You should test your pool water in the same location and lighting as the standard solution.
! Make sure to check (compare) the standard solution in the same situation you will be testing your pool water CYA sample
- (no need to dispose of it, you can reuse it, be sure to label it clearly)
- And it's suggested to "test" the calibration sample in different lighting situations
- Find the lighting/area where you see 50 "easily" or every time