Density Altitudes

L

Louis.J

New member
Jerry Sharrett mentioned Density Altitudes in the Super Shoot thread and I did not want to distract from it and posting my question over there. How does Density Altitudes affect one's tune and in what direction does on with adjusting his or hers powder charge to stay in tune. Also which one takes precedence Temperature changes or Density Alitiudes or do thy go hand in hand.

J.Louis
 
Density altitude (DA) is a calculation typically used in aviation to combine the effects of temperature, elevation, barometric pressure, and humidity all into one number to help evaluate how an airplane will perform in a given weather scenario. The general rule is that most airplanes have better performance at lower density altitudes. As the elevation or temperature increases, so does the density altitude. Changes humidity and barometric pressure changes have a smaller effect on the overall calculation. There are commercial devices like the Kestrel wind meters that actually perform all the calculations and give a number we interpret as an "elevation" which is the actual elevation adjusted for temp and humidity. Hot weather will make 2500' elevation behave like 5000'.

Another way to think of is this: hot air is less dense than cold air. Humid air is less dense than dry air. But heating up air (making it less dense) makes it drier (more dense) -- confusing yes, but DA combines the two into one consistent number.

Relative to our shooting sport, the range elevation is constant, and the barometric pressure usually does not change enough during a day of shooting to make any difference. That leaves us with temperature and humidity to track. Typically (but certainly not always) temperatures change a lot during a day of shooting while humidity remains relatively stable. Because of this, we can usually work out some rough rules for tuning our loads and rifles by correlating to just the temperature, ignoring everything else.
As the temperature warms up (i.e. density altitude rises), most folks tend to lower the powder charge a little to stay on the same node. If the humidity is also rising, you might lower the powder charge even more.

That said, some powders perform differently depending on the moisture they absorb from the atmosphere, so the adjustments may go the other way, and of course every gun has its own personality of what works for it, shooters jump nodes, your mileage may vary, etc.

My own record keeping shows that a 1 degree (F) change in temperature is worth about the same change in tune as a 10% change in humidity. The humidity would have to move 50% to have the same impact as a 5 degree change in temperature which is why we *usually* can get by with ignoring it.

Rod
 
Louis.J said:
Jerry Sharrett mentioned Density Altitudes in the Super Shoot thread and I did not want to distract from it and posting my question over there. How does Density Altitudes affect one's tune and in what direction does on with adjusting his or hers powder charge to stay in tune. Also which one takes precedence Temperature changes or Density Alitiudes or do thy go hand in hand.

J.Louis
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Louis, Rod gave a very good detailed explanation of DA. DA, simply put, is the thickness of the air. As to an airplanes ability to lift, DA is very important.
As to how and if it DIRECTLY effects gun tune, the jury is still out.

Jim Borden did a several year study on DA and tune and never really came to a DIRECT relationship. Another of the top benchrest shooters theorizes that as humidity rises more moisture enters the barrel bore and has some effect on the fouling. Another well known shooter I know uses DA to adjust his tuner.

I track DA and a few other environmental elements, and note them, just so if one day someone ties this all together I can say Ah-Ha!!
 
My take lately with temp/humidity. This is what I see at my home range in St. Bernard Parish (County) in extreme S.E. Louisiana.

Currently, we are in an extreme drought, the same as last year. Our days begin on the cool side (70 degs. or so) and heat up to the mid to high 90's. Later this summer, perhaps 100 or 101, also like last year. It's getting freakin' hot.

Now the humidity starts off in the morning anywhere from 85% to 97%. As it heats up, the humidity begins to drop. Last Friday it dropped to 37% by 11:00. That's very dry for us in this climate. All bets are off if there is rain or even if cloudy.

I lowered my Harrells by 2 clicks (N133) and it stayed in tune. The rifle did, I didn't. It shot in the very low 2's when the wind behaved.

For what it's worth.

Roy
 
There's a big difference here in degrees of tune in that one rifle may simply depart from a record setting pace while another may depart from being competitive. Before I continue I will suggest that rifles in the latter case are best left to the other competitors.

As Jerry stated, others have observed the effects of DA and have tried to correlate their observations. I have no idea of any success or of any failure but would believe that had there been either it would be widely published. The original question was/is which way to go with the powder and that has been the burning question for all these years. Thankfully, we only have three options for the powder charge...raise, lower, or leave it as is. Leave it alone is so very often eliminated by our last target measurement that we can forget that option altogether. That may be the correct option but not one we will likely choose. It is now a 50/50 proposition - which is not good enough odds since we already have a big group hanging. We're already out of the agg and now we're trying to stay in the grand (maybe the 2 Gun). So...what do we do, what do we do!

