Quote Originally Posted by Photo Engineer View Post
You must remember that it is almost impossible to check a tank for Silver content. This is especially true when using the fix for paper.
The only sensible way I know to do this is with the silver-estimating paper I mentioned. Unfortunately the typical test papers are not good at identifying silver levels much below about 1 g/l. Kodak once published a method to increase the sensitivity with a 30-second immersion, but it required "calibration" using a lab that could run silver analyses. (I don't think the method is published anymore.)

The most reliable test is "time to clear". Select a 35mm film type (whether you are using paper or not) and cut an unexposed roll into 2" pieces. Using fresh fix, determine the clearing time. Now, every time you make a run, do the same with another piece of film. When the clearing time is approaching 2x the original time, that fix (first or second "tank, tray or whatever") is shot.
I agree completely with this test to find when fixer "is shot." My issue is this: if you are trying to keep a low silver level in the final fix tank, say under 1/2 g/l, I doubt this test will show it. (I'm guessing, as I've never done fix-time vs silver content tests, but I'm very doubtful.)