Altitude Density & Temperature Change

[Lee Hachigian]

I’m confused with the concept of altitude density (AD) and how it may impact bullet velocity (in short range benchrest shooting). Questions:

Does AD change (in the same direction) as temperature changes throughout the day (all else being consistent)?
How does the bullet velocity change (up or down), if AD goes up?
How does the bullet velocity change (up or down), if AD goes down?

Thanks
Lee

 

[josh shrum]

I’m confused with the concept of altitude density (AD) and how it may impact bullet velocity (in short range benchrest shooting). Questions:

Does AD change (in the same direction) as temperature changes throughout the day (all else being consistent)?
How does the bullet velocity change (up or down), if AD goes up?
How does the bullet velocity change (up or down), if AD goes down?

Thanks
Lee

The basic thing to remember with AD is that the key word is altitude, not density.
Just as with people climbing up mountains like Everest needing O2 tanks because the air is getting "thinner", an increase of altitude density will result in less dense air.

An increase in temp will result in an increase of the AD.
As AD goes up the air is becoming less dense, therefore less drag on your bullet.
The reverse occurs as AD goes down.

In short range, especially where we can shoot sighters, this is something that you can dance with.

Josh

 

[Boyd Allen]

First of all you have your words reversed. It is DA not AD. I mention this so that it will be easier for you to find information on the subject. Secondly, there is a huge amount of information about DA online, primarily because of its importance to aviation. If you find a source that explains it in a way that is easiest for you to understand I think that you will be off to the races. DA expresses air density by saying that it is equivalent to what it is at a given altitude under standard conditions. At any given altitude the density of the air decreases as the temperature rises, and to a much lesser degree it decreases as humidity rises. This may be counter intuitve to many because as humidity rises we commonly say that the air feels heavier, when in fact it is less dense. Here is a link to a document that explains DA in considerable detail. https://commons.erau.edu/cgi/viewcontent.cgi?article=1124&context=ijaaa

 

[Gene Beggs]

First of all you have your words reversed. It is DA not AD. I mention this so that it will be easier for you to find information on the subject. Secondly, there is a huge amount of information about DA online, primarily because of its importance to aviation. If you find a source that explains it in a way that is easiest for you to understand I think that you will be off to the races. DA expresses air density by saying that it is equivalent to what it is at a given altitude under standard conditions. At any given altitude the density of the air decreases as the temperature rises, and to a much lesser degree it decreases as humidity rises. This may be counter intuitve to many because as humidity rises we commonly say that the air feels heavier, when in fact it is less dense. Here is a link to a document that explains DA in considerable detail. https://commons.erau.edu/cgi/viewcontent.cgi?article=1124&context=ijaaa

Perfect explanation Boyd. Thank you.

 

[Lee Hachigian]

Thanks guys! So as DA increases (say from morning to afternoon) my velocity will increase due to less dense air.

 

[MikeE]

That is correct.

Somewhere I have a set of cards, similar to a paper slide rule, that does DA calls, plus gives you bullet drop for a M118LR ( 7.62/308 175 grain bullet ) load. Fun to play with, and gets an understanding quickly of the different effects. Some of the hand held wind gauge weather stations also do the DA calcs, in addition to the ballistic calculator built into them.

 

[JerrySharrett]

Perfect explanation Boyd. Thank you.

Beggs knows, being an old airplane pilot that DA is very important in how an airplane performs, how much weight it can get off the ground with. In simple terms DA is the thickness of the air.

Unfortunately bullet performance isn’t that simple. Somewhere back in the early 2000’S several of us, Jim Borden, myself and several others tried to find a direct relation to DA and velocity. Krestel had just come on the market with a small hand held device that, among 6 or so other functions would automatically display DA. None of us ever came up with anything usable. Fun to play with though.

.

 

[Lee Hachigian]

I’m just trying to understand why my well tuned load in the afternoon won’t shoot well in the morning, or vis-a-versa. Is our powders that sensitive to temperature changes, or are they sensitive to air density (where temperature is one of the key variables)?

 

[Jerry H]

I believe it is the air's moisture related the friction of the bore fouling. If you push a brush through a just fired barrel in very dry air, there is a lot of resistance. If you do the same thing in very wet air, the brush just sails through. I noticed this in the mid 90s in CFBR but could never pin it down. When I got into RFBR where I could test thousands of rounds without changing barrels, it looks like it has a most important relationship to dew point, not temp, DA, RH or the rest of the weather environmental factors. The state of tune stays very stable in dry to very dry air. It is less stable in wet to very wet air. The real roller coaster ride is in between these 2 areas which is when you shoot most of the matches in.

