I like your idea about the weighing of the cartridge and loading the powder directly into it from the dispenser. I dont know if it is too difficult to make it an option, so you could do both methods, with and without the cartridge. If it is too hard or time consuming, then just forget about it, its just an idea.

Actually, the subtract from zero will need to take place whether a cartridge is used on the scale or not.

Simply dispensing powder onto an empty scale would not be a good idea; the powder would scatter everywhere - ball would be the worst, but most of what both of us deal with is extruded cylindrical powders, which would also scatter in all directions.

Flake powder might stay on the scale, but one wouldn't want to sneeze....

In any event, a container needs to be used to hold the charge, more or less centered on the scale for best accuracy. The container will have some weight, even if it is made from aluminum foil or paper. The weight of the container must be accounted for, whether it is as simple as a muffin tin, a piece of formed aluminum foil, a paper or cardboard cup, a cartridge, or whatever - the weight of the container must be subtracted from the weight of the powder charge.

From the powders you posted, it looks like most of what you are reloading for are high-powered rifles in the range of 6.5mm to perhaps 8mm.

What are the particular cartridges you are loading for, the approximate weight of empty but primed cartridges, and the approximate range of powder weights that you are using?

Please do not post your particular loads; just a simple summary is all I am asking for.

I reload in many calibers, from .380 Auto (pistol) to .30-06; powder weights from 2gr to 58gr using flake powder for light loads and cylindrical powder for high-power loads. I think that most of your loads will fall into the latter category.

I've made some corrections in your spelling that I have italicized. One of the most difficult parts of the English language is that very similar-sounding words can be used properly or improperly. "To", "Too" and "Two" are three words that are very confusing, even to those who speak the language.

"Two" - a noun; a name; refers to the number 2
"To" - a preposition; as in "I will go to the store"
"Too" - an adverb; as in "It was too high a mountain to climb"

 

Surely I wouldnt dispense right onto the scale and I think I didnt see the whole idea of yours. As you say, it doesnt matter what the powder is filled into, but it would be good to have a function, where you can set the limit in wich the cartridges should be within.

I load in Lapua or Norma cartridges. I load them with approx. 40 grains. The avarage weight I dont know atm, I would have to look at my logfile of reloading when I get home and calculate it.

 

Tell me about the function buttons on your scale.

I'm assuming it has at least "Zero" and 'Tare" controls, besides on/off.

Zero would be used to tell the scale when there is no weight at all on it.
Tare would be used to zero out the weight of the container.

 

This is the scale I got:

http://www.dealextreme.com/details.dx/sku.15762

I got tare, on, off and mode, where I can switch to gn, grams ounce and so on.

It has a calibration function too, where you can use a control weight to calibrate it.

 

Do you use the calibrate function? If so, are you using a 100g weight or a 500g weight?

How well does the weight results compare with a manual scale?

 

On the lit of the scale there is a ATTENTION thing, wich says that you shouldnt calibrate the weight before use and I havnt done that yet, on the other hand I havnt used it yet for reloading, but when I get home today I will try to compare it to my manual scale and report back how it did.

 

I see that the scale will automatically turn off after 1 minute. That could be annoying. The uC will have to be able to control the scale ON button as well as the MODE button.

I find it very odd that a 100g scale uses a 500g weight for calibration. I see that a 100g weight is an option, but the 500g is recommended for better accuracy.

The scale needs to be very accurate, at worst 2/10 grain, particularly at the low end, where a small error in a pistol cartridge load could be quite serious.

Can you weigh various samples of bits of hardware, like nuts, bolts, screws, washers, and compare the two?

 

Ive just weight a few things and when I place the weights in the middel of the scale tray, I get the same readings on the manual scale as on the digital scale. If I place it close to the edge on the digital scale, then I got 0,2 grains lower than if positioned in the middel. I think that this can be awoided by placing a tray on the scale wich destributes the weight more evenly.

The scale can measure down to 0,1 grains.

I was wondering about the calibration thing too, but atm I am not concerned, because it weighs as accurate as it does.

The auto turn-off could be annoying yes, but when you reload you will keep up a good speed, wich should take care of it in the most cases, but if you can get rid of the auto turn-off, then I think you should.

 

While general purpose rifle loads can tolerate +/- a few grains with little difference in accuracy (if the load has been properly "tuned") this could make quite a difference with pistol loads.

I suggest that there needs to be a mechanism in place that ensures the positioning accuracy of the item to be weighed. A simple tray would probably not work very well; as a matter of fact it will probably make the situation worse. This is because no surface is perfectly flat. The surface of the scale will not perfectly match the surface of the tray, so weight will be distributed unevenly across the scale's surface.

The precise centering of an empty cartridge case on the scale will help to minimize errors caused by positioning. I am thinking of a simple cylinder that could be epoxied to the center of the scale's surface, or perhaps a hex nut, so that various size adapters for different cartridge diameters could be screwed into the nut. The epoxy would make sure that the surfaces would be perfectly matched, as any gaps would be filled.

