Does humidity affect rifle tune?

[1KBR]

As I will always be reloading in my basement with stable humidity, it doesn't look like it will affect me. I have hit the button on an order for the kestrel with temperature read out, but no humidity reading.

 

[artbosco]

I was looking at the kestrel wind meters and noticed one of them can read humidity. Is there any practical real world change in barrel tune with different humidity?

Here is an excerpt from a question posed by russel m. and the answer provided by Gene Beggs [Gene was a Southwest Airlines pilot for over 30 years and is a BR shooter] in Benchrest Central that I pulled from my Benchrest file. Hope this helps:

Question: 6ppc using v 133 powder. Is it just humidity or is humidity & temp that affect tune? When the Humidity goes down I go up on the powder to keep my gun in tune? Is it that the powder kernels weigh less with lower humidity so I have to go up on the charge? Is it that the conditions change the harmonics of the barrel. Is it that the bullet travels through the air differently with the changes in conditions? russell m

Answer: Changes in air density makes our rifles to go out of tune. If the bullets are exiting when the barrel is stopped at either the top or bottom of the swing, the rifle is in tune. If the atmosphere becomes less dense due to an increase in temperature, the bullet encounters less resistance as it travels down the bore and exits early before the barrel comes to a complete stop, in which case, we can either reduce the load or adjust the tuner to compensate. I think it's much simpler and easier to use a tuner. You NEVER have to change the load and can make your adjustments at the line.

Atmospheric density is the result of pressure altitude, temperature and to a far lesser degree, moisture content. Contrary to what many believe, dry air is heavier. Steam rises and it's 100 percent saturated.

So .. altitude, temp and humidity in that order. What do we mean by altitude? Range elevation above sea level. The Midland Shooters Association range is 2800 feet above sea level; that never changes so we can forget about it. Yes, slight variations in barometric pressure can change the pressure altitude but for all practical purposes, we can forget about it.

**Temperature? This is the big one! It's actually the only thing you must concern yourself with. The temperature range from being perfectly in tune and being completely out is 20 degrees F.**

Yep, if your rifle is perfectly in tune during the first match of the day when temperature is 70 it will be completely out of tune when the temp reaches 90 if you do nothing to compensate.

To keep the rifle in tune throughout the day, reduce the velocity 30 fps for each five degree increase in temp. With most powders, N133 being the classic example, this equates to one half click per five degrees. For example, let's say your rifle was in tune for the first match; temp was 70. Your Culver type measure was set on 54 clicks. When temp reaches 75 the proper click value is 53.5, for 80, 53, 85, 52.5 and for 90F, 52 clicks. Would you have ever believed it would take two full numbers to stay in tune with a twenty degree spread?

"Now wait a minute Beggs; are you telling me relative humidity has nothing to do with it?" Yep, that's right; very little, so little that for all practical purposes we can ignore it.

 

[wwbrown]

Watch this video. Humidity is a very misunderstood subject. Especially in Hollywood.

http://www.youtube.com/watch?v=nthZ7RHD3yA&feature=share&list=PLJUaiRIEduNXoal2_PkBZi0vDCIcEPxUn&index=28

That is a fantastic video he has the science right at the precision needed for ballistic solutions.

wade

 

[wwbrown]

The video shown in the link above discusses exactly your concern, you need to worry about exterior ballistics. Exterior ballistics is the study and modeling of what the air, gravity, spin of the earth, and spin of the bullet, does to the bullet after it leaves the barrel. Watch the video and you will see that the difference in impact of a 308 from 0% to 100% humidity is 0.3 MOA. I will worry about humidity for me 1,000 shots when I have the ability to call my shots to 0.3 MOA at 1,000 yards. Since I will never live long enough to achieve that level of precision at 1,000 yards I will never worry about the humidity.

Humidity sensors are more expensive that air pressure and temperature sensors and the impact of humidity on the ballistics is so small many hand held systems only use air pressure and temperature sensors.

The other type of ballistics is interior ballistics and that is the study of what happens to the bullet while it is inside the gun, everything I state above is exterior ballistics.

A disclaimer:
I do not shoot at short range and I am not providing advice on that discipline.

 

[GSPV]

Gene is making a huge assumption there that air density effects interior ballistics. He'll be happy to tell you that as he does later in he thread that that was excerpted from.

Others, with more to base it on, like being the chief ballistican at One of our national labs will tell you that it is energy loss to the barrel.

 

[artbosco]

This is all I try to remember: The temperature range from being perfectly in tune and being completely out is 20 degrees F. I either increase or decrease my charge or use a tuner.

 

[bigedp51]

1KBR

In post #15 you clarified you were talking just about the ammunition "BUT" you still have rifles with wooden stocks and up pressure at the fore end tip.

Therefore a wooden stock has more effect than air density and the rifles "tune" and not everyone has a free floating barrel.

Just "tuning" my 2 cents.

 

[jlow]

To keep the rifle in tune throughout the day, reduce the velocity 30 fps for each five degree increase in temp. With most powders, N133 being the classic example, this equates to one half click per five degrees. For example, let's say your rifle was in tune for the first match; temp was 70. Your Culver type measure was set on 54 clicks. When temp reaches 75 the proper click value is 53.5, for 80, 53, 85, 52.5 and for 90F, 52 clicks. Would you have ever believed it would take two full numbers to stay in tune with a twenty degree spread?

