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

[TheDaleness]

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.

 

[delooper]

Regulators have duct or line to a reference pressure. So they regulate say 100 psi above reference pressure. If referencing atmospheric pressure you will have less than 100 psi reservoir pressure at higher altitude

I haven't seen a regulator that has a mechanism to a reference pressure. Is there a specific regulator you are thinking about?

 

[Franklink]

I have a unique situation of living at 5600, and shooting field target matches as low as 1250feet, and as high as 8600feet.

For the 8600 and 7200feet matches....
Generally speaking, regulated or not, a pcp will shoot to about the same impact points from 10 to about 40 yards. At distances past roughly 40-45 yards, the trajectory curve is just a bit flatter.

For the 1250feet matches.....
Same impact points out to about 40-45 yards. At further distances, trajectory curve is a bit loopier.

It's not drastic, but does exist.

 

[sb327]

Airgun regulators may have a vent to atmosphere but this is to prevent a buildup of pressure behind/between the piston. It is also a place for pressure to go in the event of an oring failure.

The main reference is a spring (spring stack). The input pressure effect on the system is what changes things the most, that’s why input orifices are quite small (generally) to have the least amount of swing during the inputs declining pressure. Some designs use a balanced input to help negate this effect and still have higher flow rate.

Point being, airgun regs I am familiar with do not reference atmospheric pressure.

Dave

 

[nervoustrig]

Point being, airgun regs I am familiar with do not reference atmospheric pressure.

Well it may be debatable whether a vented chamber produces an atmospheric reference in the strictest sense, but the absence of it is a sealed chamber that will gradually become pressurized by permeation of gas through the O-rings. When that happens, the normal counteracting forces between the two ends of the piston become upset and the regulator will not function properly. The setpoint will shift. A vent ensures this condition does not arise.