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Pneumatic Speargun Compression Ratio

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popgun pete

Well-Known Member
Jul 30, 2008
The compression ratio of a speargun determines how much force you need to apply to the spear to latch the gun given the amount of air pressure pumped into it when setting the gun up for future use. The compression ratio (CR) is defined as the initial internal volume of the unloaded gun divided by the final internal volume when the gun is cocked and ready to shoot. The volumes are the internal capacity of the chambers, i.e. their physical dimensions. (In a scuba tank this volumetric capacity is often referred to as the equivalent water volume of the tank.)

If the inner barrel volume swept by the piston during muzzle loading and subsequent shooting is denoted by Vb and the reservoir volume is denoted by Vr then the total volume of the gun is Vt = Vb + Vr. Vt is also the speargun's initial volume, so the compression ratio for the gun is given by:-

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

The air in the inner barrel has been squeezed into the reservoir, therefore the compression ratio will be a number greater than 1.

If the volume of the reservoir Vr of the gun was infinitely large then Vb/Vr would be a very small number approaching zero and the compression ratio would then be virtually equal to 1. That would mean that the force required to start the piston moving back would not change as you pushed the shaft all the way into the gun to latch the mechanism as the pressure opposing the piston would always be a constant value. This is an entirely unrealistic situation, but the larger Vr is compared to the value of Vb then the gun is easier to load as the force to be overcome during muzzle loading does not change too much as the piston is pushed along the barrel towards the rear of the gun.

On the old style pneumatic guns, which had long skinny barrels, mid-handles and a relatively small bottle type reservoir at the rear of the gun the compression ratio was considerably larger. If the volume of the reservoir was equal to the volume of the barrel then Vb/Vr = 1 and the compression ratio would be equal to 2.0. On a gun with these dimensions the final effort to push the spear to the latch position would be twice the effort to get the spear moving at the muzzle, so to have any chance of loading a gun like this you would need to set a lower initial charge pressure.

If you are wondering what it was like to shoot a high compression ratio gun then you have probably already done so without knowing it. The modern Mares "Sten" type speargun has a dual power facility which when set to "low power" closes off nearly all of the air reservoir except for a small section at the rear which connects to the inner barrel. Hence when you load/cock this type of gun, set it to "low power", take a shot and then reload the gun again without changing the power selector position the gun is functioning as a high compression ratio, low pressure gun. This is because if you checked the air pressure in the gun after the shot was taken at "low power" then you would find that the pressure had fallen below the pressure that the gun had been charged at initially. The rear inlet valve, where you attach a pressure gauge, is only connected to the inner barrel, not the forward section of the air reservoir. The reason the pressure in the inner barrel is lower is that some of the air molecules originally in the barrel are now locked up in the forward closed off section of the reservoir, hence they no longer contribute to pressure in the inner barrel. Conversely the air pressure in the forward reservoir would be higher than the original charge pressure, but there is no easy way to measure it.

A dual power (partitioned reservoir) speargun is essentially two guns in one; a low pressure, high compression ratio gun ("low power" setting) and a high pressure, low compression ratio gun ("high power" setting). So now you know that those old small rear tank guns were less powerful than modern full length tank guns given the same inner barrel length for the gun. However many of these old guns derived their shooting performance from having greater length in the inner barrel, e.g. Nemrod "Fragata", the necessarily low charge pressure for a high compression ratio gun being an advantage to get the piston moving in the initial part of barrel travel in such a long gun. Another trick was to apply a "surcompressor" after the gun was cocked, this being either a lever system that telescoped inwards the back wall of the rear mounted reservoir and further raised the air pressure in the gun, e.g. Nemrod "Crucero", or pumped water into the reservoir space (a piston inside the reservoir acted as a mobile rear wall which separated the injected water from the compressed air) to decrease the air volume and hence raise the air pressure, e.g. Mares "Superjet". Using a surcompressor temporarily raised the gun's compression ratio, so to make reloading easier their action had to be reversed before the next insertion of the spear into the gun.