This would also provide more stability/support for a cartridge that is much longer than it's diameter; for example a .30-06 cartridge vs a .45 ACP cartridge. You would not want a cartridge to tip over while it was being charged with powder.

 

Hmm yes the centering of the cartridge would be a relevant point to do something about. How each builder of this system would make this improvement is up to them, but it is clearly a problem, that needs to be mentioned in the "guide" to build such a dispenser.

Do you know how long it will take after we are done with the runthrough of the design and functionarity of the system, to draw the schematics and get the system to work? I know we would have to test it quiet a lot and compare our experiance with it, when we get so far. We have to ensure us that it is working as well as we can manage to build it and we have to point out that the powder charge that this system throws has to be check/controlled on another scale, to be sure that the cartridge is loaded properly.

 

I wouldn't even build a prototype before creating a schematic, and a board layout.

However, still a ways away from that. I don't even have a scale yet. I'm hesitating to buy a scale like the one you have until you can do some more testing on it to see if it is really repeatable, and what happens to the deviation (how much the readings change) when an item is placed in different areas of the scale platform.

You have mentioned only that the weight is 2grains (or is that grams?) different when it is on the edge of the scale platform than when it is in the center. I would like to have an idea of what area(s) of the scale are most repeatable before I decide to buy one of those, or something else.

I know that you wanted to have this project finished and working last month. It is not going to happen really fast, as safety is very important.

 

The deviation I was talking about was in GRAINS.

I will try to devide my scale (5 by 5 squares is that enough?) into sections today and make some more testing.

I will take 5 different things to way and check them with my manual scale. Afterwards I will take one of the items and place them in the different squares and write them down and post the result here.

The scale is pretty inexpensive and it is free shipped, but it takes a few weeks for it to come.

I know it is important to make it right the first time and to make it as safe as possible, so I will be patient, dont worry.

 

Five by five squares should be fine for now.

Let's assume that the edge of the platform is "off limits", or out of bounds.

Place a known weight as closely as you can centered on the intersection of the X and Y lines that form the corners of the squares.

I don't know what the size of the scale platform is; they mention 69mm x 64mm tray; is that the platform size?

 

I just tried 3 times to check the 5 x 5 squares i devided the scale into and here are the results.

 

So, the maximum resolution is 2/10 grain, and you obtained readings anywhere from 77.2 to 77.6. If the edges of the scale are ignored, it's 77.2 to 77.4.

What did the manual scale say the weight of your test object is?

My manual scale has a resolution of 1/10 grain on the markings, but you can read between the markings. It is very repeatable.

Try it again with a lighter object and a heavier object. We need to find out how much error there might be with this electronic scale.

 

Thinking more on this on the way home, if you were reading 77.2 to 77.4 (ignoring the "edge" readings") - that means there is another 1/10 grain not accounted for.

Since the resolution appears to be 2/10 grains, the error could be more than that.

For example, a reading of 77.2 might mean anything from 77.1 to 77.3, and a reading of 77.4 might mean anything from 77.3 to 77.5.

So, even ignoring your "worst case" readings, and just taking the better readings, there is still a possible range of values from 77.1 to 77.5, or 0.4gr difference from low to high - even using the same weight in the same locations.

Would you be happy with that much of a difference in your powder charges?

It might be OK with high-power rifle loads that were already "tuned", but not for pistol loads.

I think a search for a better scale is required.

 

Hmm I think I have made a very big mistake. I tared the scale every time I started over again. I think I will do the test again when I get home. The scale is 1/10 grains resolution, but somehow I didnt get 77.5 or 77.3 ect.

I have also heard that if there is a sudden change in temperature, the scale can be of by 0,1-0,2 grains from cold to warm. I got the scale from my bedroom and the temperature in there is about 18 degrees celsius and in the livingroom where I tested it is 24 degrees. If you look at the readings, they get better and the test done above took 15mins and that may have been enough. I will let it get the right temperature and try to make the test again without taring it.

 

The "tare" function should be to compensate for the weight of the container being used to weigh items in.

Is there a "zero" function along with the "tare" function?

There are no instructions available on the Internet for that scale; just what can be read from the photos of the item.

Temperature stability is another item that I had not previously considered. That is why it is important to gather as much data as possible before jumping into a project like this.

 

The scale hasnt got a zero function.

I will do some tests of the scale again today.

 

Okay, I have made new tests.

I put a weight on 16 times in the center of the scale. 13 times I got 77,4 and 3 times I got 77,6.

The scale can only make 0,2 gn increments.

I weight a bullet 20 times and got the same result every time, so I have two theories:

1. The bullet is heavier wich is better to read (139,4 gn)
2. The weight I did use in the test with 16 readings was just in the middel of 77,4 and 77,6 wich made it hard for the scale to choose.

Those 0,2 gn is for me to live with. I dont think that it has that much affect anyway for my rifle, but as you said, it may be a problem for pistol shooters.