Tech Tip: Changes in Humidity Can Alter Powder Burn Rates
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, Dew Point, Humidity, IMR, Moisture, Norma, Powder, Robert Whitley
This is a VERY interesting and valuable article. I’ve always wondered about this very thing. I’m happy to read this article sourced from a credible ballistician from Norma. We were just speculating about this in the pits today. One further question I have (if Johannson could be called on again); how long does the powder have to be exposed to a different humidity environment to have it’s burn rate affected by humidity? Typically, competition shooters go thru one or two 8 lb jugs of powder a year. Could the 2-3 months (or so) from the time a keg is ‘cracked’ until it’s gone affect it’s burn rate if stored in a different humidity environment; or does it take years for powder to absorb or expel moisture?
Thanks again for a very informative article!
-Bryan
Another question. Lets say a powder starts life from the manufacturer at 10% RH. If that powder is left open and comes up to 50% RH for a time period. This powder then is dried out back down to 10% RH like when it was made. Will it have the same burn rate as it did when it left the factory or is there a humidity/time degradation?
-Dan
I worked in the chemical industry for over twenty years and was aware of this problem.
As such I have always bought half a kilo at a time and always store in the same spot in a cupboard.
If the caps are left on tightly how does it change in moisture content
Time/exposure/effect is presented via graphs in the book, if I recall correctly.
Not knowing the velocity change makes it hard to tell if its a issue or not. I known if you leave the powder out on the scale overnight its not unusual to see a different weight the next day. As they weigh the powder it could more of a difference in weight then burn rate. Just to muddy the situation lol.
So what is the best temperature and humidity to keep my powder storage room at to keep the burn rate like it is supposed to be?