 

[JerrySharrett]

I’m just trying to understand why my well tuned load in the afternoon won’t shoot well in the morning, or vis-a-versa. Is our powders that sensitive to temperature changes, or are they sensitive to air density (where temperature is one of the key variables)?

Lee, get yourself a Kestrel 4000 series device and see if you can find any direct relationship between atmospheric conditions and load tune. Its an interesting undertaking.


.

 

[Bill Gammon]

Could you patch me the results, and "ME" only, do not want the rest of these people to get their hands on the results. EH!!!!!!!!!!!!!!

 

[Mozella]

I'm chuckling a little bit when I read some of these posts. For example, claiming that a certain factor is related to dew point but it is not to relative humidity is just plain wrong. Dew point and relative humidity measure the same thing, i.e. the amount of moisture in the air. They simply express it in different ways. Dew point is a measurement of humidity which is most meaningful for pilots. If the dew point at a particular destination airport is 48 degrees and the temperature is 52 degrees, you would expect the air to be clear below any clouds. However, if your flight plan predicts a landing after sunset when the temperature is forecast to drop to 46 degrees (2 degrees below the dew point), you might expect fog on arrival.

Pilots have to take into account countless factors to insure a safe flight, so reducing the things which require complex calculations or references to charts is welcome. Dew point is one such thing an density altitude is another. .

Density Altitude is simply a way to label the density of air in such a way that it is easy to grasp. Which would you rather get in a weather report? The air density is 1.29 kg/m3 at a temperature of 288.15 K or the density altitude is zero; i.e. sea level?

 

[K Hope]

Related to dew point or relative humidity

I'm chuckling a little bit when I read some of these posts. For example, claiming that a certain factor is related to dew point but it is not to relative humidity is just plain wrong. Dew point and relative humidity measure the same thing, i.e. the amount of moisture in the air. They simply express it in different ways. Dew point is a measurement of humidity which is most meaningful for pilots. If the dew point at a particular destination airport is 48 degrees and the temperature is 52 degrees, you would expect the air to be clear below any clouds. However, if your flight plan predicts a landing after sunset when the temperature is forecast to drop to 46 degrees (2 degrees below the dew point), you might expect fog on arrival.

Pilots have to take into account countless factors to insure a safe flight, so reducing the things which require complex calculations or references to charts is welcome. Dew point is one such thing an density altitude is another. .

Density Altitude is simply a way to label the density of air in such a way that it is easy to grasp. Which would you rather get in a weather report? The air density is 1.29 kg/m3 at a temperature of 288.15 K or the density altitude is zero; i.e. sea level?

I agree there is a definite relationship between Relative Humidity and Dew Point and that both are indicators of the amount of moisture in the air. Relative humidity is given in a percentage, not so easy to work from and relates only to the vapour phase and not to any other form of water/air combination. Dew point however is the temperature at which air can no longer hold its water vapour and a percentage starts the phase transition. Pilot or Benchrest it seems like dew point is easier to work with and tells you more.

 

[mks]

I’m just trying to understand why my well tuned load in the afternoon won’t shoot well in the morning, or vis-a-versa. Is our powders that sensitive to temperature changes, or are they sensitive to air density (where temperature is one of the key variables)?

It's both. Temperature affects muzzle velocity by affecting the combustion rate of the powder and the fraction of energy that is transferred as heat to the barrel. Temperature also affects the length of the barrel and, therefore, how it vibrates.

Density affects external ballistics and bullet drop. Perfect compensation is no longer perfect when density changes. As a really simplified example, lets say the fast bullet of a group drops 1" and the slow bullet drops 2", but launching it at a higher muzzle angle compensates perfectly to put it in the same hole. The next morning, density has doubled so that the two bullets drop 2" and 4", respectively. With the same 1" compensation, the slow bullet now hits the target 1" lower than the fast bullet. Actual changes are a lot less. For instance, the air density increase between 90 F and 50F is a bit less than 10%

 

[K Hope]

It's both. Temperature affects muzzle velocity by affecting the combustion rate of the powder and the fraction of energy that is transferred as heat to the barrel. Temperature also affects the length of the barrel and, therefore, how it vibrates.

Density affects external ballistics and bullet drop. Perfect compensation is no longer perfect when density changes. As a really simplified example, lets say the fast bullet of a group drops 1" and the slow bullet drops 2", but launching it at a higher muzzle angle compensates perfectly to put it in the same hole. The next morning, density has doubled so that the two bullets drop 2" and 4", respectively. With the same 1" compensation, the slow bullet now hits the target 1" lower than the fast bullet. Actual changes are a lot less. For instance, the air density increase between 90 F and 50F is a bit less than 10%

Question: Should I take that to mean Temperature is the only atmospheric condition that changes burn rates?

