be quickly removed for service or swapping to a different body for a change of external appearance. Basically the bodywork provides buoyancy and mass and protects the long metal tubing barrel from being bent. Reading the available published material this interesting gun has a compression ratio of approximately 2 with a start pressure of 12 bar and a cocked to shoot pressure of 25 bar..
 
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