We've had a couple of emails recently from customers mentioning that they've struggled to weigh small amounts of material with our accurate 0.001g digital scales, and it occured to me that whilst many researchers know the correct procedure, others could probably benefit from a brief guide.
This advise is important whether you're using our £27.49 mg scale or a more expensive model costing hundreds of pounds.
We get some excellent feedback on these scales and they are superb for the price, but a couple of customers emailed recently to say they were adding material to the scale pan and the reading would stick at 0.000g and then suddenly jump to a reading of say 0.010g (10mg).
This is because you should never add the material directly to the scale pan when it is on the weighing platform! Ok, if you're weighing out 500mg or 1g then it's not as critical but for very small amounts it is important.
Digital scales will often not register the increasing weight accurately as material is applied to the scale directly in a continuing manner - just to re-iterate this is the same with our £300 KERN balance that we use here in our lab, it is not to do with the price bracket of the low cost devices.
Thus one should add the material to the scale indirectly, and the following procedure represents better practice (Just to add the obligatory disclaimer - the procedure described is for informative purposes only and we will not be held responsible for any consequences that occur from following it!)
[quote:x8d0t98a]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.
), ou vaut mieux attendre? 
