PRS Humidity Test

[Mordamer]

After reading about humidity's pretty drastic affect on velocity and then hearing Bryan Litz on a podcast talking about it, I started thinking maybe it was a factor I had overlooked for my PRS ammo.

My reloading room is almost always very close to 30% humidity. This is lower than almost every place I travel to by a significant amount. After travelling to Tennessee in December and having my ammo exposed to 80%+ humidity for several days I had a slight drop in velocity that cost me a few points on day 3 of the AG cup. After returning home and having my ammo sitting on a shelf in the ~30% humidity for about a week I went to the range and discovered that my velocity was back up to where it was in the first 2 days of the AG cup.

Here is the question I plan to test: Does the powder in loaded ammo subjected to a different humidity level acclimate to that new humidity level? If so, how long does the ammo have to be exposed before the change is significant for PRS purposes?

I have 2 sets of ammo loaded and in sealed clear plastic ammo boxes. One set with a 32% humidity pack (closely matches my reloading conditions and the humidity the Varget was acclimated to) and another set with 62% humidity control pack. I have humidity meters in both that are visible through the plastic. I'm going to shoot both sets of ammo approximately 52 hours after I loaded them and sealed them in the boxes. If they both shoot very close to the same average velocity I probably don't have to worry about trying to control humidity while travelling to higher humidity areas for matches.

Either way I'll report back and update this thread. Any predictions?

 

[spife7980]

A thread on here awhile back had this guy doing the same thing

How does humidity affect powder?

You may have heard about a relationship between humidity and bullet velocity either in a book, on a reloading forum, or from a crusty benchrest shooter right after saying they get better ES/SD&#821…

chronoplotter.com

 

[Mordamer]

A thread on here awhile back had this guy doing the same thing

How does humidity affect powder?

You may have heard about a relationship between humidity and bullet velocity either in a book, on a reloading forum, or from a crusty benchrest shooter right after saying they get better ES/SD&#821…

chronoplotter.com

I read through that study, but it doesn't seem to answer the question I'm worried about. Does non-hermetically sealed ammo acclimate to humidity changes rapidly enough to affect performance at a PRS match?

To me, if it doesn't change in 2 days time I'm probably ok.

 

[Rio Precision Gunworks]

I think you’ll likely find the answer to be “sometimes” if you performed it enough times or did it with enough rounds.

As none of your ammo will be purposely hermetically sealed, some will be and some won’t be just by happenstance.

And then it would be luck of draw at a match. If the target is small enough and/or far enough away, and you happened to pick up that round or two that wasn’t sealed as well…….then the stars align and you drop a point or three.


That’s just my off the cuff thoughts. Very well could be wrong.

 

[Mordamer]

I think you’ll likely find the answer to be “sometimes” if you performed it enough times or did it with enough rounds.

As none of your ammo will be purposely hermetically sealed, some will be and some won’t be just by happenstance.

And then it would be luck of draw at a match. If the target is small enough and/or far enough away, and you happened to pick up that round or two that wasn’t sealed as well…….then the stars align and you drop a point or three.


That’s just my off the cuff thoughts. Very well could be wrong.

Yes. The thought occured to me that if some are sealed enough to not acclimate and other rounds do then it would be a worse case scenario. I'd rather have them all acclimate and be able to maintain consistency.

 

[RegionRat]

I have humidity meters in both that are visible through the plastic. I'm going to shoot both sets of ammo approximately 52 hours after I loaded them and sealed them in the boxes.

You may want to consider making the dwell time longer, even if you think you can manage travel ammo better with a shorter trip storage.

In order to know if the storage dwell time is a real risk and to make the test more clear, I would suggest you start with a longer soak for that higher humidity ammo. This will show the total potential change if you give it more than long enough to change while just sitting at home without big pressure swings and temp changes, bot of which can stimulate "breathing" within the ammo.

It is very difficult to predict how long it takes humidity to penetrate the tiny capillary leaks. There are many factors involved that include the size of the leak and the external temps and pressures during travel, and the difference of the water content of that air.

Knowing what the total potential of ammo that is just sitting is one thing you can nail down and easy if you give it time, guessing at the travel conditions is another. There could be differences based on your container, the roads or flying, etc.. all of which are difficult to simulate in combinations.

Experimentally my suggestion doesn't offer the answer to the second situation, but it does tell you what the potential limit difference could be.

I always recommend loading near as possible to 50% RH conditions because it splits the potential change to be as small as possible if you can't predict the trip. I also always recommend getting a speed and local dope as near to the important shooting as possible. YMMV

 

[spife7980]

I read through that study, but it doesn't seem to answer the question I'm worried about. Does non-hermetically sealed ammo acclimate to humidity changes rapidly enough to affect performance at a PRS match?

To me, if it doesn't change in 2 days time I'm probably ok.

Youre right now that I re-read it. Not the one I was thinking it was. I thought experiment 2 was the loaded ammo storage link... off to the search machine I go.

 

[Mordamer]

Meh... I'm gonna have to call it a nothing burger. The test needs a larger sample size to be conclusive. I only used 10 shots each. I fired all shots at 1,000 yards and elevation was very consistent. The majority of the shots hit a 10 inch plate and the misses were mostly left or right and not from vertical error. Maybe I'll repeat this with a higher top end humidity and more shots. I fired 5 shots of one then 5 of the other. I let the barrel cool and then repeated.

