Question for Bob

[Privateer]

Would warm air put into a cold gun cause a difference in the power?
The Gun in question traveled for several days in below freezing temps. The tank was filled with an AV compressor and was warm to the touch.
Allowing the whole thing to sit over night I see a considerable difference in the power I saw last night when it was cold with warm air fill.

[rsterne]

The pressure will be lower after the air cools off.... Whether that causes an increase or decrease in the velocity of an unregulated PCP will depend on how the gun is tuned.... If you normally fill to 3000and refill at 2200, and the peak of the bell curve occurs at 2600 psi.... and if the gun was filled to 3000 warm, and after cooling off was 2500-2700 psi.... the velocity should actually be higher....

In a regulated PCP, the temperature from the cooling effect of the air should make no difference.... However, in both regulated and unregulated PCPs, filling at a high temperature and then shooting at a lower temperature WILL cost you on shot count.... It takes a large difference in temperature to affect the pressure enough to notice.... a 50*F change only changes the pressure about 10%....

If you are noticing a loss in velocity compared to the same pressure reading on the gauge when the gun is cold, I suspect you have some lube on the hammer that is getting thicker when cold and slowing the hammer down.... or some other hammer related anomaly.... Less hammer strike will usually result in lower velocity, of course....

Bob

[Privateer]

That's what I expected from reading your wisdom in the past.

As the gun was cold from shipping I'm suspecting maybe the area that holds air was larger in volume due to being cold.
Thus allowing a greater volume of usable air for the initial shots power.
Warming to room temp over night the parts again expand and the following shots allow less volume thus a weaker shot.

So to start, a very cold gun and warm air. Next day everything is at room temp.

Does that make sense?

[rsterne]

Sorry, but no it doesn't.... The metal parts of the gun would not change in volume to any measurable degree with temperature.... The warm air you filled the gun with would help to warm the cold metal, and temperature equilibrium would come quite quickly....

If you shot the gun immediately after filling, the air, and hence the pressure would be above ambient.... The next day, it would be at ambient.... Therefore the pressure the next day should be less.... What effect that has on the velocity would depend on where you were on the bell curve....

Bob

[Privateer]

Thanks Bob.
I'm trying to figure out why the Gun would hit a target at 70+ yards with 2 shots then the next day not even reach that target using the same aim point and scope settings.

Maybe the gun being cold allowed the rail to warp? I don't know.

[rsterne]

Sorry, no idea for that one.... but it should not be related to effect of cooling on the air pressure, that is too drastic a change....

Bob

[nervoustrigger]

Jeff, there are a couple of scenarios that come to mind.

The first is, was it possible that the hammer assembly was colder on the second day?  If so, the lubricant is a likely culprit.  I get the sense it’s more likely the opposite was true but figured it’s with a mention.

The other scenario I have experienced is a bit more esoteric.    Had the gun been totally depressurized for a while prior to filling it with the compressor?  If so...and if the poppet has a soft seal (e.g. the polyurethane seal embedded into a QB poppet), it will develop a compression set which then requires more hammer strike to knock it open.   Meaning on day 2 you may need to re-adjust the hammer strike.  In my experience, it seems to stabilize within 48 hours or so.

[rsterne]

Jason, that could be a valid mechanism if the poppet seal is soft, like on a QB.... The hammer energy required to crack a valve open depends on the pressure on it, the area of it, and the distance the seal is collapsed and must decompress before the valve opens.... If the gun was empty for a long time the seal could initially not compress too much, and the valve be easier to open.... but after sitting it took a deeper "set".... Whether that would increase the distance the material had to flex before cracking off the seat, or in fact reduced that distance by taking a semi-permanent set.... would determine if the hammer strike would increase or decease....

I think we both agree that this is likely a hammer issue, and not a pressure issue....

Bob

[nervoustrigger]

Bob, thanks, I follow you on that.  And just to complete the thought, the compression set also moves the stem nominally closer to the hammer, effectively reducing the stroke.  That contributes to some weakening of the hammer strike when we are instead needing a bit more to compensate for poppet being harder to knock open.   Granted I don’t know how much of a contribution it represents but it’s something else in the mix.

[Privateer]

This is the seat. Some sort of "hard plastic" is all I've found out about it.
There may be a o-ring behind it.


This is the exhaust valve. The tapered area sits in the "hard plastic".

[AustinBinTX]

Jeff,

There is an o-ring behind the lip of the seat if I remember correctly.

[AustinBinTX]

If I remember correctly that rifle had  thick shims (watch battery IIRC) on the back side of the hammer spring. I believe I removed them when I worked on it, but if not, one of them may have came out and fallen into the tube.

I had them come out once while I was shooting a string so I removed them first time I pulled it apart. When they came out the rifle still worked fine, it just lost quite a bit of velocity. I don't think I put them, or one of them back in, but that's what it's sounding like.

[Privateer]

Thanks Austin.
 Think I'll take it down and look at things then.
I'd rather go through finding a heavier hammer spring then some kind of whacked spacer deal.
That would also give me the chance to check that "hard plastic" part.
I could replace it with PEEK if it's just Delrin or worse.
I just happen to have a lathe and a good chunk of PEEK.

[AustinBinTX]

The spacers were definitely not OE. The first time I pulled the rifle apart I saw them and though someone lost a battery in there.

I'll be extremely embarrassed if I did put them back in. I would normally say I'm 100% sure, but what you're describing sounds eerily similar to what happened when I found them.

[Privateer]

No problem my Friend!
 I'd be kind of disappointed if I DIDN'T have to go inside.
 
Finding the springs should be a good experience to pass on to others anyway.
Knowing that batteries were used? That is crazy!
 

[AustinBinTX]

The good news is, you should have an answer as soon as you pull the rail. The hammer spring is a bit of a pain to get out sometimes, but once you do it once or twice, it's not terrible.

[Privateer]

Thanks to Bob, Jason, and Austin!

I'll continue in the review thread I started to not clutter this Gate.