Anticipated Velocity Loss for Springers Based On Altitude (!) Increase?

How much velocity does a springer lose as elevation above sea level increases?

This topic came up for me recently in regard to an email 'conversation' I was having with a tuning customer. 

Last week I completed the velocity testing on his R1 at my place here in Texas, right before I sent it back to him. My elevation is about 85'.  He lives where the elevation is about 700'.  IIRC, the effect of approx. 600' of difference should be pretty-much negligible, but I'd like to know specifics for present and future use. I know there's a simple rule of conversion.  I seem to recall that  humidity is also a factor to some extent, but not as much as is elevation.

I know I read about this more than once on the old Yellow Forum.  I really did try to search for it myself just now.  I tried using some 'qualifiers' which I thought would narrow my results to what I was looking for.  I entered,  "springer+elevation+loss+velocity" in the search fields.  I may have confused the proper usage of what I deemed to be appropriate qualifiers.  Maybe I'm thinking of Google or some other search engine.

I say that because searching the old YF gave me over 1700 pages to look through--not what I was hoping for.  What I did see for the first several posts didn't pertain to what I was trying to find.

Searching this forum gave me fewer results, but not what I'm looking for unless I missed it.

I forget what the effect is, but Steve or one of you other guys knows this off the top of your head, or at least how to find it.  Any help would be much appreciated!

Altitude is the magic search term.

https://www.tapatalk.com/groups/yellow/finally-springer-altitude-testing-completed-with-p-t156064.html#p1467575

Ed,

I don't post much anymore but because you tuned several of my guns and contribute unselfishly I wanted to try to help with your questions.

There isn't a linear progression for loss in power at elevation.  I live in Colorado and have two homes that I shoot at regularly.  One is at 5800 feet in elevation and the other is over 9800 feet in elevation.  I've shot over a hundred springers at both elevations.

Once upon a time Dmitry was a guru at tuning springers for these elevations.  He was also unselfish about posting his results on the old yellow forum and they were VERY interesting.  Here's one of Dmitry_Smushkov posts:

https://www.tapatalk.com/groups/yellow/finally-springer-altitude-testing-completed-with-p-t156064.html

jinx

Citizen K : "Altitude".  Now why didn't I think of that?

Kevin  Lentz : Do you realize that besides posting an answer. you told a pretty funny joke?  I'm not poking fun at anything you wrote.  Let me clarify:

"Ed,

I don't post much anymore but because you tuned several of my guns (and contribute unselfishly) I wanted to try to help with your questions".

Thanks regarding the unselfish contributions, but in regard to the tuning, since I'm sure you paid me for it, it seems to me that it's me who's in debt to you!  LOL  And, I hope at least one of what I tuned is still shoot-able.  (I still have the records here of what I did).

OK,  I took (an admittedly quick) look at the post from 2009.  The graphs helped a lot.  And, if I'm looking at it right, my memory serves me pretty correctly.  This difference of from 100' above sea level to 700' above sea level (a 600" net difference) shouldn't result in too much of a difference in velocity.

Sincere ttanks to both of you guys.

Posted by: Citizen K

Altitude is the magic search term.

https://www.tapatalk.com/groups/yellow/finally-springer-altitude-testing-completed-with-p-t156064.html#p1467575

...about all we had was the rough guess that springer power was roughly proportional to air density - which predicted that, since air density declines by roughly 3% per 1000', muzzle energy will as well.

Interestingly, I just ran a rough average over all the examples in your last plot.    Although of course the case-by-case variability is huge, I get:   2.5% / 1000'

Running with that average would predict an ME loss in this example of 600' x 2.5% / 1000 = 1.5% for an MV loss of 0.75%

I.e.: 6 or 7fps or so.

The 2.5% loss of ME is roughly accurate for smaller compression chamber rifles (25 mm's and under). And the longer the stroke, the closer the rifle will follow the equation.

For larger compression chambers (26 t 28 mm's) and shorter strokes (3" and less), I have seen numbers as small as 0.25% per 1,000 FASL over the first 1,000.