The last arrangement to consider is the hydropneumatic speargun which has a flooded inner barrel when being used underwater. The compression ratio for these guns is given by a slightly different formula as follows:-

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

The difference is because the inner barrel never has any air in it, so when loading the spear the water volume of the flooded barrel is pushed back into the air reservoir space, thereby reducing the internal capacity of the reservoir by that volume. However on a firing valve operated hydropneumatic gun the spear can be pulled out and reinserted a second time, thereby pushing another column of water into the gun. So now the compression ratio is given by:-

CR = initial volume divided by final volume or Vr/(Vr - 2 x Vb), for a third spear insertion stroke it will be Vr/(Vr - 3 x Vb)

Hence the hydropneumatic gun has a variable compression ratio depending on how many times the spear is inserted, either partially or fully. However hydropneumatic guns like the RPS-3, which have a mechanical retention of the spear and are not valve operated, have a fixed compression ratio as they cannot be pumped using multiple spear insertions. The "Aquatech" spearguns can, being firing valve operated, thus they possess a wide range of firing powers if you are prepared to pump the shaft when cocking the gun.
You can see a Nemrod "Crucero" here. http://www.skindivinghistory.com/gear/spearguns/vintage/Nemrod-Crucero.html
Pulling the upper lever that is angled forwards in the photo back to lay against the gun body moves the rear tube section forwards, reducing the size of the air reservoir and raising the pressure inside it. You do this after the spear has been muzzle loaded in the usual fashion. The last cylindrical section of the gun is a balance bar, it screws into the position where the hand pump fits when pressurising the gun at the start of the season. The balance bar improves the weight distribution around the centre handle as these guns are sinkers, whether loaded or not. On the same web-site you can see a Nemrod "Fragata", both it and the "Crucero" are long pneumatic spearguns by today's standards.
The other gun that I referred to is the Mares "Superjet". It is a variation on the Mares "Jet" model of the same shape, but has a black pump handle at the rear end. This pump reduces the volume of the rear mounted reservoir by injecting water into it and thus increases the pressure of the air inside it after the gun has been cocked via muzzle loading. A photo of a Mares "Superjet" can be seen at http://www.apneateam.it/Fucili%20Storici/Fucili%20storici%20-%20Mares%20Superjet.htm.

If the centre handle casting looks familiar then it is because it is based on the Mares "Bess" spring gun handle, but has been re-engineered to provide the trigger for the release mechanism situated directly behind the casting.
Pete, great stuff mate, very interesting. On the Crucero is it a single pump affair? Does it just mechanically reduce the volume of the air chamber using a piston type of arrangement or is it an air pump? If the former then it would have the advantage of being able to be done sub surface, yes? How much was the reduction? If the gun was loaded with as much human strength/force as was available to load it using the spear, was the lever strong enough to use human force/strength to further increase internal air pressure? Or was it a case of starting with a lower internal pressure for easy loading using human strength/force and then using the lever to "top up" the pressure?

On the superjet was the pump a multi action pump with a valve designed to be used in water sub surface to inject water? If so how many pumps could it supply and what additional air pressure was generated inside the gun. As above was this just an easy load and then a top up or could more than normal internal air pressure be created over and above what a max strength load with the spear could generate.

I've seen a video (from Miles hunting tuna) where a russian hydopneumatic gun is additionally pressured with a rear mounted lever action water pump. I guess this is a sort of combination or the systems used in the 2 guns above.

Keep it up mate as I am being more inspired daily to build something.

My latest thoughts are on a hydropneumatic. What do you think the air pressure would be for a medium power hydropneumatic with an 8mm internal diameter barrel? Does 50 to 60 bar sound about right. I'm talking about a 90cm gun with equivalent performance to a single/double euro band gun.

