Gas rams & temp

[buttloaves]

I was reading an article in Airgun World concerning gas rams and temperature, Jim did experiments with differing temps to see how it effected power output, i was wondering if a gas ram is cocked and fired quickly would it produce more power than if cocked and left for a few minuets to settle down as the gas in the ram would heat up as it is compressed during cocking increasing pressure..
Wonder if this has some bearing on the fact that after cooling the gun before firing Jim saw a rise in power, perhaps the heating up of the gas as it is compressed was negating the cooling?
I think i once saw Adam Hart Davies describing a hot liqud freezing quicker than a cold one or something like that, you would think the opposite, wonder if a cooled gas will heat up more on compression than an already warm gas?
Im doing a lot of thinking tonight..

[Loki_79]

The hot water freezing experiment is not the same thing, as this is due to evaporation resulting in less water there to freeze by the time it does.

I would expect the gas ram to heat by a similar amount, but maybe the cooling of the action gives a much larger heat sink/temperature differential for the ram to cool again after heating? The pressure in the ram will certainly increase with temperature, but friction of the seals and relative expansion/contraction of metal parts will all play a role.

[severnsider]

I posted my findings on this a year or so ago. I have three Theoben 'rammers', all of which will push a pellet out at a greater velocity when the action has been cocked quickly than when the action has been cocked slowly. The difference is noticeable........ so noticeable that I gave some thought to charging the rams with nitrogen rather than air. Never did try it though, and I have no idea why this effect occurs. The volume of gas is, after all, rather small.
Blackbeard on here may recall the exchange of correspondence.

[BTDT]

The pressure in the ram will certainly increase with temperature, but friction of the seals and relative expansion/contraction of metal parts will all play a role.

It more than plays a role, Jamie, it dwarfs any temperature effect on the ram gas pressure.

I posted my findings on this a year or so ago. I have three Theoben 'rammers', all of which will push a pellet out at a greater velocity when the action has been cocked quickly than when the action has been cocked slowly. The difference is noticeable........ so noticeable that I gave some thought to charging the rams with nitrogen rather than air. Never did try it though, and I have no idea why this effect occurs. The volume of gas is, after all, rather small.
Blackbeard on here may recall the exchange of correspondence.

I checked the thread, Mike, and there's no way the velocity increases you found were caused by temperature. I think the inertia weight was possibly starting from a different position, depending on how you cocked the rifle.

[severnsider]

I checked the thread, Mike, and there's no way the velocity increases you found were caused by temperature. I think the inertia weight was possibly starting from a different position, depending on how you cocked the rifle.

Thanks Jim. I have no idea what caused the variations; perhaps it is something to do with the mysteries of the inertia weight! This has rekindled my interest in trying a nitrogen fill... or do you think that would be pointless?

[BTDT]

Thanks Jim. I have no idea what caused the variations; perhaps it is something to do with the mysteries of the inertia weight! This has rekindled my interest in trying a nitrogen fill... or do you think that would be pointless?

Gut feeling says yes, especially as you've got 76% nitrogen in there already, Mike.

Having said which, I did a test yonks ago on a nitro piston gun, and the velocities were practically identical with it acclimatised to 16C and 34C, so you never know unless you try.

OK, scratch that. I've just tested quick Vs. slow cocking with the nitrogen filled ram rifle and the difference is 70 fps

That's 11.4 ft. lb. fast, 9 ft. lb. slow.

[tryshooting]

nitrogen should not expand or contract with temp changes, thats why the fromula one teams put it in the tyres on the race cars. so if you pulled a vacum on you air tube and then charged with ofn, (oxigen free nitrogen) then if what the boffins at race hq say is right you gun should not change with temp unless its the lube or seals that are afected buy the heat change.

atb tony.

[buttloaves]

Gut feeling says yes, especially as you've got 76% nitrogen in there already, Mike.

Having said which, I did a test yonks ago on a nitro piston gun, and the velocities were practically identical with it acclimatised to 16C and 34C, so you never know unless you try.

OK, scratch that. I've just tested quick Vs. slow cocking with the nitrogen filled ram rifle and the difference is 70 fps

That's 11.4 ft. lb. fast, 9 ft. lb. slow.

Thought it might

[severnsider]

Gut feeling says yes, especially as you've got 76% nitrogen in there already, Mike.

Having said which, I did a test yonks ago on a nitro piston gun, and the velocities were practically identical with it acclimatised to 16C and 34C, so you never know unless you try.

OK, scratch that. I've just tested quick Vs. slow cocking with the nitrogen filled ram rifle and the difference is 70 fps

That's 11.4 ft. lb. fast, 9 ft. lb. slow.

So it's just down to the speed of cocking then. Very similar to what I found. It was better to adopt the "slow cock" for more consistent results. All a bit of a mystery (to me at least!); thanks for investigating, Jim.