Dmitry did a good job of tuning guns so that they would shoot, as close as possible, at most elevations. That was what he was looking for, and he achieved it brilliantly.

But if you want to get most power at high altitude for a given cocking effort (efficiency), then you need to follow different paths.

HTH

HM

Posted by: Steve in NC

Posted by: Citizen K

Altitude is the magic search term.

https://www.tapatalk.com/groups/yellow/finally-springer-altitude-testing-completed-with-p-t156064.html#p1467575

...about all we had was the rough guess that springer power was roughly proportional to air density - which predicted that, since air density declines by roughly 3% per 1000', muzzle energy...

Running with that average would predict an ME loss in this example of 600' x 2.5% / 1000 = 1.5% for an MV loss of 0.75%

I.e.: 6 or 7fps or so.

Thanks for your input, Steve.  I was hoping you might chime in on this one.  I remembered that you were one of the posters who contributed to the topic on the Old Yellow.  The owner of the rifle in question should be happy with the results he gets.

Not sure if this will answer your question specifically, but I can tell you that the velocity DOES drop as elevation increases.

Living in Phoenix, I normally shoot at elevation of about 1200 feet above sea level.  In Camp Verde, the elevation is around 3000 fasl, and at Mormon Lake (just south of Flagstaff) the elevation is 7200 fasl.  That is a 6000 foot elevation variance.  Going the other direction, when I shot with Scott over at Morro Bay, the elevation there was almost sea level, or within one hundred feet.

I haven't figured percentages as Hector has, but I have given him my findings and velocities, seeing as he built the D54 that I am shooting.  Apparently, the numbers work and he was happy to get those figures.

What I HAVE done, was chronograph my gun at each elevation and rezero (25 yards) each time.  My D54 is set for 12 fpe at sea level.  It does less than that as the elevation goes up.  I have made up velocity charts for my gun for every 10 fps velocity difference starting at 820 fps down to 730 fps.  Once I get to where we are shooting, I chrony the gun, pull the chart that is closest to that velocity, rezero to 25 yards, and then I am pretty good to go.

I have noticed that temperature affects the velocity as well; sometimes during a match.  At Mormon Lake we started the match nice and dry with a comfortable temperature.  During the matches (this past July and August), we got major rain storms that rolled in on us that caused the temps to drop considerably; 20 to 30 degrees (from 90 degrees and dry, down to 60 degrees and quite wet, aka "soggy").  Although I was able to finish each match, the POI changed with no way to check it.

Each time I returned home to the Phoenix area, re-chrony'd and rezero'd the gun and it shot right to where it was supposed to.

Like I stated, it will change with elevation differences, but taking along a chronograph, rezeroing at the new location, and having scope dope for the new elevation/velocity should alleviate any POI changes you may notice.  This may not entirely answer your question, but it should help to hit your targets, if that is your concern.

Hope this helps.

Mark Kauffman

Posted by: Mark.in.AZ

Not sure if this will answer your question specifically, but I can tell you that the velocity DOES drop as elevation increases...

...Like I stated, it will change with elevation differences, but taking along a chronograph, rezeroing at the new location, and having scope dope for the new elevation/velocity should alleviate any POI changes you may notice.  This may not entirely answer your question, but it should help to hit your targets, if that is your concern.

Hope this helps.

Mark Kauffman

Thanks, Mark. 

In this case its not an accuracy or target issue.  Instead, the owner was hoping for velocities at a certain level as a result of the tune I did on his R1.  I was actually able to exceed bis expectations, and he's very-excited about that!  But, he wondered if the numbers might not be quite as good due to the difference in our altitudes.

It turns out that even if there's a 2-3 % difference, which there probably won't be if I read it right, he's still going to be delighted.  In fact, I'm excited for him, too.  I love a good ending to a suspense novel.

Thanks again for all the time and effort I know you put into your post.  I've said before that I love the free education I get around here when it comes to things that are out of my field of expertise, and this is just another example.