As the transmittion of power require water rather than just air to flow inside the gun at high speed do you think this design will have a problem with the transfer porting from cylinder storage to barrel?

Any pearls of wisdom appreciated.



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Hi Dave, the "Crucero" lever is not a pump lever, it is just one stroke to telescope the back end of the gun and slightly reduce the length of the air chamber at the rear. The stroke length on the rear reservoir section is not that great, you can tell by the pivot pin position in the metal straps that connect each side of the gun to the lever. The lever operates something like an over the centre action toggle when it shifts that pin (one on each side of the gun) from front to rear of the second pivot pin at the base of the lever. The reason that it makes a reasonable change in chamber pressure is the chamber is not very big in diameter and is also not very long, considering the very long forward barrel on the gun.

The "Superjet" uses the same principle of effectively shortening the rear pressure chamber, but instead of using a one stroke lever with a mechanical action to make the chamber shorter in length, which you can see externally when you actually do it, the Mares gun pumps water in instead. On the "Crucero" you have mechanical advantage through the length of the lever, on the "Superjet" the mechanical advantage is by using a number of pump strokes to add water in small volume increments with each pump stroke. Water being incompressible, each pump stroke advances the internal bulkhead at the rear by the same small amount, thus reducing the internal volume of the rear pressure chamber. The "pump handle" is actually the black rear grip and the outer body tube which is in a sense a sleeve sliding over the rear pressure chamber inside it, the black grip being where you grab onto it. These "surcompressor" systems are to overcome the high latching effort limitation that you encounter with high compression ratio guns that have small reservoir capacities, but they are not necessary when you have large reservoir capacities like we have in today's pneumatic guns. "Surcompressor" systems were also used in spring guns, both lever and water pump types, and operated in virtually the same way. Sure they squeezed a bit more power out of the guns, but added to weight and complexity in an environment that loves to stuff up anything with sliding surfaces and close tolerances.

Hydropneumatic spearguns are essentially pneumatic guns without inner barrel pistons, so usually the barrel diameter is very close to the spear diameter, which means that the inner barrel diameter is small, with maybe a millimetre clearance on the shaft, it really depends on the size of the stop diameter on the shaft tail for the line slide, if the gun uses one. That small inner barrel diameter means that the operating pressure has to be higher (spear propulsive force = chamber air pressure x cross-sectional area of inner barrel) than in a pneumatic speargun of the same size. You can work out what that air pressure should be by using the ratio of the internal barrel diameters squared for a hydropneumatic compared to a pneumatic, all over things being equal. You also have to factor in the compression ratio, the formula is slightly different for a hydropneumatic, as mentioned earlier.

I do not know the magnitude of the pressure changes in these two guns as I have never owned one, but owners have sent me photos of them in various states of disassembly.

More to follow later.
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Thanks Pete. I was thinking that the hydro gun would need double the pressure based on a spear tail of 8mm (6.5 - 7mm spear) and a pneumo piston being 11mm -13mm. As 8 squared being half of 11 squared more or less then you need double (at least) the pressue. So 2 x 20 - 30 ats for a pneumo gives the 50ats - 60ats for a hyrdo.

That was my theory anyway but I was unsure or the actual figures as opposed to my theory as I have never seen let alone handled a hydropneumatic gun.

Looking forward to further revelations.

Dave, you are correct in your estimation of the pressure levels for hydropneumatic spearguns, however it is hard to pump any speargun much past 40 Bar, given that the hand pumps supplied with most guns usually have very similar pump bore size diameters. The manufacturer wants to minimise the number of pump strokes, so the hand pump bore is a compromise between gun pumping time for the user and the ability to generate sufficiently high pressure in the gun for satisfactory shooting performance. Hence while the user handbooks for hydropneumatic guns often cite impressive chamber pressures like 50 Bar, 60 Bar or even 80 Bar in the specifications, it is likely that most users never shoot their guns with such pressure levels in them, unless the guns are filled using other methods, simply because the pumping action with the supplied hand pumps becomes too difficult. If you fit a bigger tee handle to the pump then you can use both hands and exert your body weight on it, that can squeeze a bit more air into the gun, but unless the pump bore is decreased in diameter you will soon hit the pumping limit again, maybe not a lot more than 40 Bar.

