Effect of altitude and temperature on pellet speed

[dan_house]

Long story short, been trying to finalize my TX200 setup for the WFTF Worlds in few weeks

Ran 50 shots of 8.64 pellets over the chrony giving 780 fps avg @55 degrees F, yielding 11.68 ft lbs. Shot that string at 4500 ft ASL

Now I'm a bit panicked.

Phoenix is at 1086 ft ASL, Im at 4500 ft. The usual number given for Springers at different altitudes is 1.5% change for every 1000 ft gained or lost. So in my case that a 5.2% increase in efficency going to the lower altitude. Strelok Pro also says I can expect 3.7% change in every 15 degrees of temp--but it doesn say if thats an increase or decrease. Checked for the average Novemember temp in Phoenix. 75ish degrees. Rounding off for easier math., thats 7.4% change based on 55 to 80 degrees change.

One thing I cant calculate is the effect of air drag in the thicker air at the lower altitude.

And really not wanting to switch to a heavier pellet this close to the match......

If it were any other match, Id just beg out and not go. But this is most likely the only Worlds match I'll ever attend, and Ive paid for everything except gas already. Dont want to get there and be over the 12 ftlbs before even starting to sight in......

My questions are.... Am I over thinking it? meaning, the math will work out and i'll still be legal. If not, then what are some options to keep it legal at the speeds and altitude I think the gun will be performing in?



much thanx in advance



dan

 

[Gear_Junkie]

If you have an opportunity to drive a few hours to shoot the gun at the correct altitude, I would HIGHLY recommend it, especially given the venue. This will give you a chance to check the pellet energy and holdovers.

I shoot at 2500feet ASL. My FT matches are shot a few hundred feet ASL. When I was shooting my HW50, HW97, and TX200 (all between 10 - 12fpe), the pellet energy at the muzzle was about the same but holdovers were noticeably different. In fact, it was so different that it was one of the main reasons I stopped shooting springers and switched to PCP for FT.

 

[JP806TX]

Let me propose another alternative.

Start breaking in a back up spring. Set it up to shoot at around 11-10.5 FPE where you are at and bring washers.

Get to Mesa early, that way when you test the primary spring and if it's over the limit, you got options.

If you can't get there early or don't have a backup spring...go with posting #2.

Good luck Dan. I'll see you at Mesa.

 

[BrazosbyNemo]

You have some reason to be concerned. I had a house in CO at 8600’. At home in TX I am 700’. Yes that is an extreme spread, more than yours. That said my rifle was WAY off going from TX to CO (Spring rifles). FT you can’t be off by much. Get there early and spend some time getting it sorted and more importantly your confidence the you rifle is sorted.

 

[thomasair]

For pcp purposes, and many other things ...altitude alone is not super useful. Density altitude is much more refined.

The density altitude in mesa will probably be around 4500-5500ft....despite the actual altitude being much lower.

What is your density altitude?

At your temperature...I'm guessing it's pretty close to your actual altitude.

You may not have to make any adjustments.

Mike

 

[weevil]

As a biathlon competitor, I’ve spent a lot of time considering these effects.

You should not consider the temperature and altitude in an additive fashion because they are somewhat confounding. Density altitude takes into account both the elevation and temperature and therefore can be used to compute and compare actual atmospheric pressures at different locations. The DA in Mesa at 75F will be very close to 2,800 feet. At 55F in Bozeman you were shooting at a DA of 5000 feet. In terms of the effects on the projectile, the drop and wind drift will increase with a reduction in DA. It’s akin to using a lower BC projectile. Those effects can be evaluated by changing DA in Chairgun (I’m retired from competion but I tend to use that program to understand effects on my airguns). With a PCP, there is no effect on power at the muzzle, because bottle or plenum pressure is the same, regardless of DA. However, springer muzzle power output will be higher at a lower DA. The actual air pressure at a 2,800ft DA will be 1.2psi higher than at 5,000ft DA, which is ~9% higher. I don’t know if springer power output is directly proportional to input air pressure, but if so, the advice to set up at 10.5 to 11fpe comports with a 9% difference.

To answer your side question, increase in elevation and temperature both increase DA, such that at 4,500ft in my location (Utah), the density altitude can be 9000ft on a 100F summer day. Contrastingly, in the depths of winter, the DA can drop to 2,800ft. Humidity also has an effect but it is very minor compared to elevation and temp and not worthy of consideration in the western US where humidity is invariably low.

Best of luck!

 

[Scotchmo]

The reading at the Rio Salado range a couple of days ago:
Temperature: 97F
Humidity: 15%
Air pressure: 27.76 inHg

I think the elevation is about 2000ft.

 

[weevil]

Must be 2100ft. So around 1psi (~8% higher) if it’s 75F at the time of the event…

 

[Chas DiCapua]

If you can't get there early or don't have a backup spring...go with posting #2.

Ha, I love it.! Thanks for the smile.

This is slightly off topic, but not totally. What about PCP's and lower temps? At a match this weekend, My rifle was shooting about 3/4 - 1 inch low. The temp was in the low 40's (as compared to 70+). Would that make a difference? Any thoughts would be appreciated.

Chas

 

[thomasair]

The density altitude at Rio Salado last Tuesday was 5200'.

Mike

 

[gonzav71]

If you do not have a spare spring and you are over the limit, you can always use a torch and partially collapse a coil to reduce the power. Or use a Dremel tool to grind off a small section of coil.
At 11.7 ft-lbs. & 780 fps, I think you should be fine.