Boyd Allen (I think it was) just recently mentioned folks going to the line with multiple loads. If we followed this idea and had a method to quickly determine which load is best "right now" then we wouldn't need anything else.
 
Rodney's explanation is spot on.
DA, RPH, BP,=voodoo to me. I have tried to tune based on RPH and Jack Neary's explanation for the last year and a half and have decided to go back to temp based tunings based off of chronograph readings and target shapes/sizes. (No disrespect to Jack at all, but I think there is a difference in tuning in Ohio vs. in the Rocky Mountains). I'm no Einstein by any means, but I am a very critical thinker and I can't figure this humidity thing out to save my life. I can't measure any effect on powder weight either in the bottle or the hopper or in the cases. I can't see any difference in internal combustion effects. The only thing I can say I have seen any measurable difference caused in humidity changes is in external ballistics and optical physics. But how to seperate that data from DA and/or temp changes since they almost always go hand in hand?:confused:

Last weekend in Colorado, I tried tuning my first load of the morning based on the humidity instead of temps. Now, I've got hard data for this range going clear back to 2008. Temps, powder charges used, what worked, the whole nine yards. According to my notes, I should have used around 52.5 clicks of N133 for the 62 degree temps we had. But since it had just finished raining a few hours before the first group, I loaded lighter as per Neary's recommendations. High humidity requires lighter charges according to the theory. Well, my first group was a .447" pure vertical slot. Way too light. Went up a half number and it got better (.364") but still too light. For the third group, the temp was still in the sixties so I went up a whole number to 52.5 clicks and it punched out a round .285". I feel that if I'd started where my notes said I should have based on temps and temps alone, I would have done better right from the start. But just to make sure I wasn't missing something, I asked Lowell Frei, who was sitting behind me, if he loads by humidity or temps. He said temps only. He never could figure out how to tune off of humidity either. And he's had 30 years in this game.

Anyway, yesterday I got to shoot some of that leftover ammo from Colorado over my Chrono. It was ammo that had the right powder charge in it for tune but had bullets seated inconsistently due to a seater mic top that fell apart (whole other story). The load that worked out there was clocking at 3245 fps. Then I began to tune for yesterday's conditions. With XBR and H322, it just so happened that the best loads with those powders shot 3255 and 3252 respectively. Very interesting. Temps were 65 to 75 degrees and I didn't care what the humidity was. So, I have come to the conclusion that these particular bullets in this barrel like the rpm's that 3250 provides and as long as I can keep them going that speed, they shoot like a ppc should. Keeping them at that speed is going to depend on the temperature that the powder is burned in.
Note: If you live east of the rocky mountains, disregard everything I just said. I'm sure it's different out there. I myself have never shot a competitive rifle east of Denver and never lower than 1160 feet elevation.;)
 
One thing I question, especially at bigger shoots, a shooter will set up and test 3-4 loads, then they will leave their gun in its bags and load 3-4 more loads. This repeats sometimes through 50-75 rounds without cleaning the barrel. Then they thoroughly clean the barrel and the next day shoot for record expecting to get the same results they did when the barrel had dozens of rounds down the tube.
 
Last week at the Super Shoot morning temps started out in the low 70s and ramped up to high 80s-low 90s each day. Humidity would start at around 75% and drop to low 40% range by the last match. I found that Wed-Fri I could shoot my first two groups (warmup and match 1) with 29.1 grains of 133, third group with 29.4 and then jump to 30 grains for the last three groups and the gun stayed in tune. Saturday the temp ramped up a bit quicker and I would have been better shooting 29.4 for the second and third groups. Never did get 29.7 grains to shoot...no clue why. I record temp and humidity for every group, so I'll look at my book and post the actual temps and humidity when I have a chance.
 
I had a DA Meter for several years and learned quit a few lessons from it. The first lesson was if I had the correct ammo and good equipment the DA ment very little to my success. The second lesson was the KISS method {Keep It Simple Stupid}...my brain can only handle a limited amount of information, so if you don't see enough improvement to justify the device don't use it. And third...the only people that really succeeded with the DA Meters was the people that sold them!
Just my 2 cents worth.
 
I bought a Kestral a number of years ago and have had a good bit of trouble dealing with it. For one thing, the indicated DA seems to keep drifting quite a bit so I don't know how one would be able to know precicely what the DA is.

That said, I can see my RF rifles go out of tune when I know the DA is changing. Either I don't have a Stopped Muzzle or I don't have big enough selection of ammo, one.

With CF rifles, I shoot Score only and pre-load, so just go with what I find in testing. Considering most Score Shooters either pre-load or seem to load the same charge all day, well, pre-loads work as well or better than what they are doing, unless they have glommed onto a better load than I have. I gave up on tuners and worrying about Tune during a Match day. It seems to work most of the time for me, anyway.
 
