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October 16, 2008, 05:38 PM | #1 |
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Do You Calculate VMD of Individual Powders?
Discussion about the use of the Lee Auto-Disk in a different thread got me thinking about my own reloading practices, and I was curious if I'm the only one who takes the time to figure out the VMD of specific lots of powder as I use them?
What I do basically is set up my Auto Disk with the largest CC single disk (1.57), drop 5-6 charges at that CC, and then weigh each to verify the actual charge in grains. From there it is just a little math using the formula: Charge in grains x VMD= CC. In this case, you divide 1.57 (the volume of the charge) by the weight of the charge in grains, and you have the VMD of that powder. As an example, the current lot of Unique I'm using has a VMD of .1283 even though Lee lists Unique's VMD at .1092, so that the 1.57 CC disk drops a 12.2 charge rather than the 14.4 charge the Lee charts show. Similarly, my H335 has a VMD of .0669 while Lee lists it at .0645, and the 1.57 disk drops a 23.5 grain charge versus the 24.3 grain charge their charts list. After I figure out the VMD of my lot, I then label the canister with the VMD of that powder so that basically all I have to do is multiply that VMD with my desired charge weight, and in turn I know what CC I need on my Auto Disk to get that exact charge. So I guess I'm wondering, am I the only one who does this and does anyone else do something similar to aid in their loading process? This has helped me use my Auto Disk with less frustration and more predictability, and also enabled me to use powders like TAC for which Lee has no information listed. |
September 24, 2019, 02:14 PM | #2 |
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VMD as it relates to powders and lots within a particular type.
I have read the many posts regarding VLD values and charts mostly regarding LEE powder measures and disk type volumetric powder dispensers. I too am interested in noting the VMD of all my powders and like the fact that Lee has taken the time to publish these relative values of the many types of reloading propellants. Reloading in this respect is similar to baking; you wish to use the same ingredients for future success. I am familiar with the many types of both volumetric and weight based powder measuring and own several types and brands. My Belding and Mull of 45 years does an outstanding job as a volumetric measure especially for long stick powders such as H4831. RCBS has a beam scale with digital readout for a weight based measure in grains with Lee demonstrating both their disk type and drum type auto measures. Reloading powders are deliquescent or have the ability to both absorb and decline moisture. Powder charges in the summer months tend to weight more in a given volume than during the winter but may have less powder. This is due to heat and humidity adding moisture to the powder giving a reading in weight based on volume a higher setting. This same volume (CC or cubic inches) will differ in actual weight in that it will weigh less in drier cooler winter. If a load created in the winter is fired during the summer, the weighed powder charge will show more powder weight, hence a hotter load as there is no filler of water.Those who wish to use VLD incrementation must be aware of this and use VLD only as an approximation as to which disk or drum to use in measuring. This is also important when working up max or near max loads. As a reloader and with knowledge of physics regarding volumes,density and weight I realize the many variables present and make decisions based on past knowledge and the advice of manufacturers.
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September 24, 2019, 02:48 PM | #3 | |
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Quote:
FWIW I'm a stickler for my charge weights and the discs , dippers or any none adjustable volume based measure has never thrown a charge I want , not one ! So I went to the charge bar for my auto disc and auto drum as well so now that math really doesn't mean anything to me because all my measurers are adjustable . I get it though . I love knowing little facts like the thing you describe . I do that stuff all the time but in this case , it's not much help to me . Also doesn't powder weight change with in the same bottle based on how dry or humid it is out and how much the powder has absorbed through out that's bottles life . wouldn't that throw off your math a bit depending on what time of year you test the powder or if your the type to leave it in your powder measure ? resulting in the powder in the bottle likely have more or less moisture in it then the stuff left in your powder measure ?
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September 24, 2019, 03:24 PM | #4 |
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If you peruse pages for the powders Western distributes, they give VMD's with a tolerance for their Ramshot and Accurate lines. The Ramshot is all labeled ±3.6%, but a couple of the Accurate powders vary as much as ±5.6% from one lot to the next, and some as little as ±2.2%.
