How do I understand what the real accuracy of a laboratory balance is ?

I need to buy a laboratory balance to measure 400g +/- 0.02g. That is, when I put an unknown weight on the balance I want to be absolutely sure that the reading I get is within 0.02g of the actual weight. And let's make an assumption first, and I know it's only an assumption that the balance is in perfect calibration.





The problem I have is that the manufacturers don't appear to quote "accuracy" on the brochures or data sheets. They will quote, 'readability' or 'repeatability' or 'resolution' or 'precision'. How do I convert these 'values' into real life accuracy. Thanks|||Look for a balance with the specifications of 400 X 0.02 g.





The upper end of what the balance will measure is always the first number. The resolution tells the accuracy of the calibrated balance. This second number (after the X) is also called the readability.





The repeatability or precision tells how often the balance will give the same mass reading in statistical terms.





The AdamLab ACB-1200H listed below is 1200 X 0.02 g for only $ 480.|||Haven't worked with lab balances for many years. Aren't you going to need a set of calibrated weights (mass standards)? Your accuracy requirement is about level R111 Class F1 (NIST grade S-1), which is pretty high accuracy. Cost will be more than $500 per set. You're going to need careful handling of the balance and the mass standards -- temperature, humidly, dust, white gloves, face mask, magnetic field, etc.





The balance, on the other hand, should cost only about $300 - $800, depending on features.





Once you have the mass standards with an accuracy of (for example) 2.5 mg, and the balance repeatability is 10 mg, the total readout error is about 10.4 mg. In other words, the RMS error is dominated by the repeatability.|||It's years since I had anything to do with chemical or analytical balances but at that time the accuracy used to be denoted by a +/- sign, e.g. Accuracy +/- 0.001 gms. Most balances were guaranteed accurate to one less than the number of decimal places it would read so if it read to 3 places it would be accurate to 2, good balances were even better.





If you get a decent balance and use it within the specified temperature range, out of draughts, away from sources of vibration, make sure it doesn't get knocked and get it calibrated regularly it should be OK.





Be aware though that many substances absorb atmospheric water to some extent so if you weigh something on a very dry day the reading may not be the same as if you weigh it on a very wet day.