To the OP: Lower the cartridge (preferably an MC cartridge, because they have big magnets) until it is barely above the weigh pan but not touching it. Does the gauge show a negative weight value? If so, that indicates it contains some low level of iron; the magnet in the cartridge is pulling UP on the pan. If the weigh pan is magnetic, even a little bit, that messes things up. I used one of the popular digital scales for years before realizing that it had the problem, which was only evident when I tried to adjust VTF for an Ortofon MC2000 cartridge, which has a humongous magnet situated in its belly, near to the platter or any weigh pan. I subsequently bought one of the 2 or 3 gauges made by Ortofon, the weigh pan of which so far seems to be non-ferrous, although it is metal. This problem seems so easy to avoid in designing such a gauge. But apparently the sellers do not care a fig. As long as they're selling.