So I have a question about rationale – The author suggest “reduce the velocity 30 fps for each five degree increase in temp”. Is this to compensate for a 30fps increase in velocity due to each 5 degree increase in temperature so that the “barrel time” remain the same? Seems like an awful lot of MV change for a 5 degree temperature change?

 

[amlevin]

To keep the rifle in tune throughout the day, reduce the velocity 30 fps for each five degree increase in temp. With most powders, N133 being the classic example, this equates to one half click per five degrees. For example, let's say your rifle was in tune for the first match; temp was 70. Your Culver type measure was set on 54 clicks. When temp reaches 75 the proper click value is 53.5, for 80, 53, 85, 52.5 and for 90F, 52 clicks. Would you have ever believed it would take two full numbers to stay in tune with a twenty degree spread?

So I have a question about rationale – The author suggest “reduce the velocity 30 fps for each five degree increase in temp”. Is this to compensate for a 30fps increase in velocity due to each 5 degree increase in temperature so that the “barrel time” remain the same? Seems like an awful lot of MV change for a 5 degree temperature change?

This is most likely what works in "his" rifle. Now will it work in anyone elses?

 

[jlow]

To keep the rifle in tune throughout the day, reduce the velocity 30 fps for each five degree increase in temp. With most powders, N133 being the classic example, this equates to one half click per five degrees. For example, let's say your rifle was in tune for the first match; temp was 70. Your Culver type measure was set on 54 clicks. When temp reaches 75 the proper click value is 53.5, for 80, 53, 85, 52.5 and for 90F, 52 clicks. Would you have ever believed it would take two full numbers to stay in tune with a twenty degree spread?

So I have a question about rationale – The author suggest “reduce the velocity 30 fps for each five degree increase in temp”. Is this to compensate for a 30fps increase in velocity due to each 5 degree increase in temperature so that the “barrel time” remain the same? Seems like an awful lot of MV change for a 5 degree temperature change?

This is most likely what works in "his" rifle. Now will it work in anyone elses?

I would agree with that, but my question was more general in nature as I don’t use V133 anyway.

So again, when the temperature changes, your MV is going to change and of course it depends a bit on the “temperature sensitivity” of your powder and perhaps on the change in air resistance as it comes down the barrel. I personally have not seen that degree of MV change even with a 30 F change in temp, so the question is what is the basis for his change? Is it normalizing MV or something else? Do you see where I am coming from?

 

[BoydAllen]

I think that a lot could be learned by spending a day at the range shooting and retuning as the temperature rises, shooting all shots over a chronograph, and logging the in-tune loads, their velocity, the temperature, and the humidity. The fly in the ointment in all of this is that if one is involved in a sport where loading at the range is the common practice, the temperature and particularly humidity change will change the powder, as is sits in the measure, and cases waiting to have bullets seated. The direction of this change, will depend on the condition of the powder at the start of the day, and the ambient conditions, and how they change. This is one of the main reasons for LT32's creation, as an attempt to duplicate that lot of surplus 8208 that came to be known as T powder. That powder was much less sensitive to ambient conditions, with regard to its burning characteristics, a and had broader nodes than 133.

 

[jlow]

Agreed! Thanks.

 

[Dave Brown]

I remember reading this article. The test was so extreme that it would be hard to duplicate out of the test tube. I won't leave powder in my scale after reading that and return it to the jug immediately after weighing charges.

I don't leave my powder in the chargemaster overnight and place it straight back into its original container after finishing a reloading session. Once you load the round do you think humidity has any effect on the powder inside say for example you let the rounds set outside for a full day?

 

[dcali]

The *only* people I've ever heard say that humidity is an issue are point blank benchresters. This makes me think it has something to do with their equipment/methods rather than the powder. Since so many of them seem to like powder measures and load at the range, I wonder if it's not a static electricity thing that impacts the way the measures work. When it's dry, powder can get "sticky" in that it clings to itself in strange ways. Static potential is *very* sensitive to humidity.

Just an observation. I'm sure someone with more experience with BR would have better insight.

 

[Olde Man]

I am going to throw a real world twist into the mix. The key to this is that everyone should be using the term relative humidity. This means the amount of water in the air is relative to the air temperature at that moment. A relative humidity calculator will yield the following. 80F and 60%RH , 79.5 F and 61%, 79F and 62%RH and 78.5F and 63%RH all contain the same amount of water vapor. The difference is that the air has expanded or shrunk to change the % of water relative to the volume of air. This will change how you look at the sealed container and the air conditioned loading room in the basement. The temperature is also a much bigger player in the air density calculations. A simple example. Have you wondered why the refrigerator door is harder to open the second time you use it? The first time you open it the cold air runs out the bottom and warm air moves in at the top to fill up the space that the cold air previously occupied. You shut the door and the new warm air cools and shrinks in size creating the vacuum that make it difficult to open the door the second time.

 

[mikecr]

Some of observed could be do to stability changes, as a direct result of air density changes that a 30% drop in humidity contributes to.
But I always wondered if air density in the bore matters to muzzle velocity, and associated barrel timing. Do point blank BR guys ever look at this over a chrono?