32% Humidity Shots
2900
2879
2880
2877
2876
2884
2889
2869
2894
2877
AVG=2882
ES=31
SD=8.868483523

62% Humidity Shots
2899
2886
2868
2882
2888
2915
2893
2901
2875
2882
AVG=2889
ES=40
SD=12.96495276

 

[memilanuk]

Running the numbers thru a spreadsheet...

Anova: Single Factor
SUMMARY
Groups	Count	Sum	Average	Variance
0.32	10	28825	2882.5	87.38888889
0.62	10	28889	2888.9	186.7666667
ANOVA
Source of Variation	SS	df	MS	F	P-value	F crit
Between Groups	204.8	1	204.8	1.494042312	0.2373553324	4.413873312
Within Groups	2467.4	18	137.0777778
Total	2672.2	19

Assuming I did that correctly, it looks like there is no meaningful difference (P-value not <0.05, F < F-crit), or at least not enough to pick out of the noise, given the data.

 

[Huskydriver]

@Doom thoughts here?

 

[secondofangle2]

The case that I filled with 150 g of water two weeks ago today has not lost even a fraction of a grain of weight in my garage where the temperature is between 80 and 105° and the relative humidity is 20%.

My conclusion is that at a stable atmospheric pressure, and across those temperature and humidity ranges, it is going to take much longer than two weeks for any moisture to escape the case.

The only way traveling to a match is going to have an effect, is if flying with the ammo in the unpressurized cargo area of the airplane causes offgassing of moisture

 

[RegionRat]

The only way traveling to a match is going to have an effect, is if flying with the ammo in the unpressurized cargo area of the airplane causes offgassing of moisture

Most of the modern commercial planes do a decent job of controlling pressurization. For perspective, a standard atmosphere is about 1013.25 mb or 14.7 PSI for sea level. So, even if you took the ammo to a rough vacuum, the pressure difference is only 14.7 PSI.

At about 35,000 feet at -60C, the pressure is roughly 1.86 PSI, or a difference of 12.84 PSI.

To see if neck and primer seal holds pressure, a vacuum bell jar with a roughing pump will answer the question pretty quick, but required a vacuum grade oil.

We used to degas PFPE oil and then drop the ammo into the beaker and pump down to watch for bubbles. No bubbles meant it was going to take a very long time to dry/wet the propellant. Lots of bubbles meant someone screwed up.

The problem with that test at home is, even if you had a bell jar and roughing pump at your disposal, the PFPE is very expensive oil and not something you can get a HomeDepot.

 

[ma smith]

Holy smokes $250/kg you aren't kidding

What is the density of PFPE oil?
between 1.8 and 1.9 kg/liter

Krytox PFPE Oils

Krytox PFPE Oils Krytox PFPE oils are sold by the 2 lbs (1 Kg ~0.5 liter) containers for use in chemical processes which require inert mechanical vacuum pump oil fluids. Krytox 1506 (H11307018) oil is specified by Edwards to meet Rotary Vane chemical inert vacuum…

highvacdepot.com

 

[JerkyFreak]

How low do you need the vacuum to go?!

You could in practice use any hydraulic oil for the pupose of checking a cartridge. You could probably use water in a pinch. It's not like you are taking your bullets to space.

 

[Doom]

The results show no statistically significant difference in the mean velocity. For a single tailed assumption of 2882<2889 using Welch's T-Test the p is 0.09. For 2882 not equal to 2889 the p value is 0.18.

 

[secondofangle2]

Yeah but it’s approaching stat sig and we need a power calculation for the next experiment

 

[RegionRat]

How low do you need the vacuum to go?!

You could in practice use any hydraulic oil for the pupose of checking a cartridge. You could probably use water in a pinch. It's not like you are taking your bullets to space.

Trying not to side-track the thread... but the only point to bringing up the vacuum issue was as a way to quickly spot the integrity of a sealant on a cartridge.

Not necessary for a home experiment, but can stop you from wondering if the round is "sealed" or not.

How low? Any roughing pump will do in a pinch, remember we are only leveraging the 14.7 PSI of the pressure inside the cartridge this way.
If it shows bubbles, it means there is a gross leak and you can expect trouble down the road.

 

[Bandit320]

Most of the modern commercial planes do a decent job of controlling pressurization. For perspective, a standard atmosphere is about 1013.25 mb or 14.7 PSI for sea level. So, even if you took the ammo to a rough vacuum, the pressure difference is only 14.7 PSI.

At about 35,000 feet at -60C, the pressure is roughly 1.86 PSI, or a difference of 12.84 PSI.

To see if neck and primer seal holds pressure, a vacuum bell jar with a roughing pump will answer the question pretty quick, but required a vacuum grade oil.

We used to degas PFPE oil and then drop the ammo into the beaker and pump down to watch for bubbles. No bubbles meant it was going to take a very long time to dry/wet the propellant. Lots of bubbles meant someone screwed up.

The problem with that test at home is, even if you had a bell jar and roughing pump at your disposal, the PFPE is very expensive oil and not something you can get a HomeDepot.

Most airline cabin pressures never go above 8,000’ pressure altitude. Otherwise the pax’s would be passing out. Only exception I know of is if you’re talking off or landing at an extremely high altitude airport; Bogota, Quito, LaPaz, etc. Even then as soon as you’re airborne it comes right back to 8’000 feet or so.

If you’re really worried about it just put your ammo in a marginally inflated large viploc bag with a humidor pack that matches your loading conditions. One of those bags people use to compact their clothes for storage would work.