I got these instructions from an RC site selling half decent scales :-
1) Turn on the unit, and calibrate it using the instructions included with the device. After callibrating, check again that the 10g calibration weight gives a reading 10.000g. If it doesnt, then re-calibrate, making sure there are no drafts or vibrations that are effecting it.
2) Next, place the "scale pan" (for want of a better term) on the weighing platform and make a note of this weight of the pan - it should be in the zone of 1.197g with the HA-20's pan but check what the reading is for your pan. And weigh it twice to double check the figure.
3)Take the pan OFF the weighing platform and then add some of the chemical onto it. This is the key bit of information - do not add material to the scale pan when it is on the weighing platform, it will most likely not register the weight accurately as the chemical is added! Only after that, place the scale pan back on the weighing platform.
4) Make a note of the new reading and then subtract the weight of the scale pan, (e.g. in this example 1.197g) from the total weight shown. So for example, if the scale now shows a reading of 1.215g, then you know you have 18mg of material on the pan.
5) If this is too much and you wanted to weigh out less, say you only want to weigh 13mg, then take the pan off the weighing platform and remove some material. Then place the pan back on the weighing platform and take a note of the new reading. If it now reads 1.210g then you know you have the desired 13mg!
6) But suppose you really wanted 20mg. Ok, take the scale pan off the weighing platform, add a little more chemical and place it back on the platform. If the new reading is now 2.217g then you've likely got your 20mg. If not then take the scale pan off again and repeat until you hit the target.