Hydropneumatic spearguns need to be extremely well built, so constructing one is not a job for anyone who does not understand the forces involved, especially if they intend to use high pressure air in the gun. A pressure of 600 psi in a speargun is normally considered to be high operating pressure, plastic mouldings and light gauge aluminium tubes can cope with that level of pressure, but when you go up to 1,000 psi or 1,500 psi you enter a whole new ball park. If you want to use pressures up to 600 psi (40 Bar) then it is better to stick with pneumatic spearguns as their larger diameter inner barrels can do more with the available pressure, but a hydropneumatic gun with better or comparable performance has to use higher pressure air, particularly as it is also inherently less efficient than a pneumatic speargun and will dissipate more of the stored energy (from cocking the gun) within the gun when it shoots.

The gun that Miles was showing in his video will be the Ukrainian, not Russian, "Black Sea" speargun. That gun started off life as a "surcompressor" equipped gun where the water pump further raised the air pressure in the already cocked gun. When the gun fired it pushed the spear out followed by the extra water that had been inserted by the "surcompressor". That gun rapidly evolved to a new form where the gun was cocked using only the "surcompressor". Inserting the spear into the muzzle played no part in cocking the gun. That required the gun to be at an elevated pressure before any loading effort had commenced, either 100 Bar or 120 Bar, the water being injected into the gun by the rear lever operated pump was only that required to clear the spear from the muzzle, hence no more water left the barrel after the departure of the spear.
Peter, Have you ever run Nitrogen gas in your guns?
I know that air @ sea level is 78% Nitrogen, 21% oxygen and 1% other gases
including water vapor, N atomic wt @ 7. Any thoughts?
I'm thinking less water vapor, less heat transfer (barrel in sun) and less oxidation of any remaining rubber in old air guns. Lighter barrel wt.:)
Cheers, Don
Not much benefit using nitrogen, except for the fact that you are filling the gun from a pressurised vessel. The presence of oxygen in the air is not really an oxidisation problem inside pneumatic spearguns, especially with oil sloshing around inside them.
You could fill a pneumatic speargun from a carbon dioxide cartridge, that would save on pumping the gun up. I have never tried it, but if the cartridge was large enough then the gun would fill with gas at 900 psi (60 Bar) provided that there was still a drop of liquid carbon dioxide left in the cartridge once the gun was connected up to it. There is a barrel-less pneumatic speargun called the "Vlanik" which is sort of an air analogue of the Russian hydropneumatic "Kobra", both use a stationary sealing system in the muzzle which operates on the body of the spear shaft and they are forward latching guns. The "Vlanik" needs high pressure air/gas as the spear shaft acts as the barrel (volume or displacement of the shaft entering the high pressure reservoir), hence the gun can be filled with carbon dioxide gas. The other option is to use the "Vlanik" hand pump that looks to have a long body and a very narrow bore, so a lot of pump strokes required! Gas flow inside the "Vlanik" is not an issue as essentially the gun body is just a gas tank with nothing inside it. When the spear files out of the muzzle a tail cap on the end of the shaft detaches and stops the gas escaping through the muzzle opening by plugging it. When you muzzle load the gun the spear pushes the tail cap back into the gun and the muzzle seals take over the gas sealing job. If the tail cap falls off inside the gun then you have a problem as next time the shaft shoots the gas will escape through the muzzle. With a small diameter shaft you can understand why the pressure requirements are high, especially after considering what has been previously written on this thread.
Thanks Pete, I'm not filling my AirB with anything but a hand pump, I want to
my fish killing muscle, powered. I was thinking more for one of my old air guns that I might bring back from retirement.:)
Cheers, Don
With a dual power ("Sten", "Cyrano", etc.) speargun's power regulator switched to "low" power the volume of compressed air available for spear propulsion is restricted to that held within the gun's "pre-chamber". The "pre-chamber" consists of the tank volume behind the power regulator's partitioning bulkhead combined with the volume remaining in the inner barrel tube rearwards of the conical rubber piston seal when the gun is cocked, plus that of any interconnected cavities incorporated in the rear handle moulding. If we denote this "pre-chamber" volume by "Vp" then the compression ratio for the gun in this switched to "low" power state becomes:-