Pete Wass said:
I bought a Kestral a number of years ago and have had a good bit of trouble dealing with it. For one thing, the indicated DA seems to keep drifting quite a bit so I don't know how one would be able to know precicely what the DA is.

.
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Pete. the DA does constantly change during the day. You must keep the barometric pressure reading (one display) and the altitude (another display) set to current conditions then the DA reading will be correct. You can set the baro display by setting the altitude reading (small letters under the baro readout) if you know the altitude of your location. Then use that baro reading to set the altitude. (again the small letters under the altitude display. Doesn't matter which you set first as long as they are both correct, then the DA reading will display correctly.

It is best to know the MSL altitude of each range. Then when you get there you can set the altitude display first to get the current baro. These settings, to be accurate must be set several times each day for the DA to be correct. All that said, why bother on centerfire unless you know something most of us don't??

Some rimfire guys swear by the DA to determine which ammo they use since they know the actual velocity of each ammo lot.
 
It seems to me that the winners seldom worry about altitude but rather attitude...that is the attitude of the wind flags. Good shooting...James PS Bad picture...the flags are hidden by the scope.

t4wvtd.jpg
 
Phil Deese said:
And third...the only people that really succeeded with the DA Meters was the people that sold them!
Just my 2 cents worth.
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Phil, you're spot on with that.

Also, my comment above was not correct. I really think that as the temperature went up, I went down on the measure. I put the info in my book backasswards.:(

I have been getting on the DA bandwagon lately. But I am seriously reconsidering. I am going to concentrate on temps only. As you stated, K.I.S.S.

Very good thread.

Roy
 
Have not been here for a while, but it really is great to know people still think about DA......and rightfully so. The LACK of paying attention to DA causes more aircraft accidents at high altitude airports, especially when flights originated at low altitudes, than anything else! Sea level shooters at high altitute matches soon find out they can't trust their flags, and hold off is often too much compaired to what they are used to. Velocities are higher etc.
Ambient temperature is the single most important factor in Density Altitude, and with my load of H322 and a Beggs tuner, a 3 degree rise in temp could be compensated for by turning the tuner One Clock Position toward the action. Simple as that, and repeatable.
Google "Koch Chart" and study it.
Bryan
 
IMO, the fly in the ointment, as far as this whole density altitude thing goes, and for that matter being able to spot tune precisely from observed conditions of whatever, is that the burning rate and pressure characteristics of the most popular powder, 133, are significantly altered as its moisture content varies. On the other hand, once tune has been found for a given powder condition, in a given set of ambient conditions, adjustments for ambient conditions can be made by load or tuner. There are significant exceptions to this that have been reliably reported, where no usable/competitive load could be found, and shooters had to change to another powder. The information on the variability of the powder came from a published study that I thought was well enough done to produce usable results. The rest came from multiple conversations with experienced competitors. Comments? It seems to me that the whole point of trying to reproduce T is to have a reliable source of a powder that is relatively immune to the conditions that it is exposed to, which makes staying in tune, in a variety of conditions, much simpler.
 
When I spoke of drifting, when I turn the device on , after having set the altitude and BR, the DA wanders around quite a bit and does not stop switching from one DA to another.It never seems to settle in and remain somewhat stable. I understand the fact that DA goes up and down but not rappidly, like every second a bunch.
 
Pete Wass said:
When I spoke of drifting, when I turn the device on , after having set the altitude and BR, the DA wanders around quite a bit and does not stop switching from one DA to another.It never seems to settle in and remain somewhat stable. I understand the fact that DA goes up and down but not rappidly, like every second a bunch.
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Pete, density altitude (DA) is not an exact quantity. It's normal for the DA meter to fluctuate a bit. Variations of plus or minus 200 feet are typical. I do not make tuner adjustments for less than 500 feet change in DA which would require a quarter turn to compensate. Hope this helps.

Gene Beggs
 
Gene Beggs said:
Pete, density altitude (DA) is not an exact quantity. It's normal for the DA meter to fluctuate a bit. Variations of plus or minus 200 feet are typical. I do not make tuner adjustments for less than 500 feet change in DA which would require a quarter turn to compensate. Hope this helps.

Gene Beggs
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Thanks Gene,

Let me see if I can be a bit more specific so I can tell if my Kesteral has a problem. When I turn it on and let it aclimate, after, say two or three minutes there will be a numbersort of appear. That number never stays constant going up and down by hundreds of feet at a time to the point that I can't seem to tell what is the base number. I can see that as the DA changes the device would chase it but shouldn't it sort of settle in during the first couple of minutes to some number that would obviously be a base number?

Thanks,

Pete
 
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