Powder Moisture Content — Did You Know?
Variations in moisture content change the burning rate of a powder and thereby chamber pressures and muzzle velocity. The moisture content of the Vihtavuori N100 and N300 series powders is usually around 1%, while the N500-series’ normal moisture content is 0.6% because of the added nitroglycerine.
So what difference does moisture content make? Here’s an example. In a test, a [Vihtavuori] powder sample was dried by heating it, losing about 0.5 % of its weight. Cartridges were then loaded with the dried powder and fired using a pressure gun. Chamber pressures and muzzle velocities produced by these special cartridges were compared to those produced by cartridges loaded with untreated powder. (The powder charge and bullet were of course the same in both sets of cartridges.)
After Powder Drying:
Pressure Increased 11% from 320 MPa to 355 MPa
Velocity Increased 2.6% from 2526 to 2592 FPS
Comparing results showed chamber pressures increased from 320 MPa to 355 MPa with the dried powder, and the muzzle velocity increased accordingly from 770 m/s to 790 m/s (2526 to 2592 FPS). And note, this is only one example, of one caliber and loading. The difference might be much higher depending on the cartridge and loading combinations.
Recommendation: Store powder below 68°F in 55-65% humidity.
What does this tell us? Well, it seems we need to forget the old saying “Keep your powder dry”! Instead, focus on proper powder storage, at a temperature below 20°C/68°F and humidity between 55 and 65%. Safe reloading everybody!
Tech Tip sourced by EdLongrange. We welcome reader submissions.
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.
In this video, Bryan Litz of Applied Ballistics talks about Density Altitude and the effect of atmospheric conditions on bullet flight. Bryan explains why you must accurately account for Density Altitude when figuring long-range trajectories.
Bryan tells us: “One of the important elements in calculating a fire solution for long-range shooting is understanding the effect of atmospherics. Temperature, pressure, and humidity all affect the air density that the bullet’s flying through. You can combine all those effects into one number (value) called ‘Density Altitude’. That means that you just have one number to track instead of three. But, ultimately, what you are doing is that you are describing to your ballistics solver the characteristics of the atmosphere that your bullet’s flying through so that the software can make the necessary adjustments and account for it in its calculations for drop and wind drift.”
Bryan adds: “Once you get past 500 or 600 yards you really need to start paying careful attention to atmospherics and account for them in your ballistic solutions”. You can learn more about Density Altitude in Bryan’s book, Applied Ballistics for Long Range Shooting (Third Edition).
General Scientific Definition of Density Altitude
Density altitude is the altitude relative to the standard atmosphere conditions (ISA) at which the air density would be equal to the indicated air density at the place of observation. Density altitude can be calculated from atmospheric pressure and temperature (assuming dry air). Here is the formula:
Air is more dense at lower elevations primarily because of gravity: “As gravity pulls the air towards the ground, [lower] molecules are subject to the additional weight of all the molecules above. This additional weight means the air pressure is highest at sea level, and diminishes with increases in elevation”.*
Both an increase in temperature, decrease in atmospheric pressure, and, to a much lesser degree, increase in humidity will cause an increase in density altitude. In hot and humid conditions, the density altitude at a particular location may be significantly higher than the true altitude.
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.
Here’s a smart new product that monitors the temperature and humidity inside your gun safe — with a convenient LCD display unit located on the outside of the safe. You don’t need to string wires or cut a small hole in your safe — there are two separate components, one inside and one outside. The sensor unit (on the inside) communicates wirelessly with the display unit (on the outside).
The new GoldenRod Wireless Hygrometer was designed to display the temp/humidity in your safe without the need to open the safe. NOTE: the wireless LCD display can show BOTH in-vault AND in-room humidity and temperature levels. You can attach the display to the vault door with its built-in magnet, or simply place the display unit on top of the safe using the handy flip-out kickstand. The unit costs just $20.89 at Amazon.com.
GoldenRod Wireless Hygrometer Specifications and Features:
Measures In-Vault and In-Room humidity from 20% to 95%.
Measures In-Vault temperature range from 14°F to 122°F.
Measures In-Room temperature range from -4°F to 158°F.
Records Min/Max temperature and humidity history.
Product Tip from EdLongrange. We welcome reader submissions.
Tests Show Burn Rates Vary with Water Content
Here’s a smart new product that monitors the temperature and humidity inside your gun safe — with a convenient LCD display unit located on the outside of the safe. You don’t need to string wires or cut a small hole in your safe — there are two separate components, one inside and one outside. The sensor unit (on the inside) communicates wirelessly with the display unit (on the outside).