CR = (Vb + Vp)/Vp = Vb/Vp + 1

where "Vb" is the volume of the inner barrel swept by the sliding piston during the shot. "Vp" is a sub-part of the main reservoir "Vr" and as "Vp" is usually smaller than "Vb" and much smaller than "Vr", so Vb/Vp will be larger than 1 and hence the "switched" compression ratio will be greater than 2. Compare this figure with a typical pneumatic speargun compression ratio of 1.10 when the full capacity "Vr" of the tank surrounding the inner barrel is used for the shot. While the one-way valve in the partitioning bulkhead ensures that higher pressures in the "pre-chamber" always pass through into the forward section of the reservoir regardless of the power selector setting, that does not happen going the other way. Thus shots will be always taken with the same cocked pressure level in a dual power speargun, but the two different compression ratios available will mean that the discharged pressures will be different (cocked pressure divided by the respective compression ratio). The discharged pressure of a pneumatic speargun represents the base pressure which has to be to loaded against and the higher that discharged pressure is the more powerful the gun will be. The most powerful muzzle loaded guns are high pressure, low compression ratio guns principally because loading a high pressure, high compression ratio gun by hand would be next to impossible.

From the above formula we can see that the compression ratio will be increased if, with the power selector set to "low", the "pre-chamber" is reduced in volume with respect to "Vb", hence for a longer barrel version where the "pre-chamber" is of the exact same size as that used in a shorter model of the same gun the low power shot will be weaker in proportion to the gun's high power shot as "Vb" for the longer gun is that much larger. As far as I know manufacturers do not adjust the bulkhead stand off legs, or cylindrical insert spacing length in the modern design bulkheads for their rear handle guns; that would require also fitting different length power selector control rods to the different length models, so just how low the "low power" performance is depends to some extent on the length of the gun.
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Just to show that anything is possible check out this Hang Fung Industrial Co. Ltd. "Aqua Gear" pneumatic speargun with a power regulator installed in a 30 cm model! The "pre-chamber" in this gun must occupy most if not all of the reservoir tank! At the other end of the scale the gun is also produced as a 140 cm model. A Seac-Sub "Asso" clone by the look of it, but manufactured in Hong Kong.

Why it is so incongruous to see such a small gun as the "Aqua Gear" 30 cm model with a power regulator is that it is too short to ever need the "easy loading" feature and the difference in "high" and "low" power shooting performance will be negligible as the reservoirs being used are nearly the same volume and the compression ratio as a consequence will not change by any useful amount. I therefore assume that Hang Fung Industrial currently only make the power regulator equipped rear handle, so all their models are fitted with it regardless of whether the guns actually need it.
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The "low" power shot from a partitioned reservoir speargun is theoretically greater than half the power achieved with the gun set to "high" power, but the difference in power is determined by how much difference there is between the respective compression ratios available at each power regulator setting. The higher the compression ratio then the steeper the gradient of the pressure change with the piston being pushed along the inner barrel into the gun. The smaller the "pre-chamber" volume is compared to that of the front tank the closer you get to half power from a partitioned reservoir gun as the compression ratio becomes very high on the "low" power setting. Some examples are shown in the attached diagrams, the horizontal axis is the piston travel distance and the vertical axis is the internal pressure in the gun, on the left hand side the gun is cocked and on the right hand side it is discharged. The energy available for each shot is indicated by the area under the graph, in each case this is the area of the rectangle combined with the area of the right angle triangle sitting directly above it. Losses due to friction and throttling are not considered here, so the energy output will be less than that indicated by the graphs. The "high" power shot is throttled by air passing through the transfer port while the "low" power shot does not involve the transfer port at all, so that affects the relative power output of the two power settings.


