Chilling down the mag

[Aod]

as a side note:

 

Your GBB runs off of pressurised gas, not the liquid. it is better for efficiency to NOT completely fill the mag, and to leave room in it for expansion. 4-5 seconds is all it needs, not till it sprays out BC the mag is 'Full'

Sorry, That is positively untrue. Most GBB pistols will not finish a whole mag if only filled for 4/5 seconds. While having an expansion chamber is beneficial for efficiency, the trade-off for less overall liquid propellant in the magazine is a poor one.

[Stuey]

Egad. This is what I was hoping to get from my gas system thread while back, but I only got a bunch of people telling me basic stuff I already knew.

 

No, the fact that you noted ("filling a mag has the effect of making it colder") is completely irrelevant.

 
What you are trying to establish is a pressure differential; lower pressure in the magazine than in the bottle, encouraging flow from the higher-pressure area (the bottle) to the lower-pressure area (the magazine). The reason that filling a magazine makes it colder is irrelevant is that what is making the magazine colder is adiabatic cooling, which starts in the bottle, not the magazine, and affects all the fluid in the bottle. The part that is getting cold is the fluid in the bottle (where the pressure is dropping), not the fluid in the magazine (where the pressure is increasing). The cold you're feeling in the magazine is simply due to fluid, cooled by adiabatic cooling in the bottle, draining into the magazine. The bottle is just as 'cold' as the magazine, though with a larger volume to 'absorb the cold', you won't notice it as much. Therefore, even though the magazine may be cold when you're done filling it, that doesn't mean that it is now at a lower pressure than the bottle; there is no pressure differential.

 

This bit makes sense, I hadn't realised the cooling was happening in the bottle...it did strike me as odd that putting gas into something would cool it but I took it as read.

 

It does keep going until the magazine is full (or very nearly full) of liquid. If you'd like I will film me filling a P226 magazine up, and you can watch it spitting fluid at me as it begins to overflow.

 

 

You're absolutely correct. As soon as there is liquid in the magazine, the pressure inside is the vapour pressure for the temperature of the fluid. That is an equilibrium of its own. As gravity pulls more of the denser liquid from the bottle into the magazine, more of the space inside the (fixed) internal volume of the magazine is occupied by liquid, which reduces the space available for the gas (compressing it). As the gas is compressed, it passes the vapour pressure and condenses, reducing the pressure of the gas zone and again reaching equilibrium. If you fill it indefinitely (and in the absence of designed-in safety measures), the magazine will eventually completely fill with liquid.

 

I've been filling my mags until they spray liquid out the valve, as do most airsofters I think - does that mean they're now full of liquid? I've been labouring under the impression that the liquid transfer stops when the mag is some fraction full of liquid, and it's the mechanism behind this I've been trying to figure out.

 

The temperature of the system isn't low enough to decrease the pressure of the gas pocket in the mag to shrink enough that all space will be filled with liquid. Get everything cold enough and you should be able to transfer fluids like it was tap water in a canteen.

 

I had that thought myself, if you could cool it below -42 or whatever, you could just pour the propane in. I don't think you have to lower the temperature to shrink the pocket though; as soon as the pressure exceeds the vapour pressure the gas will start to condense to lower the pressure.

[Im going space]

Sorry, That is positively untrue. Most GBB pistols will not finish a whole mag if only filled for 4/5 seconds. While having an expansion chamber is beneficial for efficiency, the trade-off for less overall liquid propellant in the magazine is a poor one.

Quite the opposite. Time to find the GHK/Samoon video.

 

We also tested across a range of pistols, limited filling led to more shots by quite a margin EVERY time.

 

Those first few shots where you get the liquid spray? Bad. This leads to much faster cooldown. Test it yourself.

 

Edit: samoon.com.tw/gasification-4-seconds

[Pepperisit12]

Also a lot of pistol magazine fill valves are quite long. So when they start spilling liquid out the top there is still a small area for gas to expand from. Obviously when you turn the mag over this bubble of gas moves to the top.

