Hmmmm! Seems I'll have to quit raking leaves and keep up with this DA thread more often.  I'll try and answer everyones questions that I can.

Forrest,

  Well, yes I do have a Kestrel. Got that a few days back. we've been on other topics and forgot to mention it to you.

  As I mentioned in a past message the instructions are not all that clear on some points. Changing the altitude after setting the current barometer setting causes the barometer setting to change. So set the altitude first and then the barometer, and I HOPE I got it right. The altitude is something you set wherever you want to start recording from.

  This might be where the discrepancy you noted came in. When I saw it I wasn't satisifed with the reading either because the temp. on the Kestrel didn't match what my out door thermometer read (very accurate). So just went with the reading on the Kestrel as I figured it knew what it was doing with any condition it was recording internally.

  One thing I'd like to go over with you again, which I'm sure some have thought about. That is where does humidity come into the formula. You think it isn't needed, as temp. will bring the widest swings in DA. I'll take your word for it, as you've had more experience with this idea. But, considering the considerable amount of humidity changes in the Midwest and South during the Summer, I'm not to sure about this. You live, and shoots, in country with low humidity, where I think if it gets about 50% it's considered very humid, whereas in my part of the country 50% would be considered a "dry" day. I routinely see changes of 40% difference in the course of a days shooting. What's your thinking on this?

 
Joe,

  What I think I should do to answer your question on how much DA variation we can expect in a day, is to just keep track of it for a day and see. It's something that I've been wondering about to. I'll use the Kestrel, and record the temp., barometric reading, PA, & DA. That way there's enuf info for both Forrest and I to calculate it out the "hard" way with the E6-B. I'm also curious as to how much variation there is between the Kestrel and the E6-B. One of the things I've commented on to Forrest was the seeming "rough" calculations you get with the E6-B. The numbering is so close together and pretty general in how accurately you can set it, that I got to wondering if it was going to be accurate enuf for the short ranges we shoot at. Thus the reason for buying the Kestrel.

  As for a mathematical formula for figuring out DA.... I'm sure there is one since I would assume that the E6-B had to be set up according to one. But the PA has to be figured out manually, so that would require two formula. But I think it would be more complicated than all but the most avid math weenie would enjoy. Considering the lack of being able to do really fine settings with the E6-B, this is a pretty "rough" WAG as it is.


  Bob,

  Well, you know I have to pay for this instrument somehow. maybe a slight charge for every time you look at it would be appropriate.  How does $20 a eyeball sound to you?

  Actually it'll be setting on the shooting bench at the matches, or any time we shoot together, so you can look at it any time you want. If it pans out, by matching up with the E6-B, I might just loan you that to play with so you can keep track of things at your own pace.

PETE

Forrest, & others interested,

  Here's what I came up with today using both the Kestrel and E6-B Flight Computer. Remember my altitude is 970 ft. above sea level.

  9 AM  Bar. 30.25  54 deg. F.  90% Hum.

Kestrel  PA - 672  DA - 785
E6-B     PA - 670  DA - 590

  11 AM  Bar. 30.20  56 deg. F.  87% Hum.

Kestrel  PA - 672  DA - 785
E6-B     PA - 670  DA - 810

  11:30 AM  Bar. 30.22  57 deg. F.  82% Hum.

Kestrel  PA - 683  DA - 919
E6-B     PA - 670  DA - 820

  2 PM  Bar. 30.14  64 deg. F.  82% Hum.

Kestrel  PA - 751  DA - 1455
E6-B     PA - 770  DA - 1020

  4 PM  Bar. 30.14  63 deg. F.  70% Hum.

Kestrel  PA - 751  DA - 1377
E6-B     PA - 750  DA - 980

  5 PM  Bar. 30.13  61 deg. F.  75% Hum.

Kestrel  PA - 765  DA - 1243
E6-B     PA - 760  DA - 975

  The interesting thing is that the PA for both the Kestrel and E6-B are reasonably close. I can't explain why the discrepancies in the DA tho unless it was my not being able to exactly enter PA & temp. C. in the E6-B.

Forrest,

  No the Kestrel doesn't tell you whether you are above or below sea level. At least I don't think so. Maybe when it does go below sea level it will indicate the reading with a "-" sign.

  What I'd really like to know is what conditions does the Kestrel use to figure DA. It's obvious that it calculates PA by the same method we use manually, but it is also obvious that it's using something different for DA. Some of the problem of course would be the lack of being able to exactly enter figures considering the closeness of the numbers etc. Also should probably use a mathematical conversion for F. to C., as I was just getting as close as I could with the graph provided, which left a little to be desired.

  But with all that said, I think if you use the same procedure you'll still get acceptable results. Altho DA is not an arbitrary figure if you use the same method to achieve it, then it can be used to advantage. Your results at 1000 yds. bears this out.

  Your thoughts?

PETE

OOPS wrong button....to continue....

Much has been said here and I would like to put in my two pennies.

