Humidity and Powder Burn Rates — What You Need to Know
Most shooters realize that significant changes in temperature will alter how powders perform. That’s why you want to keep your loaded ammo out of the hot sun, and keep rounds out of a hot chamber until you’re ready to fire. But there are other factors to be considered — HUMIDITY for one. This article explains why and how humidity can affect powder burn rates and performance.
We’ve all heard the old adage: “Keep your powder dry”. Well, tests by Norma have demonstrated that even normal environmental differences in humidity can affect the way powders burn, at least over the long term. In the Norma Reloading Manual, Sven-Eric Johansson, head of ballistics at Nexplo/Bofors, presents a very important discussion of water vapor absorption by powder. Johansson demonstrates that the same powder will burn at different rates depending on water content.
Powders Leave the Factory with 0.5 to 1.0% Water Content
Johansson explains that, as manufactured, most powders contain 0.5 to 1% of water by weight. (The relative humidity is “equilibrated” at 40-50% during the manufacturing process to maintain this 0.5-1% moisture content). Importantly, Johansson notes that powder exposed to moist air for a long time will absorb water, causing it to burn at a slower rate. On the other hand, long-term storage in a very dry environment reduces powder moisture content, so the powder burns at a faster rate. In addition, Johansson found that single-base powders are MORE sensitive to relative humidity than are double-base powders (which contain nitroglycerine).
Tests Show Burn Rates Vary with Water Content
In his review of the Norma Manual, Fred Barker notes: “Johansson gives twelve (eye-opening) plots of the velocities and pressures obtained on firing several popular cartridges with dehydrated, normal and hydrated Norma powders (from #200 to MRP). He also gives results on loaded .30-06 and .38 Special cartridges stored for 663 to 683 days in relative humidities of 20% and 86%. So Johansson’s advice is to keep powders tightly capped in their factory containers, and to minimize their exposure to dry or humid air.”
Confirming Johansson’s findings that storage conditions can alter burn rates, Barker observes: “I have about 10 pounds of WWII 4831 powder that has been stored in dry (about 20% RH) Colorado air for more than 60 years. It now burns about like IMR 3031.”
What does this teach us? First, all powders start out with a small, but chemically important, amount of water content. Second, a powder’s water content can change over time, depending on where and how the powder is stored. Third, the water content of your powder DOES make a difference in how it burns, particularly for single-base powders. For example, over a period of time, a powder used (and then recapped) in the hot, dry Southwest will probably behave differently than the same powder used in the humid Southeast.
Reloaders are advised to keep these things in mind. If you want to maintain your powders’ “as manufactured” burn rate, it is wise to head Johannson’s recommendation to keep your powders tightly capped when you’re not actually dispensing charges and avoid exposing your powder to very dry or very humid conditions. The Norma Reloading Manual is available from Amazon.com.
| Real-World Example — “Dry” H4831sc Runs Hotter
Robert Whitley agrees that the burn rate of the powder varies with the humidity it absorbs. Robert writes: “I had an 8-lb. jug of H4831SC I kept in my detached garage (it can be humid there). 43.5-44.0 gr of this was superbly accurate with the 115 Bergers out of my 6mm Super X. I got tired of bringing it in and out of the garage to my house for reloading so I brought and kept the jug in my reloading room (a dehumidified room in my house) and after a few weeks I loaded up 43.5 gr, went to a match and it shot awful. I could not figure out what was going on until I put that load back over the chronograph and figured out it was going a good bit faster than before and the load was out of the “sweet spot” (42.5 – 43.0 gr was the max I could load and keep it accurate when it was stored in less humid air). I put the jug back in the garage for a few weeks and I now am back to loading 43.5 – 44.0 gr and it shoots great again. I have seen this with other powders too.” If you have two jugs of the same powder, one kept in a room in your house and one somewhere else where it is drier or more humid, don’t expect the two jugs of the same lot of powder to chrono the same with the same charge weights unless and until they are both stored long enough in the same place to equalize again. |
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Tags: Burn Rate, Humidity, Norma, Powder, Propellant, Temperature, Water Content
Interesting article but it lacks the all important answer to how much of a difference in fps are we talking about.
@ Guy4064 : Variation in fps can vary due to size of charge , amount of humidity increase or decrease in the powder used . I’m sure that storing powder in my un-air conditioned garage is not a wise thing to do in Arizona . Especially in the summer months .
Another variable to consider is ; How close to maximum is someone loading their favorite load , as that can easily take your load out of the , “sweet spot” . Don’t think there is a one size fits all rule in play here .
I don’t understand how a sealed container can be effected by outside humidity or temperature changes. If you left the powder in your hopper or the cap off the container, it would obviously undergo some absorption or reduction based on humidity. Sealed up it shouldn’t be effected.
This article poses the obvious question. Why haven’t powder manufacturers researched containers which do a better job of isolating powder from the effects of environmental changes in the expected range of conditions under which their products are stored?
Alternatively, someone else might explore the manufacture of what amounts to a humidor for powder jugs. Store the powder in it. Remove it to load and then return it to your temperature and humidity controlled storage device.
What would also be interesting to investigate would be the effect of air travel on loaded Ammunition. Even in an aircraft which maintains the same air pressure as the passenger compartment the usual cabin pressure altitude is 8,000 feet.
The containers are designed specifically for safety in the event of a fire. That material has to breathe.
This article is very general and most know that this has an effect. The article also only give a sample-of-one anecdotal example without quantifying the change or any supporting data.
The anecdotal example also lacks specificity in what other measures were taken to eliminate other variables which could create velocity change (temperature, bore condition,chronograph error etc.)
I don’t see much difference in powder containers made of cardboard vs. plastic when it comes to resistance to fire, they’re both going to burn. While cardboard’s outdated as a powder packaging material I can fully understand why metal cans aren’t used anymore.
I find it hard to believe that 4831 could turn into 3031. You would think old 30-06 ammo would be blowing up 30-06 rifles more than we hear about. Here in N.Az we have radical temp. extremes and very low humidity. Kind of makes me nervous about my powder storage methods. Every oz. of my powder is in a safe in my garage but after reading this I don’t know
I hope you keep the door open on your safe in the garage. If not, you have a bomb out there.
Water vapor passes through just about everything, including plastic. Exceptions are metal and glass. Note metalized polyester used in food packaging to keep moisture out. Once its in the cartridge, and the cartridge is sealed, there is a very limited path for moisture ingress. A plastic jug is like a screen door to water vapor.
Primarily powder moisture content is not affected by relative humidity but by partial pressure of the water in air. Relative humidity is not a measure of partial pressure (dew point is the direct measure) but is a measure of the percentage of total moisture that the air can hold at a given dry bulb temperature.