Weight, however, does not offer a more accurate approach unless the powder is kept in reasonable humidity (50-70% range is good, and should reflect your environment). Norma points out that dropping the humidity powder is stored in from to 0% from 80% will raise the burn rate 12%, and that can, obviously, throw published weight or volume-based recipes off. IIRC, powder can absorb about 1.5% of its weight in water in near 100% RH. Norma also mentions that when a cartridge is exposed to a change in humidity, water molecules are so small they can still get in and out of the case and over about 12 months the powder in the cartridge will equilibrate to the moisture level it would have if it were stored in that changed cartridge humidity before it was loaded. Some folks who load and test in the humid season and then, to preserve the ammo, seal it into a bag with a desiccant to store it for a number of years will eventually find it pretty spunky and no longer tuned to their gun if they open it up to shoot it without waiting another year. If I were building some ammo for long-term storage today, I would desiccate a sample of the powder for the load workup before loading to store it like that. The plus side of checking the disk weight is that I would bet you'll find, at least for stick powders, that the VMD number you get with a big cavity is not the same number you get with a small insert. In the small insert, any corner packing issues the powder has in the disc cavity will be a larger percent of the whole. I think I would use a small, medium, and large disc and see if you can't find the trendline fit to go by in Excel. If there error isn't significant (it probably won't be with spherical propellant) then you have your number and can ignore the rest. You also want to know how the powder handling gear behaves. Here is the same charge of stick powder dispensed with different drop tubes. As it packs tighter, the effective burn rate is decreased because the flame front has a harder time passing through the smaller grain interstices. What VMD would you use?
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September 26, 2019, 01:41 AM | #5 | |
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Quote:
Would one of you fine gentleman care to enlighten the casual reader?? The usual convention is that the first time a term is mentioned, it is written out, with an explanation, if needed, then after that its ok to just use the initials. thanks
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September 26, 2019, 02:00 AM | #6 |
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Volumetric Measured Density
https://www.titanreloading.com/powde...md-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.
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September 26, 2019, 09:30 AM | #7 |
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I make graphs for each powder that have the settings of my 10X powder measure and charge weights.
Any charge weight can then be just dialed in on the micrometer head of the 10X. Saves a lot of time. Of course I double check with a scale. |
September 26, 2019, 04:34 PM | #8 | |
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Do You Calculate VMD of Individual Powders?
No because I have no need to. I weigh my powder charges. That said in this read on the subject they do mention: Quote:
Finally as Unclenick's images clearly show it depends on how compressed the powder actually is. I have used Lee Dippers and the Lee VMD chart only to find out that my actual charge weights do not necessarily agree with the chart. The below data was collected using a 2.8cc Lee Dipper. The powder is H335 the numbers reflect both leveled and non leveled charges. Just my take on all of it. Ron Ron |
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September 26, 2019, 09:44 PM | #9 |
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Reloadron : H-335 might be the best powder you could use for volume testing . Try that test again but this time use IMR 4350 or 4064 and see if there is a difference . My guess is the SD will be so big most would never load those powders that way . I know when I'm throwing either of those powders I can get as much as a +/- .6gr so and ES of 1.2gr . Now that's not the norm but I have seen it . I think I avg maybe +/- .35gr
My take is if the powder companies or the manufacture's that print reloading manuals thought volume was important or best they would write there load data using volume rather then weight . Seeing how none of my manuals (5) use volume in there load data , I'll keep with weighing my charges even when throwing from a powder measure . I think it's a cool little fact but has no importance to me .
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September 27, 2019, 07:16 AM | #10 |
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Metal_god, maybe later today I'll give that a try. I know I have some open IMR 4064 laying around here. I am guessing just as you do. Also, like yo0u I see it as interesting but it really does not pertain to me.
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September 27, 2019, 11:16 AM | #11 |
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OP is 11 years old. Why anybody would bother with something as useless as The Volume Measured Density is beyond me.
Those Lee scoops can vary the powder charge plus or minus a full grain. And a cubic centimeter is a metric unit of LIQUID measure.
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September 27, 2019, 11:26 AM | #12 |
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Been reloading over 50 yrs and have never heard of VMD before! What the H*ll is that?
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September 27, 2019, 01:14 PM | #13 | |
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Quote:
CC is commonly used for liquid volumes but it is not a unit of liquid measure.
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September 27, 2019, 01:33 PM | #14 | ||
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Metal_god, I ran some numbers with IMR 4064 and got pretty much as expected.