 

I am assuming that you cannot fire gbb pistols upside down as only liquid propane would come out?

[PureSilver]

I've been filling my mags until they spray liquid out the valve, as do most airsofters I think - does that mean they're now full of liquid? I've been labouring under the impression that the liquid transfer stops when the mag is some fraction full of liquid, and it's the mechanism behind this I've been trying to figure out.

 

Liquid spraying out the top means that magazine is more or less full of liquid, barring maybe a few bubbles of gas trapped at the 'top' (the bottom) of the inverted magazine outside the gas inlet tube. The liquid is heavier than the gas; if it's coming out the 'top', it's only because all the gas has been displaced or condensed first. It's like filling a bottle from a tap until the water starts to overflow the mouth of the bottle, essentially.

 

I don't think you have to lower the temperature to shrink the pocket though; as soon as the pressure exceeds the vapour pressure the gas will start to condense to lower the pressure.

 

That's my understanding. However, the only pressure you can exert that exceeds the equilibrium pressure is that of gravity - the pressure of the weight of the liquid pressing down. That means that the pressure in the magazine is always fractionally - very fractionally - higher than that of the pressure in the bottle, owing simply to the weight of the liquid in the bottle pressing down upon it, the same way that the pressure is higher in the bottom of a bottle of water than it is at the top. It follows that everything below the level of the liquid/gas interface is under pressure to be liquid, not gas, and therefore (as you say) any gas, even that trapped in pockets inside the magazine, should condense eventually.

[Aod]

Quite the opposite. Time to find the GHK/Samoon video.

 

We also tested across a range of pistols, limited filling led to more shots by quite a margin EVERY time.

 

Those first few shots where you get the liquid spray? Bad. This leads to much faster cooldown. Test it yourself.

 

Edit: samoon.com.tw/gasification-4-seconds

My personal testing with real-world style usage has shown the opposite of what you say.

[Im going space]

We went through most of WEs range of pistols, plus all the TMs we had on site at the time. (1911, sig, hk45, desert eagle, 5-7) all showed an improvement on the near full mag

[Stuey]

Interesting, perhaps worth a try on my GBBs. How did you determine nearly full for the different guns? Or did you just use a 4-5 second charge on all? From memory 4-5 seconds is easily enough to completely fill a 1911 mag.

 

Liquid spraying out the top means that magazine is more or less full of liquid, barring maybe a few bubbles of gas trapped at the 'top' (the bottom) of the inverted magazine outside the gas inlet tube. The liquid is heavier than the gas; if it's coming out the 'top', it's only because all the gas has been displaced or condensed first. It's like filling a bottle from a tap until the water starts to overflow the mouth of the bottle, essentially.

 

It's just occurred to me - those tubes on the inside of the fill valves, although they won't stop the filling process when the liquid level reaches the bottom the tube, they will cause liquid to spill out the valve at that point, prompting users to stop filling. Reckon that's the idea? Then the designers could set the length of the tube according to what's a good amount of liquid for that mag, and tell people to stop filling when the liquid comes out (pretty sure that's what TM say in their manuals).

[renegadecow]

At room temp filling of liquid stops just about when it reaches the end of the fill nozzle because of said gas pocket. Gravity on the liquid alone cannot surpass the pressure of the pocket.

[blobface]

Ow no, what have I started....... 

You've started an interesting back and fourth between people who have experience in their stuff and further enforces the reason why I like this forum. 

[xrayhead]

You've started an interesting back and fourth between people who have experience in their stuff and further enforces the reason why I like this forum. 

 

I will admit, not been here long but yes very enjoyable forum! 

[Im going space]

Interesting, perhaps worth a try on my GBBs. How did you determine nearly full for the different guns? Or did you just use a 4-5 second charge on all? From memory 4-5 seconds is easily enough to completely fill a 1911 mag.

 

Filling up one mag and timing it off that, ive been doing the hk45 ones for 6.