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What if some water left or entered the vacuum barrel
- Thread startertromic
- Start date
Thread Status: Hello
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It can take a long time to get an up-to-date response or contact with relevant users.
There is always some water inside before you start pushing on the shaft with the hand loader. This diagram was done some time back when there was discussion on the then new Salvimar muzzle. The stop ring is not shown in the muzzle. As long as not much more water gets in then any excess will be blown out the skirt of the vacuum cuff nozzle. Water flowing backwards only occurs if the water cannot flow out fast enough and the spear is still stuck in the piston. It has to be a function of piston velocity and water hammer that distorts the "O" ring instantaneously so not much water gets into the gun’s interior spaces. Oblivious to this happening and with further shots the user adds more each time until he fixes the problem by changing the vacuum cuff for a new one. And changes the oil in the gun.
Conclusion - Pneumovacuum Evil! At least in the form in which it is used in spearguns! Installing an emergency valve on the muzzle immediately solves all problems with pneumovacuum when excess water enters the barrel due to wear on the pneumovacuum cuff! But alas, no one wants to do this! But it's simple! Absolutely without water in the barrel is impossible! There must be some water to lubricate the 0-ring of the piston along the barrel when fired!
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O-ring of the piston is mainly lubricated with the oil inside the speargun, especially in pneumo-vacuum spearguns. Even there would not be a wear of vacuum cuff the water left inside is enough to build a high pressure during braking the piston. I made a calculation about what is going on when water interfere with the operation of regular shock absorber. The braking distance of the piston is shorter, instead of being 3 mm it is about 2 mm.
The holes for the water escape valve should be larger. I was using rubber tubing over the regular water escape ports on my Mirage and also on my Cyrano 850 and 1100.
The present area though witch the water runs forward, between the shaft end thickening and the shock absorber (8 and 8.5 mm) is equal to one hole of ID 2.8 mm. For emergency valve there should be at least two additional holes of same size, better four..
Example.
For damping 4.65 J pistons energy, water will damp 3.13 J and the regular absorber 1.52 J. That damping (braking) would be on
distance of 2 mm. Without water in muzzle this distance would be 3.07 mm. Longer braking is better for the life of the piston and shock absorber.
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It is a big mistake to think that the O-ring of the piston slides through the oil when the piston moves during the shot! If the barrel is polished to a mirror, then there is no oil in front of the piston and only water in the barrel participates in sliding! For this, a small amount of water is needed in the barrel of pneumatic vacuum guns! In my emergency valve, the total cross section of the holes is equal to the cross section of the barrel minus the cross section of the harpoon! The oil in the gun is necessary to ensure tightness and lubrication of the cocking trigger, as well as to facilitate loading the gun when the O-ring of the piston really slides over the oil film! For example, in Zelinka, 3 - 5 ml of ATP oil is enough!
I agree what you said about emergency valve and the total cross section of the holes. I am not sure that the O-ring on the piston can 100 % wipe of the oil film in a barrel... I have no experience with mirror polished stainless steel or titanium barrels. With my Tomba vacuum system and common aluminum barrel there is no water in a speargun and everything is working fine.
The Normal category speaks of insufficient research of the Subject of Dispute, because some average value of the sliding friction force is taken! It is enough to inject a spray - mineral oil or a decoction of flax for ecology into the barrel after loading and check the speed of the harpoon! The result will be amazing and will mark the beginning of the use of such injection using a special container on all air guns! For guns without air vacuum, this does not make sense! Good luck with the Tomislav experiment! When you patent - do not forget to mention me as the author of the Idea!
Zahar, it is not wort the effort to lower the friction of the piston that way. The static friction of the piston is anyway about 3 % of loading effort. I had measured it years ago. The dynamic friction must be even lower, in theory. I could not measure it... but your idea is something new. However something like that could not be patented (as I know). It might be maybe technical improvement...
Guns are assembled with lubrication on the "O" rings at the factory to enable sliding them into position on the components. You can push the piston back when trying the spear for loading effort on land, but if you are going to do that a number of times then a few drops of oil should be added to the inner barrel first through the muzzle mouth or relief ports as the seals will need lubrication once what was originally on them wipes off. When the piston moves at shooting velocity it needs lubrication as do the vacuum seals if fitted and that needs water, or on land a few drops of oil. (Don't shoot guns on land!) Vacuum barrel guns when cocked are not truly dry inside if loaded underwater as some moisture always surrounds the piston forwards of the piston front seal. It can accumulate there over a series of shots even if the piston nose leans tightly on the shock absorber anvil after each shot. In wet barrel guns this area dries out right back to the front piston seal via the relief ports.