The same 2.8cc dipper was used. @ T. O'Heir Quote:
Quote:
The point of the discussion is to point out that yes, obviously the Lee Powder Dippers will have a margin of error up to a grain of powder and that the VMD of powder can vary lot to lot at time of manufacture. <EDIT> I see 44 AMP covered the volume thing before I clicked and posted. </EDIT> Also the image I posted is mine and linked to a domain which I own. Ron |
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September 27, 2019, 08:03 PM | #15 |
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Reloadron , am I reading that right . you got an ES of 3.3gr ? If so that's way more then I thought it would be . I was thinking 1.5gr max as a ES . My powder measures do much better then the dippers .
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September 27, 2019, 08:49 PM | #16 |
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Looking at things the 35.7 may have been a screw up on my part. Just doesn't look right compared to the other 9 numbers. Taking that out the numbers are more of what we would have expected. That was likely a me screw up.
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September 28, 2019, 11:04 AM | #17 |
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I'd rather be actually shooting or reloading.
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September 28, 2019, 11:35 AM | #18 |
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If I take out 35.7, the SD drops to 0.50 grains. If I replace it with 38.7, the SD becomes 0.48 grains. The means are 38.49 and 38.51, respectively. The ES in both instances is 1.4 grains or 3.64%.
Everyone who has worked with the scoops knows it takes some practice to use them repeatably. When you first try measuring throws from them, you quickly appreciate how well powder measures actually do.
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September 28, 2019, 05:29 PM | #19 | |
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Quote:
Yes we can talk about the scales +/- .1gr variance but the same scale is used for both so it's moot because it should be statistically off the same amount on both powders over time and I've noticed this differences over many years of loading those same powders .
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September 29, 2019, 03:27 PM | #20 |
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The spherical powders pack down very little. I should make up a set of photos for the same drop tube lengths with H380 and append that older photo with it. Once you saw the two side-by-side you would immediately understand why powder measure vibration causes sticks to throw more irregularly. I pulled a box of Federal GM308 down one time and found a 0.4 grain (±0.2 grains) spread in the 4064 charges in them. Still shoots well at that spread, though.
I can also throw 4064 ±0.2 grains with my JDS Quick Measure. It is designed not to cut grains as a standard drum measure does, so the vibration the cutting causes that packs powder down at the bottom of the hopper is omitted. It will usually throw sticks ±0.1 grains by avoiding that vibration, but occasionally I get one that is outside that by 0.1. It is guaranteed not to be worse than ±0.2 and thus far has lived up to that claim. I have friends who say they can make their old RCBS Uniflows or other iron drum measures throw sticks ±0.1 grains, but I haven't been able to make it happen. Whenever a grain is cut, that throw is usually good, but the next one is heavy from the vibration settling the incoming powder, and I have to dump it and throw again. The only drum measures I know that will compete with the Quick Measure at times is the Lee Perfect and its metal version. This assumes the polymer wiper is working and preventing grain cutting. I have one copy of the Perfect that does work like that and one that still cuts grains on occasion. More recently, I got one of their Classic Cast versions, and it seems to work as advertised. If I get a chance, I will do a side-by-side with it and the Quick Measure to see how they compare.
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October 1, 2019, 10:55 PM | #21 |
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WHY?
Why do you think you need to calculate the VMD?
It is used to dial in charges on adjustable powder measure (like any and all except the Auto-Disk), which you must weigh anyway (no pun intended). Since the goal is the correct weight, why does it matter if your VMD is X and Lee's Table says it is Y?? You are going to adjust your powder measure setting to get to the intended weight. VMD may help you get closer quicker than pure trial and error. But VMD cannot make the Auto-Disk adjustable. Volume, the way reloaders use it, is a means to get there , and not the end result (intended weight). If you can look it up in a table, and use a scale, your end does not justify your means. I think it is pointless, but, probably more fun than trying to use and cleaning up after the Auto-Disk, until you go back to one that works. I think your problem is that Auto-Disk thing and its chambers. IT IS NOT ADJUSTABLE! You cannot dial it in with the VMD. Only thing you can do is chose which chamber. Get rid of it right now! Put in a box in the garage. Order a Lee Perfect Powder measure for $25.47 https://www.amazon.com/LEE-PRECISION...s%2C198&sr=8-1. I own two. I also bought a Lee auto-drum powder measure to use on a progressive. I also have a Lyman#55.
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