Checking Accuracy of Milligram Force Restoration Scales



  • I'm new to the forum and would like to ask a question regarding precision magnetic force restoration milligram scales, like the A&D FX-120i or Sartorius 64 and equivalent. These scales do not require frequent re-calibration like cheaper load cell or strain gauge scales. But, being digital, a prudent reloader would do a routine accuracy check using some kind of test weight typically close to the combined weight of the powder charge and pan, and probably a periodic (annual or semi-annual) re-calibration. If your goal is to trickle and measure precisely accurate powder charges that take full advantage of the scale's accuracy potential of 0.02 grains (or less for the Sartorius), you will need a certified test weight, typically about $100 for a 50gram or 100gram weight. These certified weights though need to be handled with kid gloves (literally), or forceps, as oils from your skin can affect the weights. Much cheaper non-certified weights, ANSI Class 7 for example, are available, but a 100 gram test weight is manufactured to a fairly sizeable tolerance of +/- 100 milligrams (+/- 1.54 grains). If I understand this stuff correctly, re-calibrating your scale with a low tolerance test weight like the Class 7, may restore scale linearity but will result in a scale that is "inaccurate" by some amount up to the full manufacturing tolerance. Using an inaccurate test weight, like Class 7 weights, or coins, or bullets may cause you to recalibrate needlessly. Of course if you have access to another certified microgram scale you could weigh the cheaper Class 7 or other homemade weights, and offset your calibration to reflect the "true" weight of your Class 7 test weight. But most people do not have access to such scales. What test weights and practices do you use to routinely measure your milligram scale accuracy?



  • The A&D can self calibrate. According to the manual it has an internal mass. Truthfully I didn’t read the whole thing, (I’ll go check tonight) but you don’t need the external weight in the same way you do with other balances.

    That said I did buy a 50 gram external weight. I’m unsure of the level it’s certified to, but it cost about 40 dollars and I wear gloves to handle it. Prior to beginning my reloading I put it on the scale. It always reads 50.000 grams.

    Next I take my home made weight for whatever charge I’m loading. The auto trickler needs a charge placed on the scale to “learn” what weight to throw to. I made mine by wrapping bare wire around a pencil then trimming it down till it got to my desired weight. I have one for each of my developed loads and I keep them in little labeled bags. I put this on the scale, and copy the weight into the scale. So far it always weighs what it did when I made it a couple months back.

    That’s my method. It occurs to me that I don’t need the scale to be dead on to the true value, just consistent, which so far it seems to be. No drifting like the chargemaster.



  • The thing is with reloading, accuracy is not nearly as important as precision. If you always use the same scale and same check weight then your loads will be very consistent. Who cares if the scale reads 41.50 when the actual charge is 41.54 as long as fh scale ALWAYS reads 41.50 and as long as you always use the same scale for load development and loading.

    I have a 100g check weight that I only handle with gloves. I don’t know how accurate the weight is and don’t really care. The check weight is consistent and always calibrated the scale the same...therefore my loads are always the same.

    Promethius is the same principle. It is EXTREMELY precise and repeatable. It will give very expensive lab scales a run for their money on precision. It however would not be considered extremely ultra accurate.



  • I made some check weights out of brass brazing rods. Cut a piece and sand/grind to the weight you want.
    I made some close to targetted load weights.
    As 3d said:

    If you always use the same scale and same check weight then your loads will be very consistent.

    I like your wire weight idea, tackyp.


 

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