 

[Boyd Allen]

I can think of one top shooter who tends to shoot a lot of sighters and may wait until very late in a match to shoot his record group. I always assumed that the sighters were shot to learn what the result of variations in the wind did to bullet impact, and that may be true, but what if there is a second reason, to keep the condition of the bore in a more consistent state? I have read that one powder's fouling can change in the bore to some degree during a pause in shooting beyond some period of time, and that that can affect bullet impact location. Perhaps keeping the fouling fresh is a part of the strategy that I had not thought of.

 

[Dave Coots]

It's both. Temperature affects muzzle velocity by affecting the combustion rate of the powder and the fraction of energy that is transferred as heat to the barrel. Temperature also affects the length of the barrel and, therefore, how it vibrates.

Density affects external ballistics and bullet drop. Perfect compensation is no longer perfect when density changes. As a really simplified example, lets say the fast bullet of a group drops 1" and the slow bullet drops 2", but launching it at a higher muzzle angle compensates perfectly to put it in the same hole. The next morning, density has doubled so that the two bullets drop 2" and 4", respectively. With the same 1" compensation, the slow bullet now hits the target 1" lower than the fast bullet. Actual changes are a lot less. For instance, the air density increase between 90 F and 50F is a bit less than 10%

Say for example you have a 21 3/4" barrel. The temp is 55* at the start of the match at 8AM. At 3 PM the temp peaks at 85*. How much did the barrel grow in length?

Later
Dave

 

[Boyd Allen]

I get a little over .0035" but this is a calculation that is new to me. Looking up the coefficient of thermal expansion of 416 steel, in the range of 0 to 100 degrees Celsius it is 9.9 microns per meter per degree Celsius. The difference in temperatures in Celsius works out to 16.6 degrees, a micron is .000039 inches. 21.75 inches converts to .552 M. Check my math and procedure. I am not an engineer. Boyd

 

[Lee Hachigian]

Testing - What’s going on?

Last weekend, we shot a 200 yard Score Match at WWCCA. Since it was a fun match and cold, most of us come preloaded. I asked the guys if I could set up a chronograph. I had loaded up 50 rounds of V-133, weighing each round for my 6ppc. It was somewhat mild this day with temperature starting at 37*, which resulted in a lot of fog (we still had plenty of snow on the ground). The match started a little late due to the fog (we couldn’t see the rings), so the first shot was fired around 11:00 am, and the last shot close to 1:30 pm.

My average velocity for the first match was 3390 fps. By the 5th target my avg. velocity was 3324 fps. The temperature went from 37* to 39.5*. The Density Altitude when from -667 to -280. I didn’t write down the humidity.

Yesterday afternoon, I went to the range to fireform some brass. Again all the rounds were weighed. I shot 10 rounds every half hour. Starting velocity was 3089 fps, and ending velocity was 3090. DA went from -220 to +156, and temperature went from 45* to 49.5*. Humidity went from 60% to 48%.

I can’t draw any conclusions why the velocity changed during the weekend, but didn’t yesterday. Any thoughts?

Lee

 

[GeneT]

Last weekend, we shot a 200 yard Score Match at WWCCA. Since it was a fun match and cold, most of us come preloaded. I asked the guys if I could set up a chronograph. I had loaded up 50 rounds of V-133, weighing each round for my 6ppc. It was somewhat mild this day with temperature starting at 37*, which resulted in a lot of fog (we still had plenty of snow on the ground). The match started a little late due to the fog (we couldn’t see the rings), so the first shot was fired around 11:00 am, and the last shot close to 1:30 pm.

My average velocity for the first match was 3390 fps. By the 5th target my avg. velocity was 3324 fps. The temperature went from 37* to 39.5*. The Density Altitude when from -667 to -280. I didn’t write down the humidity.

Yesterday afternoon, I went to the range to fireform some brass. Again all the rounds were weighed. I shot 10 rounds every half hour. Starting velocity was 3089 fps, and ending velocity was 3090. DA went from -220 to +156, and temperature went from 45* to 49.5*. Humidity went from 60% to 48%.

I can’t draw any conclusions why the velocity changed during the weekend, but didn’t yesterday. Any thoughts?

Lee

It is much more likely that conditions affected your chronograph more than your actual rounds. Assuming you're measuring about as close to the muzzle as you can get I don't think DA will play a noticeable role. Temperature will, but the temperature of the barrel probably varied by quite a bit more than ambient temp numbers you've reported. This may be an intractable problem once your readings get very close, but what kind of chrono, and what were your standard deviations?

GsT