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Scales with higher resolution typically require a calibration weight with tighter specifications. For instance, the UniqueTek High-Precision Electronic Powder Scale, which has a resolution of 0.02 grains (0.001g), comes with a 20g calibration weight rated Class F2 (OIML Class F2 fits in between ASTM Class 3 and Class 4). For a 20g calibration weight, the allowable tolerance is only 0.8mg (0.0008g) compared to a Class 6 tolerance of 3.0mg (0.003g). If I substituted a 20g calibration weight from another scale that was not made to meet the Class F2 standard, my calibration could be off.
So why not just buy a calibration weight set made to tighter tolerances? In short, cost! I was given a set of calibration weights made by Ohaus. They are brass weights rated Class 6 and sold for about $40.00. The cost of calibration weight sets with tighter tolerance is excessive, especially if you want a set that comes with a certificate documenting the exact value of each calibration weight! Here are some examples:
– Class 6 brass = $40.00 / $139.00 with Certificate
– Class 4 stainless steel = $320.00 / $815.00 with Certificate
– Class 1 stainless steel = $600.00 / $1225.00 with Certificate
To make things even more difficult, some calibration weights are 2-piece assemblies. They are hollow and have either a plug in the bottom or a handle that is screwed on top. These are purposely manufactured a bit light and then grains of metal are added to calibrate it to the correct mass. If the plug falls out or the handle comes unscrewed and you loose the grains of metal from the inside, your calibration weight is worthless.
Fortunately you can buy individual calibration weights. So if your scale came with a 2-piece calibration weight and it came apart, buy a replacement. I recommend that you contact your scale manufacturer and determine the correct size and tolerance classification of calibration weight for your scale. You can usually buy replacement calibration weights directly from the scale manufacturer.
Tip #6: Air Currents
Some electronic scales come with a cover. On some of these, the cover isn’t just to keep dust off when not in use. Some covers are designed to be used during measurements and provide a valuable function by blocking air currents that can affect accuracy (the owners manual will tell you if your cover is intended to be closed during measurements). You can identify covers that are designed to be closed during measurement as they may have a hole in the top. The hole allows you to add powder with the lid already down.
As you can imagine, closing or opening the cover after placing a pan of powder on the scale will likely upset the measurement. A few scales come with a “draft ring,” which is usually a glass ring that fits around the weighing pan to protect it from air currents. Make certain it is in place for all measurements Even if your scale has a draft ring or a cover designed for use during measurements, make sure it is located away from air currents in the room. Or, better yet, make a draft shield on three sides. I made one out of mat board (the stuff on which artists mount photographs).
It is rigid, easy to cut with a utility or X-ACTO® knife, finished on both sides and available in lots of interesting colors. It is cheap and can be found at art & craft stores or office supply stores. Make it tall enough to extend at least 3 inches above the weighing pan. Adding a draft shield may not be enough. I had an air conditioning vent that blew in the direction of the reloading bench and the draft shield just didn’t help enough. I finally found an inexpensive plastic deflector at the local hardware store that attached to the face of the wall vent and deflected the airflow away from the reloading bench. Simple, elegant, cheap …. and it worked!