I did a similar thing last year. I got new dpd powder at the beginning of the year, but still had some old studd left from the year before. I tested tho old grey stuff against the new and came up with the same results - but by mid year (about now) I did the same test again and came up with about a 1/3 decrease in the reading with the old powder. It was really dark grey and clumpy by then. Unfortuantely, there seemed no way to tell by eyeballing it when the stuff really started to deteriorate in quality.