The overnight chlorine loss test

Why test overnight?

During the day, sunlight destroys chlorine. At night there's no sun, so any chlorine your pool loses overnight is being eaten by something in the water: algae you can't see yet, organic waste, or ammonia. That makes the overnight chlorine loss test (often shortened to OCLT) the most sensitive check there is for a pool that looks clean but isn't.

How to do it

1. After sunset, once the sun is well off the water, test free chlorine and write it down.
2. Add nothing overnight. No liquid chlorine, no tablets in the skimmer or feeder, and turn the salt cell off for the night.
3. Leave the pump on its normal schedule and any cover how you usually would.
4. Before sunrise, test free chlorine again.
5. Subtract the morning reading from the evening one.

Use a FAS-DPD test kit. Test strips and color-match kits aren't precise enough to see a 1 ppm difference reliably, especially at higher chlorine levels.

Reading the result

• 1 ppm or less: pass. Nothing in the water is consuming chlorine. A tiny loss is normal.
• More than 1 ppm: fail. Something is eating chlorine. Even if the water looks perfect, there's usually early algae somewhere. A SLAM is the fix.

During a SLAM, passing the overnight test is one of the three signs you're done, along with combined chlorine at 0.5 ppm or less and crystal-clear water.

Using it to diagnose chlorine loss

If your pool keeps running out of chlorine, the overnight test splits the problem in two:

• Small loss overnight, big loss during the day: it's sunlight. Check your CYA; stabilizer is what protects chlorine from the sun, and below about 30 ppm it burns off fast. A cover helps too.
• Big loss overnight as well: it's demand in the water, such as algae, organic load or ammonia. Treat that first; more stabilizer won't help.

Tips for an accurate result

• Test at the same spot and depth both times, away from return jets.
• Start with chlorine at a level you can read precisely; very low readings make a 1 ppm change hard to see.
• Write the exact times down. Your local sunset and sunrise shift through the year.