# Expansion of Nitrogen

1. Nov 29, 2009

### sdymanski

G'day
Could anyone give me the formula for expansion of nitrogen?
Cheers

2. Nov 29, 2009

### Staff: Mentor

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3. Nov 30, 2009

### sdymanski

Well, I'm a shooter, and few of my friends noticed a change in the POI when temperature changes. Rifle scopes are filled with nitrogen to stop them from fogging up, my theory is that when nitrogen expands, it affects the accuracy. Now I need to prove it, and find out specifically how it affects it and by how much.
Cheers

4. Nov 30, 2009

### Staff: Mentor

If I understand situation correctly scope is just a sealed tube filled with gas, so you would need to know how rigid the tube is, as basically there is no expansion - this is a constant volume case. Sure, tube is not infinitely rigid, so it will yield to some extent, but intuition tells me changes will be neglectable. Basically pV=nRT should work, you know temperature and volume, and n is constant.

However, intuition tells me that temperature change in general changes so many parameters that blaming just nitrogen & scope is wrong. When temperature changes air density and viscosity changes as well, so ballistic curve changes. I suppose muzzle velocity is also to some extent dependent on the temperature, that again means changes to the ballistic curve. No idea how scope mount is made, but it can be affected by metal thermal expansion as well. So - many things to consider, nitrogen inside seems to me to be between those least important.

There are thick books on the subject, these things were researched to the finiest details for military purposes. There are also programs that calculate ballistic curves. Have you checked if your observations are not easily explained in terms of known effects?

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5. Nov 30, 2009

### Staff: Mentor

If the problem is missing high when it is warm out, that would be consistent with warmer air being less dense and causing less drag. For the same reason, there are more homeruns in baseball in the summer than in the spring and fall.

6. Nov 30, 2009

### chemisttree

http://www.longrangebpcr.com/Accuracy.htm" [Broken]

Last edited by a moderator: May 4, 2017
7. Nov 30, 2009

### alxm

Given that it's a sealed container, it wouldn't be expanding but just changing the pressure as temperature/ambient pressure changed. While that'd change the refractive index slightly, I agree with Chemistree here in that other effects are almost certainly more significant.

To begin with, thermal expansion of the barrel/ammunition. Given that the barrel and bullet are made out of different materials (with different thermal expansion rates) the bullet's going to sit tighter or looser in the barrel depending on temperature, for instance.

8. Nov 30, 2009

### sdymanski

Thanks guys :), I'll take a look at different factors.