i've been testing mine about every other week... on tantia... i will get a good calliper reading to compare the numbers some time before the end of the month and then once a month after that... i would like to do underwater weighing, but i'm claustrophobic... doubt i'd be able to do it properly, i'd freak out, it would be like being in a revolving door underwater... eeek, i'm getting nervous just thinking about it!!!