Why did my PCP Airgun Lose Velocity At a Higher Altitude, even though all settings were the same?

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TheDaleness

TheDaleness

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Dec 15, 2022
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Santa Barbara, CA, United States
I had my Impact tuned at 990fps at sea level. I went to 3000 feet and it was shooting at 965. My first thought was, the air up here is thinner, so the air in the barrel is thinner, so the slug is moving through thinner air. So shouldn’t the projectile be moving faster? The air in my bottle isn’t any thinner. My reg is at 150 bar at sea level, and it’s at 150 bar here at 3000ft. I’m clearly missing something.

I’m sure this has been discussed elsewhere, but I searched and wasn’t able to find anything discussing the physics. Feel free to send links to any discussion or articles that break it down.


**EDIT:
ADDITIONAL INFO (should have included this in original post)

Gauge Used: Huma Air 28mm Digital Gauge https://www.huma-air.com/Digital-Mini-Pressure-Gauge-for-FX-Impact-28-mm

ATMOSPHERIC CONDITIONS WERE MEASURED WITH a device called SENSOR PUSH https://www.sensorpush.com and the iPhone’s built in barometric sensor and GPS for elevation.

Elevation change: +3000ft

Temp Change: -8 Degrees F (60F to 53F at 3000ft)

Humidity Change: 7% (54% at sea level, 47% at 3000ft)

BP change: -3.8 inHG (30.5 inHg at sea level, 28.2 inHg at 3000)

CHRONOGRAPH: **The same chronograph: FX Pocket Radar Chrono) was used at both elevations. https://utahairguns.com/fx-pocket-chronograph/

POI CHANGE: -.75” (far zero: 50y) Point of impact decreased by about .75” at 100y, which is what I calculated would happen with that decrease in FPS, so I don’t suspect the chronograph was reading “off” at either of the elevations.
 
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You first have to determine what your pressure gauge is really telling you.

  • Absolute pressure is referenced to perfect vacuum. It indicates how much higher your process pressure is than that of a process in vacuum, so measurements are always positive. Absolute pressure readings are commonly used in vacuum processes, such as vacuum deposition.
  • Gauge pressure is referenced to local atmospheric pressure. It indicates how much above or below local atmospheric pressure your process pressure is, so measurements can be either positive or negative.
 
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**EDIT:
ADDITIONAL INFO (should have included this in original post)

Gauge Used: Huma Air 28mm Digital Gauge https://www.huma-air.com/Digital-Mini-Pressure-Gauge-for-FX-Impact-28-mm

ATMOSPHERIC CONDITIONS WERE MEASURED WITH a device called SENSOR PUSH https://www.sensorpush.com and the iPhone’s built in barometric sensor and GPS for elevation.

Elevation change: +3000ft

Temp Change: -8 Degrees F (60F to 53F at 3000ft)

Humidity Change: 7% (54% at sea level, 47% at 3000ft)

BP change: -3.8 inHG (30.5 inHg at sea level, 28.2 inHg at 3000)

CHRONOGRAPH: **The same chronograph: FX Pocket Radar Chrono) was used at both elevations. https://utahairguns.com/fx-pocket-chronograph/

POI CHANGE: -.75” (far zero: 50y) Point of impact decreased by about .75” at 100y, which is what I calculated would happen with that decrease in FPS, so I don’t suspect the chronograph was reading “off” at either of the elevations.
 
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jmohme said:
You first have to determine what your pressure gauge is really telling you.

  • Absolute pressure is referenced to perfect vacuum. It indicates how much higher your process pressure is than that of a process in vacuum, so measurements are always positive. Absolute pressure readings are commonly used in vacuum processes, such as vacuum deposition.
  • Gauge pressure is referenced to local atmospheric pressure. It indicates how much above or below local atmospheric pressure your process pressure is, so measurements can be either positive or negative.
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Ah I did not know this. Thanks for that.
 
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sb327 said:
What was the type of chronograph used? Was it the same chrono?

Maybe that has something to do with it.

Dave
Click to expand...
CHRONOGRAPH: **The same chronograph: FX Pocket Radar Chrono) was used at both elevations. https://utahairguns.com/fx-pocket-chronograph/
POI CHANGE: about -.75” (far zero: 50y) Point of impact decreased by about .75” at 100y, which is what I calculated would happen with that decrease in FPS, so I don’t suspect the chronograph was reading “off” at either of the elevations.
 
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darylm said:
I’m sure a temperate and humidity change had something to do with it as well, not just altitude.
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Agreed, I'm curious what percentage each of those factors, and the others mentioned, are responsible for the velocity change. Very complicated. Would require lots of math. As a result of this experience I got the Strelok Pro app. Hopefully this will be sufficient and I wont have to retune when changing hunting locations that have a significant change in elevation and/or atmospheric conditions.
 
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I regularly shift from 5000-10000 feet and see no change in fps. Plus, POI is higher. Perhaps it was simply that transportation moved something in the gun, like the hammer spring tensioner?

The effects I notice are all positive. BC is higher and slugs shoot better because less twist is needed to stabilize them. At 10k ft, a regular FX liner shoots slugs as well as a slug liner does at sea level. The only potential downside is that the speed of sound is lower so speeds must be kept under 1040 to avoid the crack. That’s fine by me, given the positives.
 
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weevil said:
I regularly shift from 5000-10000 feet and see no change in fps. Plus, POI is higher. Perhaps it was simply that transportation moved something in the gun, like the hammer spring tensioner?

The effects I notice are all positive. BC is higher and slugs shoot better because less twist is needed to stabilize them. At 10k ft, a regular FX liner shoots slugs as well as a slug liner does at sea level. The only potential downside is that the speed of sound is lower so speeds must be kept under 1040 to avoid the crack. That’s fine by me, given the positives.
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Yes that is exactly what I expected was going to happen when I went to the higher elevation. You could be right about something shifting in the gun. I'll keep experimenting.
 
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jmohme said:
You first have to determine what your pressure gauge is really telling you.

  • Absolute pressure is referenced to perfect vacuum. It indicates how much higher your process pressure is than that of a process in vacuum, so measurements are always positive. Absolute pressure readings are commonly used in vacuum processes, such as vacuum deposition.
  • Gauge pressure is referenced to local atmospheric pressure. It indicates how much above or below local atmospheric pressure your process pressure is, so measurements can be either positive or negative.
Click to expand...
Not questioning the accuracy of your statement because I don't know, but a question regarding point 2. Wouldn't the gauge pressure be referenced against sea level? The gauge is a closed system between the air in the system, the hole in the gauge and the little tube pushing the air against whatever resists it (a spring, some other mechanism) to move the dial.

How would you expect a digital gauge with a pressure sensor to operate versus a analog gauge with a dial?
 
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weevil said:
It’s also worth noting that the difference in absolute outside air pressure between sea level and 3000ft is negligible compared to the 100+bar in a plenum. So I don’t see how a reg or gauge could produce any such effect.
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Agreed, the change in barometric pressure was 3.8 inHg, which is only 0.13 Bar.
 
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Gauge pressure is pressure above sea level pressure. absolute pressure is the pressure referenced to zero pressure which is a vacuum. So you’re tire pressure gauge reads zero when it isn’t attached to tire and say 30 psi when it is. If you have absolute gauge it would read about 15 psI when not attached to tire and about 45 psi when it is on same tire. Pressure is often annotated as psia when measuring absolute pressure
 
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