Edwal Hypo-Check allows you to text fixer for exhaustion due to silver saturation in the darkroom. One to two drops will test the fix, and if the Hypo-Check produces a cloudy substance in the water, the bath is exhausted and the fixer should be discarded. If the fix does not produce a cloudy substance in the liquid, no harm has been done to the bath. 3/4 ox. bottle provides 800 tests.