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Air Rifle Pressures

  • 02-12-2022 11:41am
    Registered Users Posts: 1,987 ✭✭✭

    If an air rifle is sitting for a long time without being used will the pressure change on the pellets side of the regulator providing there is no leaks. Due to temperature change is what I am thinking of.

    And having asked that do they leak anyhow because of sitting idle for long periods.


  • Registered Users Posts: 34 Ferns83

    Hi Minktrapper,

    You can calculate pressure change due to temperature using the ideal gas law.

    (Pressure 1 X Volume 1) divided by Temperature 1 is equal to (Pressure 2 X Volume 2) divided by Temperature 2.

    (P1xV1)/T1 = (P2xV2)/T2

    You can remove Volume as the volume of the tank does not change. This will give:

    P1/T1 = P2/T2

    rearrange this to get:

    (P1/T1) x T2 = P2

    For example if you had a 200 bar tank at 20 degrees C which increases in temperature to 30 degrees C (converted to SI units gives 20000000 Pascals and 293 degrees Kelvin and 303 degrees Kelvin):

    (20000000/293) x 303 = P2

    20682594 = P2 in Pascals

    Converting to Bar will give 206.8

    So a 10 degree C change has increased the pressure by 6.8 bar in this example.

    Hope this helps,

  • Registered Users Posts: 5,099 ✭✭✭Mech1

    I thought so.

  • Registered Users Posts: 1,987 ✭✭✭minktrapper

    And would a 6.8 bar change in air pressure change poi at 50 yds.

  • Registered Users Posts: 34 Ferns83

    I don't know to be honest.

    I'll give you my best guess though.

    The 200 bar I used above is the max pressure my air arms s510 can be filled to, however I believe that it does not deliver 200 bar. If it did, it would lose power after 1st shot. That is slightly irrelevant to your question though because no matter what pressure it delivers there would be expansion due to temp change.

    If it did deliver 200 bar then 6.8 bar is an increase of 3.4%. I doubt you would notice a different POI at 50 yards if you are talking about powerful PCP. Maybe a 3.4% would make a difference with 12ftlbs break barrel etc...?

    Another thing to consider is if the gun has a regulator fitted. I believe that the reg would stop the 6.8 bar increase.

    Only a guess though. Might make for an interesting summer evening of testing.

  • Registered Users Posts: 1,987 ✭✭✭minktrapper

    Why would the regulator stop the increase in pressure. The regulator regulates the amount of air going in to the chamber. I don't think it can release air. One could of course put the rifle in to a freezer to drop the temperature.

    On the subject of leaking. I presume if the chamber was leaking then the regulator would allow air in to the chamber. But then again maybe a sudden drop of pressure might be needed to open the valve.

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  • Registered Users Posts: 34 Ferns83

    Sorry, yes you are right. I didn't pay enough attention to your 1st question. I was thinking along the lines of continuous shooting.

    If the air was past the reg then it wouldn't do anything.

    I still dont think you would notice a change in POI at 50 yards for such a small increase in pressure. Maybe out at 100?

    I have an Crown mk2 coming in the near future and if I get a chance over the summer i will adjust the pressure by small percentages (will record speeds over chrony to back up adjustments) and see if there is any change in POI.

    Happy shooting,

  • Registered Users Posts: 34 Ferns83

    Ok, so I've gone down a bit of a rabbit hole here....

    I did a bit more thinking about my last post and realised I was wrong again. By measuring it with a chrony it is not the same as a direct pressure increase or decrease. Rather it would be a speed decrease as the pressure is not directly linear to speed (although there is obviously a big correlation).

    It would not be possible for me to measure the different drops based on pressure change but I can do it for speed.

    I had to get off the phone and fire up the laptop. Below I have used Chairgun pro to see the differences.

    The 1st screenshot is shooting at 922.9 feet per second using 18 grain pellets. This is what I have actually measured my gun doing in the past. To allow me to compare it, I have set a zero point of 100 yards. Then looking at 50 yards the pellet is 7.32 inches high. This is shown in the 2nd screenshot in the 1st column.

    The 3rd screenshot is a SPEED reduction (not pressure as I cant do this) of 3.5% and it is shooting an 18 grain pellet at a speed of 890.6 feet per second. As you will see the POI is now 7.78 high.

    This may seem counterintuitive as we may expect a drop but it is a drop in reverse so it hits 100 yard zero.

    So, going back to your original question.... I still dont know, interesting topic though!

  • Registered Users Posts: 539 ✭✭✭tikkamark

    My s510 xs can be filled to 250bar and I’ve often done so and left the rifle in the safe for weeks if I hadn’t time to go out with it and came back to it and it would still be sitting spot on at 250bar same goes if I had a good nights shooting and came home and put it away with 150 or 100 bar and it would still be spot on a few weeks later.

    if you’re seals are good there won’t be an issue…..a regulator only regulates the pressure from your tank to shot chamber to be completely consistent regardless of tank pressure,it won’t affect poi if you’re rifle is stored full.

  • Registered Users Posts: 39,322 ✭✭✭✭Mellor

    The regulator ensures the air is delivered at a constant pressure. That’s the point of the regular. If amount of air let into the chamber is always the same, the pressure will be the same. As the volume of the chamber isn’t changing.

    An increase in pressure due to temperature change would also be controlled by a regulator.

    In a unregulated gun, changes in temperature would cause changes in pressure. But the impact of that would be far less than the change in pressure from letting off shots. The example above was an change of 3.4%. First to last shot might be a change of 50%. Wa more significant.

  • Registered Users Posts: 1,987 ✭✭✭minktrapper

    Can the regulator release air if too much pressure builds. Most of the diagrams I have seen of regulators let in air to a certain pressure and seal the air inlet under its own pressure. I am no expert by any means. I would think that every regulator works differently.

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  • Registered Users Posts: 39,322 ✭✭✭✭Mellor

    A regulator let’s in air and is closed when the plenum chamber reaches a set pressure. If the tank pressure changes, due to temp, filling etc. the regulator controls that.

    If you’re worried about pressure changing inside the plenum over time. That could only affect one shot, then the regulator is regulating current tank pressure. And the impact of the change on that one shot would be tiny, as the power should be probably optimised for the regulator - ie it should be designed to behave like an unregulated gun at optimal pressure.

  • Registered Users Posts: 73 ✭✭Bushwack

    The fault your enquiring about is more to do with regulator creep which is common enough on some regulators manufactured. This can be down to bad design or bad bevelled washers or bad seals,rough machining and so on. After all Huma ,Lane and the like would not exist if this was not a common enough problem. A good regulator should only let a set pressure enter the pre chamber space that is the setting the regulator is set at . You can tell if a regulator is acting up if you get vertical stringing in your groups or if your first shot shoots high on the group. If you think you regulator is acting up you should really get a regulator tester. I think it's a good investment if your shooting a regulated rifle.