These moisture testers are rather cunning. A weighed sample of the subject is put into the body of the instrument, and a measured volume of crushed calcium carbide is put in the lid. The lid is clamped on, the whole lot shaken up, and the calcium carbide reacts with the water in the sample to produce acetylene gas. In the base is a pressure gauge calibrated in percentage moisture content.
So my sample from the pot read 3.5%
A sample from a new bag of finer grit that I'd been given read 0% (*)
A bag from my pile of bags gave 3.5%
The next 10 bags from my pile gave 0%
So to cut a long story short I'd been unlucky and chosen a damp bag
When you are dressed up in the blasting helmet and gloves, and are loading bags into the pot 'on the fly' you don't really get a chance to feel how it flows, but comparing the damp bag with a dry bag it is actually quite obvious with ungloved hands.
An 8x4 of hardboard was laid out on the concrete outside the workshop - the damp bag contents spread out in the fine warm day we are having today, and raked about with a garden rake. Within 20 minutes it was down to 1.5%,and leaving it over lunch my sample showed no perceptible moisture at all. Reading specifications from grit suppliers web sites it looks like a maximum of 0.2% is what it should be
(* I am fortunate in having a professional grit blaster renting parking space at the farm and he kindly gave me the finer grade grit left over from blasting bits of Hastings Pier which is rising phoenix like after it's disastrous fire of a few years back)