Overnight chlorine loss test (OCLT)
Also called: OCLT, overnight FC loss test.
The overnight chlorine loss test (OCLT) measures free chlorine after sundown and again before sunrise. If free chlorine drops more than 1.0 ppm overnight, with no sunlight to blame, something organic such as algae is eating the chlorine.
Why it matters
The test runs at night so there is no sunlight to blame for the loss. If the number still falls, your chlorine is losing the fight whatever the daytime reading says, and the next step is a SLAM. It is the way to confirm the classic mystery of a green pool whose chlorine reads fine. It is also one of the three exit tests for a SLAM: you are not finished until the overnight loss is 1.0 ppm or less. It takes a FAS-DPD drop kit, because strips cannot read small overnight losses.
In numbers
Test FC after sundown and again before sunrise. Losing more than 1 ppm overnight means algae is still feeding.