The purpose of the overnight chlorine loss test (OCLT) is to determine if you have algae or other organic contaminants in your swimming pool water. If you do, you need to SLAM, or continue SLAMing, the pool.
All that you need to do to complete the OCLT is to test your FC twice with an FAS-DPD test: once after the sun has gone down, and once before the sun comes up. Chlorine is consumed by two things: sunlight and breaking down organic contamination in your pool. By testing at night, we rule out sunlight. Any chlorine consumption overnight must be due to algae or other organic contaminants in the pool.
This test is most useful to confirm that the SLAM process is complete, and can also be useful if you are unsure if there is a problem or not.
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.
How do I perform the overnight chlorine loss test?
- If you have an SWG, tablet chlorinator, or other chlorine feeder, shut it off completely.
- Turn your pump on at least 30 minutes prior to ALL testing.
- After the sun goes down, and at least 30 minutes after your last chemical addition of the day, perform the FAS-DPD test and record your nighttime results. If your FC is 3 ppm or higher, do not add any more chlorine to your pool. If your FC is less than 3 ppm, add enough liquid chlorine to achieve 3 ppm FC, wait 30 minutes, then retest with the FAS-DPD test.
- The next morning, ideally before there is any direct sunlight on the pool, perform the FAS-DPD test again and record your daytime results.
If your daytime FC level is within 1 ppm of your nighttime FC level, you passed the OCLT and your water is clean. There isn’t any living algae or other organic contamination in the pool that requires a SLAM.
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.