In a pneumatic-vacuum charged barrel, the amount of moisture in front of the piston depends on the position of the gun Top - Bottom!
Under a partial vacuum the water may enter a vapour or droplet state inside the barrel, but unless someone does some experiments with a glass cylinder there is no way to find out. Experience to date does not seem to indicate that vacuum barrel guns damage their seals by running "dry" so there must be some lubrication as the piston advances in the barrel. My Taimen does not show any problems, it only has one piston "O" ring and a polyurethane head which gets wet when exposed at the muzzle entrance. The Taimen barrel is polished “to the mirror”.
To calculate the water pressure (p1) in front of the piston, when it hits the shock absorber, if there is some water left, there is a very simple formula derived from Bernoulli's equation:
p1 = p2 + 1/2 q v2^2 - 1/2 q v1^2
p1 = p2 + 1/2 q (v2^2 - v1^2)
p2 - ambient pressure of water where spearfishing
q - specific density of water (1020 - 1030 kg/m3 for salt water)
v1 - speed of the piston/water being pushed by the piston
v2 - speed of the water being pushed between the shaft and the shock absorber - through the annular gap
For the example given at the beginning v1 = 30 m/s and v2 = 207.27 m/s
After calculation we get the result for p1 = 218.66 kgf/cm2 or 214.4 bar. 1kgf/cm2 = 0.98 bar
Even a sealing gasket is OK if water would not be empty from the speargun over pressure would happen, for a very short time but enough to push some water behind the piston into the barrel.
I case of Mares shock absorber during the first 1 mm of compression of shock absorber actually only the hydro damper is working.
Regular mechanic dumper can absorb only 0.59 J because for 1 mm compression the max force is only 70 kgf. Peak force for the water is 199 kgf and after 1 mm 159 kgf.
p1 = p2 + 1/2 q v2^2 - 1/2 q v1^2
p1 = p2 + 1/2 q (v2^2 - v1^2)
p2 - ambient pressure of water where spearfishing
q - specific density of water (1020 - 1030 kg/m3 for salt water)
v1 - speed of the piston/water being pushed by the piston
v2 - speed of the water being pushed between the shaft and the shock absorber - through the annular gap
For the example given at the beginning v1 = 30 m/s and v2 = 207.27 m/s
After calculation we get the result for p1 = 218.66 kgf/cm2 or 214.4 bar. 1kgf/cm2 = 0.98 bar
Even a sealing gasket is OK if water would not be empty from the speargun over pressure would happen, for a very short time but enough to push some water behind the piston into the barrel.
I case of Mares shock absorber during the first 1 mm of compression of shock absorber actually only the hydro damper is working.
Regular mechanic dumper can absorb only 0.59 J because for 1 mm compression the max force is only 70 kgf. Peak force for the water is 199 kgf and after 1 mm 159 kgf.
You again forgot that the pressure created disconnects the harpoon from the piston even before the piston hits the shock absorber and the increase in volume reduces the pressure between the piston and the shock absorber! The area of the piston shank is much larger than the area of the annulus of the O-ring!
No, I did not forget that. That is why I calculate on 1 mm braking distance from hitting the shock absorber. The shaft separates from the piston in that interval.
Both 1mm and the separation time from the piston are important! Lots of gaps in your formulas! Go on! Maybe you will find the right way!
The exact braking distance nor the separation time is not important! What is important is the initial speed of the water leaving the muzzle,
in this case 207 m/s. The peak pressure in front of the piston mainly depends only on that speed and the density of water.
I made a measurement, how deep the shaft tail end (the cone) goes into the piston in my Mares Cyrano.
After inserting the shaft into the piston, at the end of loading the speargun the shaft goes into the piston additional 0.8 mm deeper.
That means that the water can not replace the shaft cone in a piston before the shaft leaves the piston for at least 0.8 mm.
After that water can enter behind the shaft into the newly created empty space and the pressure of water in front of the piston will drop.
After inserting the shaft into the piston, at the end of loading the speargun the shaft goes into the piston additional 0.8 mm deeper.
That means that the water can not replace the shaft cone in a piston before the shaft leaves the piston for at least 0.8 mm.
After that water can enter behind the shaft into the newly created empty space and the pressure of water in front of the piston will drop.
It remains to calculate the force with which the closed water will push the harpoon out of the piston and the duration of this force! As the pressure increases, the spear will break away from the piston before the pressure reaches a critical value for the passage of water through the O-ring into the receiver! The force of disengaging the harpoon from the piston can be measured by scales!