SD's are confusing and are not reliable to determine accuarcy, they will help with consistency. My main venue is ML Slug Bench Guns. I started in the game with instructions on how to load by pushing the bullet gently down to just kiss the powder. Not good, large groups, large SD's. Began to load and shoot over the chrohograph and found that as I increased the pressure on the powder my SD's came down to a remarkable 5 on average and are still there. Result being that I began to get some consistency. This new found low SD works for all of my powder charges even the big bad groups, allowing me to identify other problems.

Sample size- When I worked at GM they sent me to school to learn Statistical Process Control where I learned it only takes 5 samples to establish the wonderful "Bell Curve" which can tell you anything about anything, such as our wonderful polls.
They preferred a sample of 25. I hear of increasing 5 shot groups samples from 20 to 4000+ in order to find that "Accurate Load"...Great....but now you have a shot out barrel. A load aquired through statistical analysis will not stand up to the variables we have to deal with, unless you shoot in all conditions. The best load is not one load it is really a range of powder charges that fit the changing conditions. As a ML'er I change my powder as the conditions demand, a 15% change in humidity requires a change 10 degree change in temp, a barometric pressure change, all require attention to powder load.  Statistical charts, I don't think will be reliable. These solutions require a data bank acquired through experience. This experience I am working on for ASSRA shooting which I have recently have taken up.

Statistical charts or mathematical formulas are not going to be a shortcut to the 'top of the heap' only experience.

That said! Statistics and math can help you understand what is happening and what you have to learn. I can't teach you wind reading but I can point out things that are happening and what to look for and sometimes why. But you still have to do it and build your own database. If you want to learn to shoot in a 15mph wind cause that is what you have to shoot in at EG than you better get out and practice in a 15mph wind. And you can't do what I am doing with my .45 cal 540 grain slug with your .32 cal 185 grain slug. That's why you need to acquire your own database.

Remember the proof is the group in any given situation, not the statistics.

PA & DA sounds like another tool, that will not be of use to everyone in all situations. I for one like to learn these things, and yes I do 'play' repeat 'play' with statistics. This is another idea that requires establishing a database. I am not clear on this idea and I hope the thread continues. I have to learn more to ask questions, so keep it coming.

Just my opinion!........Ed

PETE,

Pete has provided data (below) comparing E6-B and his Kestrel.

I got out my E6-B and ran his numbers with the following results:

 9 AM  Bar. 30.25  54 deg. F.  90% Hum.

Kestrel  PA - 672  DA - 785
E6-B     PA - 670  DA - 590

Here, I get just about right at Sealevel DA on my E6-B which makes sense since 59 degrees is a "Standard Day" at SL and 54 degrees at PA of 670 feet is very close to that. Any difference is pocket change.

 11 AM  Bar. 30.20  56 deg. F.  87% Hum.

Kestrel  PA - 672  DA - 785
E6-B     PA - 670  DA - 810

Again, here the data is very close to a standard day at SL. The +810 feet DA (Or the +785 either) seems high to me since it would have to warmer than data indicates to have a DA as high as 1000 feet above SL.

 11:30 AM  Bar. 30.22  57 deg. F.  82% Hum.

Kestrel  PA - 683  DA - 919
E6-B     PA - 670  DA - 820

Here, and for the remaining points of data on Pete's post I get essentially the same DA as he got for the E6-B.

 2 PM  Bar. 30.14  64 deg. F.  82% Hum.

Kestrel  PA - 751  DA - 1455
E6-B     PA - 770  DA - 1020

 4 PM  Bar. 30.14  63 deg. F.  70% Hum.

Kestrel  PA - 751  DA - 1377
E6-B     PA - 750  DA - 980

 5 PM  Bar. 30.13  61 deg. F.  75% Hum.

Kestrel  PA - 765  DA - 1243
E6-B     PA - 760  DA - 975

Keep in mind that the E6-B is rather crude in its presentation of data. The marks in the DA window are small, close together and difficult to read with precision.

I take this to mean (so far as avaition uses are concerned) that differences in DA, although important, are not subject to such need as far as aircraft preformance is concerned that a pilot must calculate for the nearest 100 foot difference: The nearest 1000 foot difference is good enough for the performance information required for such uses.  As a relativly high-time pilot myself I find this to certainly be the case so far as I'm concerned.

In the shooting I do over long range I keep data for DA for each rifle I shoot, recording sight elevations for actual Density Altitude on the rifle range at the time. This data is kept with the rifle for regular use as conditions change.

As I have been doing this for some years now I find the sight elevations do repeat for a given DA. Data are kept for 500 foot increments in DA which I find more than close enough to use as a practical thing on the firing line.

Good evening,
Forrest

Forrest,

  Your calculation results are interesting and I'm glad for the most part they agree with mine. Possibly the difference you noticed on the 11 AM reading might be because I handled the Kestrel while getting the readings. I had it sitting out in the garage, in the shade. Since the day was dark and misting I usually had to move it to see the display. I've found that you have to be very careful in how you handle it. Even if you grab it as far away from the temp. probe as you can, you can watch the temp. change over a short time. Best to look but don't touch.  I forget now but the Kestrel displays out to at least one decimal point on all readings, and I was rounding things to the nearest whole number, which might have added a little of the differences.

