I usually had difficulty getting consistent results when testing specimens for SG. There was nothing wrong with the scales - they would consistently and repeatedly record the same weight for a stone to within a tiny fraction of a carat - but even when carefully following the instructions of a professional gemologist I had some difficulty. I did usually end up averaging the results of repeated tests on stones that I already knew the identity of to something close to what I knew that stone should be but I often remained doubtful of stones I wasn't certain of.