If we look at a VMD (Volume Measure Density) powder dispenser I guess we should look at the variables, the things that can or may change and what effects those changes. I have several means of measuring a powder charge at my disposal. I guess the
RCBS Uniflow is as good as any for an explanation. The dispenser has a cylinder, an adjustable cylinder and the cylinder has a volume. The cylinder volume set by the user. When we "throw" a powder charge we refer to it as metering and as was mentioned and as we all know some powders "meter" better than others. Once we adjust the volume of our powder throw our main concern is not just the weight of the charge we throw but how well the device repeats. Repeatability is the big deal here right?
VMD Explanation
The Volume Measured Density (VMD) of a powder is the volume in cc's (cubic centimeter) that one grain of powder occupies. This can be used to calculate the dipper, disk cavity or powder measure setting required to obtain a desired weight of powder.
The charge weight in grains * VMD = Charge Volume in Cubic Centimeters
According to the nice people at Lee (they are big on this volume thing) the VMD for IMR 4895 is 0.0728 so if I want to throw a 40 grain charge I need a volume of: 40.0 * 0.0728 = 2.912 cc. While this looks good and simple we run into problems.I can set the volume of my cylinder to get the correct charge weight but getting repeatability can be a problem. My dispenser has a hopper full of powder, a column of powder. That column of powder has a weight pushing down and with each cycle of a handle the weight of the column pushes powder down into my cylinder's set volume. Unfortunately with each powder throw the weight of the column is reduced. Enter the ever popular baffle. The idea of using a
powder baffle is to keep the weight of the powder pushing down a constant. Even with this improvement it comes down to, as was mentioned, some powders meter better than others. Spirical and Ball powders tend to meter better than stick powders. Everyone is familiar with the "crunchy" sounds powders like IMR 4064 or IMR 4895 make as the powder measure cuts those sticks of powder. The volume of the cylinder isn't going to change but how much powder we get in it can. My observations here are that better machined powder throws like
THE HARRELL POWDER MEASURE tend to do a better job of repeatability than those made of more plastic than machined steel. They also carry a hefty price tag to match their reputation for repeatability.
Finally there is the powder itself. I can't say I read it on the Internet so it is fact but in the link I used earlier for VMD they also mention:
It is very important that you repeat this process with any new container of the same powder because the powder companies allow themselves a 16% tolerance between batches. This can result in over charging if you work from the same setting and the next container of powder you get is more dense.
So while the volume of the cylinder is not going to change, not enough to matter, there are plenty of other variables acting on the measurement process, some we can control for example the use of a baffle and some more difficult to control. The question becomes will any of the variables have enough effect on the repeatability of our process to matter. Using a good VMD dispenser I doubt it if we just follow good hand loading practices.
In the time it took me to write this about 4 more good post happened. Darn, I type slow.
Rule 3:
What about altitude, relative humidity and of course temperature?
Absolutely they will have an effect, the question is will the effect be enough to come in light of the intended application? I doubt it as long as the temperature and humidity are in reasonable limits. I define that as a creature comfort zone.
Just My Take....
Ron