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Well just to complete the "high power/low power" story here is a bit more.

Some pneumatic spearguns have "low" and "high" power settings that don't vary the gun's compression ratio. In those guns there is an internal transfer port that can be changed in size by rotating a segment inside the gun that works as a shutter closing off one transfer port and exposing another of a different size. The lowest power setting uses the smallest transfer port size, virtually a pin hole in terms of its diameter, while "high" power uses the largest. The throttling effect of the port slows the flow of air moving from one section of the gun to another which removes energy from the shot. These guns are usually equipped with a rotating knob which is incorporated into the butt at the rear end of the gun in order to adjust the shooting power, hence they are often referred to as "power dials". The "power dial" is directly connected to the revolving shutter inside the gun by a mechanical linkage. Regardless of the power setting used all the air inside the gun expands for each shot, so the muzzle reloading effort is the same after every shot. However such guns should be loaded with the "power dial" set to maximum or you will be forced to squeeze air through the smaller ports as you compress the air inside the gun by moving it from the inner barrel into the air reservoir or tank. There is no point in making the job any harder for yourself and you do not save any energy by using the other power settings.

There are other arrangements that revolve a shutter where a port is progressively closed off rather than switching between a set of various sized port holes. In the Russian "Taimen" the single port is transversely drilled through a side mounted rod that revolves through ninety degrees so that the port either faces directly in line with the internal airflow direction or is turned away from it virtually blocking the airflow off, or at some intermediate position in-between. In the Technisub "Grinta VP" revolving the entire air tank by twisting it on the gun body rotates a plastic section just in front of the rear grip which acts as a shutter inside the gun that gradually closes off the airflow to the inner barrel. Although these various internal shutter arrangements are energy-wasters, unlike the partitioned air reservoir systems, they are handy if you don't want to destroy your speartips if shooting close to rocks. Or, if diving in a lake, sinking your speartip into a log where you may have trouble removing it after narrowly missing what was hiding underneath the sunken log.
I was recently looking at a graph of pressure changes in a hydropneumatic speargun which used more than one loading stroke of the spear in the inner barrel to charge the gun up for shooting and noticed that the graph shown was curved rather than a straight sloped line which we usually see in the "right angled triangle sitting on top of a matching length rectangle" energy graphs. Now those graphs of pressure change as the piston moves along in the inner barrel are mainly for guns which have a compression ratio of around 1.10 and the sloped uppermost line is more or less straight in that region. But if we consider a pneumatic speargun as one very long tube of the same diameter as the inner barrel with the unused or unwept section of the tube being the air reservoir then we can draw a graph for the pressure changes in the gun where we progressively use more and more of the length of this long tube.

The piston (or the spear tail for a hydropneumatic gun) travel distance X in the inner barrel of a pneumatic gun is directly proportional to the volume V changes, but as the pressure P multiplied by volume V is a constant for a given mass of gas then it follows that P is inversely proportional to X and that explains why the pressure versus spear tail/piston travel graph mentioned above was curved as the graph is of the form Y = 1/X for a larger range of pressure changes.

The revised energy graph attached below shows this curve for the low power case and explains why the shot is about "half power" because an area of the "green" energy graph is lost from under the blue straight line that represents the low power compression ratio (that line has an increased slope), but does not indicate the actual pressure changes acting behind the piston. Most guns operate in the initial flat section (red box) of the Y = 1/X curve, but not guns with small reservoirs compared to the swept volume of the inner barrel, or the low power option on a dual power pneumatic gun (green box).
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