Chlorine is consumed by two things: sunlight and breaking down organic contamination in your pool. By testing at night, we rule out sunlight. If chlorine is still being used up, it must be because of algae or other organic contaminates in the pool.
Of the common chlorine tests, this test can only be done with the FAS-DPD test. Test strips, and the OTO and DPD tests, do not have enough precision to determine if small amounts of chlorine are being consumed.
To perform the overnight FC loss test:
- If you have an SWG, tablet chlorinator, or other chlorine feeder, shut it off completely.
- After the sun goes down, and at least 30 minutes after your last chemical addition of the day, test your water for FC using the FAS-DPD test making sure your FC level is at least above 3ppm.
- Record that result. Do not put any more chlorine in your pool.
- The next morning, as early as practical and definitely before there is any direct sunlight on the pool, perform the FAS-DPD FC test again and record the results.
If your FC level remained the same, or went down by 1.0 or less, the water is clean. There isn't any living algae or other organic contamination in the pool.
If you lost more than 1.0 ppm of FC, then there is something in the water that needs to be removed and you should SLAM, or continue SLAMing, the pool.
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