  On your observations regarding the .22.

  I hadn't thought about using the .22 for testing DA, but now that you mention it, I think you're right. We have one 200 yd. match for the .22, and Lee Shaver has calculated that for his .22 Shilouette game that the drops, and wind deflection over the 200 meter course equals almost exactly the flight characteristics of the .40/65 at the standard BPCRS distances.

  So, I'll be able to see if there are any effects at both our standard 100 yd. matches and the 200 yd. one.

  I hope no one is in any hurry for the results tho since we're pretty well done with the outdoor shooting around here, altho I might get around to going up to the enclosed firing points a few times during the Winter. We'll see!

PETE

Ed,

  I threw in a bunch of extra figures more for Forrests benefit, so he could do independent calculations, rather than being necessary info for what I was trying to get across to everybdy else.

  To explain further...... You will have to calculate Pressure Altitude (PA) before you can enter that into the E6-B Flight Computer.

  Figuring PA is easy. What you need is to know the altitude of your location and the barometric pressure. Then adding or subtracting from your altitude will be determined by whether your barometric pressure is higher or lower than the "standard" of 29.92. Think of 29.92 as being like the constant used in the Greenhill formula.

  Example...... Lets say your altitude is 1000 ft. above sea level (ASL), and your current barometric pressure is 30.22. Subtracting 29.92 from 30.22 we get .3. Now in this case your current barometric pressure is higher than the "Standard" so you will subtract 100 ft. for every .1, which in this case would be 300 ft. So your current PA will be 700 ft. ASL. If your current barometric pressure is lower than the "Standard" then you add 100 ft. for every .1 difference.

  In order to figure out what the Density Altitude (DA) is you will need the E6-B Flight Computer. It has provisions for entering the PA and your current temp. in degrees Centigrade. A Fahenheit to Centegrade conversion chart is located on the E6-B. After you do this then an arrow will point to the DA.

  Once you get used to doing the necessary calculations it's a lot faster to do than to read.

  DA is what you will be putting into your log book, along with the sight settings, and keeping track of over time, and then will refer to it for sight changes if they are warranted.

  Forrest feels that for the shooting he does he doesn't need to correct for changes less than 500 ft. Part of what I need to find out is if this is "fine" enuf for Schuetzen ranges, or do we need to go to some lesser amount like 100 ft., or smaller. Only a lot of shooting next Summer will tell.

  So you can see from this I didn't need to put in the info I gave in the test message other than the DA. But as I said, that was for Forrest's information so he could check what I was getting with the Kestrel and E6-B against what he got using his E6-B. Altho I think I'm familiar enuf with the E6-B to know I'm figuring PA & DA right with it, the difference in data between it and the Kestrel had me worried I might be doing something wrong somewhere.

  The problem, if you want to call it that, with the Kestrel is you can set it on the table and watch the DA change 10 or 15 ft. in less than a minute. It's constantly changing!

PETE

I think I understand, you will calculate the DA for every target shot and enter the sight settings, so that after a decent amount of data THEN you will be able to calculate sight changes based on the collected data.

Ed,

Not exactly.

The way I do it is to go shooting one day at a given and semi-carefully chosen DA, say here in Sheridan at 41 degrees F which is the Standard Day at 4000 feet.
 
I'll go out and shoot all ten targets we have, ranging from 395 out to 834 yards and record the elevations at that DA.
 
Then, as the season changes I'll naturally have opportunity to shoot the same course of fire at 4500 DA and 5000 and so forth. I record these settings on my sight card that is kept with the rifle. If a fellow were so-minded I know these numbers would allow him to  calculate some useful way to predetermine sight setting elevations. For me, the simple shooter, the actual numbers acquired in actual shooting are most valuable of all.
 
With these settings written down I then may depart Sheridan for some other location. Once there I compute the DA at the time I'm due to step up to the firing line and use the sight elevations I have available on the card. The DA of course does not make a difference say between Sheridan or Forsyth or Bozeman: Downrange performance will be very, very close to that specified on the DA elevation card and one may interpolate between numbers if the DA falls somewhere in between the recorded data on the card.

Ed: At this point do you know which direction to change per data, if DA goes up will the sights have to come down or up?

F:  As Density Altitude increases the air becomes thinner. Adjust your sight down as DA increases from your "Base Line" of data. 

As an aside everything that uses air is affected by DA. The amount of lift provided by an aircraft wing for one. And the amount of power produced by the internal combustion engine in your car (Non-tubo) and the amount of energy provided by your very own internal combustion of food in your body is affected. That is to say that if it is a 7000 foot density altitude day here in Sheridan (easy in the summertime) a man gets tried a whole lot quicker than he does when things are nice and cool.

Good morning,
Forrest