Would like to add my 2cents. Over the years, working with various scales, balances, vacuum chambers, etc. in chem. and physics labs I've learned a few things which usually are left out of the books and courses. The gram which is taught in measurement labs is usually the "apparent gram", meaning that it is approximate to a true gram but has corrections for the gravitational force and barometric pressure (air bouancy) for the particular altitude of measurement. Unfortunately, it is difficult to measure a true gram of mass unless one is in a good vacuum and has corrected for gravity. Most science departments have a special vacuum chamber for measuring mass, but we still have to correct for gravity. Because of this bouancy (air) and gravity thing, all ASTM 1 and Certified Weight sets (the expensive stainless steel sets) are corrected or altered by the mfgr. to comply to a certain geographic area. For example, a set of gram wts. made in Switzerland isn't so accurate for use in NY State; so all my wts. are made in Philadelphia. Since the gram as we use it has already been corrected for these force-related factors (apparent or corrected gram), it doesn't make much difference which unit is used for hammer wts.(my opinion only). Some articles which explain this better and with equations are available from Troemner, Philadelphia; Mettler, Hightstown, NJ; and NIST, Boulder. BTW, definition of the gram and apparent gram was changed in 1975 by International Convention; just enough to drive us nuts, so now we have to use two different sets of weights-----nothing is sacred! The history of wts. and measures is pretty wild. Fred Scoles, scoles@oswego.oswego.edu
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