light powder charges

[Vintage stuff]

Hello there, when I am using a new powder I will set the micrometer on the powder measure(RCBS or Redding both with micrometers) to the manuals start to max in half grain increments write the settings down and stick them on the powder measure and have noticed that over time the same micrometer settings throw lighter charges (as much as 2 1/2 - 3 grains on the original 39 grain setting) as if the powder is drying out so to get the same weight you would have to increase the volume, the question is would the energy produced by a set Volume of the fresh powder be the same as the energy produced by the older lighter powder in that same Volume or is the energy produced dependent on the weight. what are your views Dave

[FredB]

You should never believe the settings on the powder measure. The measure should be set up with scales every time you use it.
Fred

[snayperskaya]

You should never believe the settings on the powder measure. The measure should be set up with scales every time you use it.
Fred

^This....I could never get a consistent charge with the measures I tried, especially with powder like Vihtavuori N140, so now I just weigh every charge individually.i does takes more time but for me it isn't an issue as I don't tend to load that many rounds at a time, and I actually find it quite therapeutic!

[Vintage stuff]

Fred, I check weigh the first 5 or 6 throws of the measure on a 5 0 5 scale and regularly throughout the session, I also use an rcbs powder check die and any that don't line up get weighed, the reason I brought up the lighter powder throws as the powder gets older is (A) has anyone else noticed this and (B) that I was curious as to whether if the acetone or whatever solvent is used evaporates off does the powder loose some of it's energy. Dave

[WelshShooter]

I also use a micrometer setup for my RCBS uniflow, but only use it for ball powder. I record the micrometer setting purely for quick setup on my next load session. I always verify the load weight on the balance beam rather than rely on the micrometer, for the reason you've found out.

Even with ball powders, I've noticed that it's not "dead on" every time I reuse the powder, but I've never really thought about trending any difference since the tub was opened.

[hitchphil]

naw....problem solved take up Match Rifle .......... you get 999.9% full charge when you compress the round in

[rox]

the question is would the energy produced by a set Volume of the fresh powder be the same as the energy produced by the older lighter powder in that same Volume or is the energy produced dependent on the weight. what are your views Dave

No one seems to have answered the question directly, but you're onto an interesting & valid point. Solvents used in propellant manufacture do evaporate over time, causing the bulk density to change but with no significant effect on the energy content. Consequently, a particular weight of an old powder can have a higher energy content than the same weight of that same powder when it was new - and taken to extremes a 'safe' charge by weight can become an excessive one.

We use weight because it offers an easy way to achieve a calibrated & absolute measurement that we can compare with data from various sources, and while it offers a reasonably consistent energy quantity over the course of a loading session, there is scope for variation in energy over time as powder changes (and from batch to batch). In such a situation I propose the opposite to the previous advice: using the volumetric measure to identify the required weight of powder to duplicate an established load (or use the volumetric measure, full stop). This assumes your volumetric measure can be set with sufficient repeatability, and usual processes for consistent measure operation (settling cycles, averaging, consistent level, use of baffle, consistent stroke etc). In reality, however, if powder is stored in good conditions (sealed containers, low temperatures, small quantities/not stored for excessive periods) the effects are likely to be minimal, and in practice, if the powder has changed too much then the proper and safe procedure would be to work-up the load again.

It's interesting to note that almost all other chemical sources of stored energy (e.g. fuels) are volumetrically metered.

As a side note, on the rare occasions I load using a volumetric measure I am always amazed at the resulting performance - even back to 1000 yards, and it's only because I can dispense by weight with very high accuracy & speed that I continue to do so almost exclusively. Perhaps, now my competition season is over, it's time for some investigation and comparison.

Here is some more detail, including practical examples that suggest an observed difference in energy content of 7% between the same weight of different batches of N150 due to variations in bulk density:
http://thehunterslife.com/forums//showt ... hp?t=13541

[hitchphil]

Weight is a relative measure of chemical mass (moles for those that remember their chemistry) given the propellant does not need gaseous oxygen to react & give off its energy, then molar mass is the way to measure energy input.

To measure it you need a thing called a bomb calorimitor (not as dangerous as it sounds) its a sealed vessel where the reaction takes place & temperature change & pressure are accurately measured (so PV-nRT can be calculated & Gibbs free energy calculated from that).

The other way is a Differential Scanning Calorimitor - heat a very small sample up in a sealed can & measure the heat liberated (the exotherm vs TDegC the area under the curve generated is Energy) . I used to have access to these & now wish I reloaded then, some quiet unclaimed overtime would have been had!

These are how fuels, chemicals are tested. Volume is fixed so eliminated from the calcs.

Measuring energetic compounds in volume is just convenient, but its at the mercy of: packing density (related to shape of the powder), settling (vibration is used to do that), temperature (both the material & the measuring vessel) and changes in chemical properties.

e.g. the expansion coefficients of some polymers is in the 100's of ppm (micron/meter) so 1DegC change in temperature of the measuring hopper can result in disproportionately larger changes in it volume. (larger funnily enough as they get hotter because circles (i.e. a cup) expand circumfrentially) so a load poured on a 10DegC day will have measurably less compound than on a 30DegC day.

So for bulk chemical stuff - (bombs -should anyone be making any this week?) we work in 10's/100's Kg's & for liquids we work in volumes Liters or Gallons.

NB the pic above has been done in real life & the variation in elevation achieved at 1000yds out of a rifle was impressively small!

[Vintage stuff]

Thank's Rox and Hitchphil for the replies and that link makes for interesting reading and answers my question in that a given volume of powder from the same tub whether fresh or older and weighing less for the same volume generates the same energy. Dave