Overnight Chlorine Loss Test — log sunset & sunrise FC and auto-calc the loss.
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Pass criteria (typical guidance):
FC loss ≤ 1.0 ppm overnight
CC ≤ 0.5 ppm
Water is clear
Tip: Do the sunset test after the sun is off the pool and after circulating ~30 minutes.
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Date
Sunset FC
Sunrise FC
Loss
CC
Status
Notes
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Understanding the Overnight Chlorine Loss Test (OCLT)
The Overnight Chlorine Loss Test is one of the best ways to determine whether algae or another organic contaminant is still consuming chlorine in your swimming pool. Because sunlight destroys chlorine during the day, the test is performed overnight when the sun is no longer affecting chlorine levels.
How the Test Works
Test and record your Free Chlorine after sunset, allow the pump to circulate overnight, then test again before sunrise. The difference between the two readings is your overnight chlorine loss.
Passing the OCLT
Free Chlorine loss is 1.0 ppm or less.
Combined Chlorine (CC) is 0.5 ppm or lower.
The pool water is crystal clear.
Why It Matters
If you lose more than 1 ppm of Free Chlorine overnight, something in the water is still consuming sanitizer. Continue the SLAM Process until all three passing conditions are met.
Important Notes
Perform the evening test after the sun is off the pool.
Run the circulation system overnight.
Take the morning sample before direct sunlight reaches the pool.