Beams only measure within a very narrow window. If you set your scale to weigh 40gn for example, you dump powder in and nothing happens until you get close to 40gn, when the beam finally begins to lift off its rest. Then you trickle it up to be 40gn. It is very precise in that regard. But if you have bumped the adjusting weights without noticing, so it is set to weigh 42gn, or 52gn, you really have no indication that it has moved. This is less of an issue with a low density powder that almost fills the case, if you are two grains over you should see the difference when you look into the case, but not everybody has learned to do that. With a higher-density powder you can dump that 42gn or 52gn charge in the case and not notice.
I recommend that if you are using a beam scale make yourself a check weight that you can drop in the pan before you throw every charge to confirm it is still set to where you wanted it. My mate is still very new to reloading, using the Lee beam scale, and he is using a piece of paper as a check weight.https://enoughgun.com/forum/viewtopic.php?f=2&t=16888&start=30#p293731
With a digital scale there are still ways of messing it up, but I don't recall ever having one self-adjust itself. I don't run them off batteries as that can cause issues when the battery charge decreases - stick with mains power only.
And charge all your brass together, then look across the tops with a torch and confirm that all the powder levels are similar before you seat the bullets.
I also recommend using powder dippers to dump your charges into the pan. Choose the dipper that is just below what you want, throw the charge in the pan and the scale should come up very close to the weight you want, then trickle it up to where you want it. If you use a dipper that throws 39.4gn of whatever powder you are using, when you dump that into the pan the beam should move off its rest - if it doesn't then you know to look for a problem.
bluehorse wrote:While not strictly about compressed loads my subject is powder scales . I had a st of battery scales that I used and calibrated too but they were not 100% consistent so they are put aside politely . ( not thrown to outhouse where they belong } A consistent load is needed for accuracy . I have my balnce scales from long ago that are still good . I discovered that the flat bottom of the pan can give crook inaccurate readings if the powder is not central. The amount of discrepancy depends on the volume of pwder not central . Verifying a load from a thrower then becomes a nightmare if a charge must be centrally located ! I played with the load in the flat bottomed pan and think variations can be as much as 5 tenths of a grain so checkweighing quickly in conjunction with a powder thrower aiming for 2 tenths accuracy is a painstaking exercise .
My fix for this was to use a small ball pein hammer to dome the bottom in the middle so the powder will fall centrally . It does so long as the dish is in the cradle and the charge is all central . . The dome is not all over the bottom but big enough for my loads . I often shake the tray to make the charge centrally located in this tray . It is cumbersome when I am in a hurry but gives better results .
Hope readers can find this useful .
Peter
P.S electronics are faster by a country mile while they are accurate but so many things can upset the electronic tools . The problems arising from inaccuracy or a false zero or calibration are not easy to identify so I like the old fashioned tools more . A foul up is more easy to identify and fix . Tools across the board are the same .. Some tension wrenches do not like being dropped especially on the part that has the scale stamped on it . I have 1 that is a big name but the handle grip with the scale on it is a pressed on fit and dropping this tool can reposition the scale in relation to the mating mark ..Beware ! calibrate tools in some way . Check zero or check with test peices supplied also . Find a method of zeroing scales on your bench often . It is a few minutes work that may save days of frustrations or foul ups .