As best I can tell, telling a customer who can't keep a chlorine reading in their pool that they have "chlorine lock", is sort of like the doctor who carefully listens to their patient complain of irritated skin, sagely nods, and then states in the most serious voice that the patient has "dermatitis". As a rule, they then neglect to mention that "dermatitis" just means irritated skin!

The only difference is that "dermatitis" always means irritated skin, and "chlorine lock" tends to mean whatever that dealer -- or the dealers in that area -- think it means.

I've seen it applied to high stabilizer / high chlorine pools as in, "Oh, you have algae because your stabilizer is so high that your 'chlorine is locked'.

It's also applied to cases of chlorine that disappears rapidly. It sounds like that's your problem.

I know of three reasons why this can happen:
1. No stabilizer and lots of sun.
2. Use of sodium bromide (Yellow Treat, No-Mor Problems, etc.) and sun.
3. Fall stabilizer converted to spring ammonia.

If your test results are correct, that rules out #1.

You'll have to tell us whether #2 applies.

Usually with #3, you'll still have fairly high combined chlorine.

So . . . we have confusion here.

Oops! Thought of a #4 -- use of chlorine remover OR peroxide as in, Baqua-Shok.

Anyhow, best bet: Add 2 gallons of plain bleach per 10,000 of water in your pool this EVENING, after the sun is well on the way to sunset. Next morning, BEFORE there's much sun, test FC and CC and report the results.

If your problem is #1 or #2, the chlorine will still be there (no sun!). If not, we can at least rule out those two.

Also, report ALL the chemicals used this season AND (if you can) test results at closing last fall AND whatever chemicals you 'closed' with.

PoolDoc