Very Low Chlorine Level

[Rednekk]

We just had a new 19,000 gallon AGP installed. I finished filling it this past weekend, and as per the dealer's instruction, I shocked it (3 pounds), put in algaecide (10 ounces), and put 5 1" chlorine tablets in our floating chlorinator.

I've tested it each day since, and my test kit is not showing any chlorine reading at all. The water in the tester stays perfectly clear after the drops are inserted.

If it means anything, the PH is just about perfect, according to the same test kit.

Any input will be appreciated. Thanks!

[chem geek]

If you are using a DPD chlorine test, the kind where the sample turns pink/red that you compare in intensity, then this can bleach out above 10 ppm FC. You can try diluting your pool water sample with distilled or filtered water (not tap water, since that has chlorine) and remeasure or you can use an OTO chlorine test or FAS-DPD chlorine test for verification. 3 pounds of Cal-Hypo in 19,000 gallons would be around 12.2 ppm FC while 3 pounds of Dichlor would be around 10.5 ppm FC so I don't think these are so high as to bleach out the DPD test (though it's possible). 5 1" Trichlor tabs are only 5.4 ppm FC so not very much and won't last long unless your pool is covered.

Do you remember what you used to shock the pool? Was it Cal-Hypo or Dichlor or chlorinating liquid? Did you initially add any Cyanuric Acid (CYA aka stabilizer or conditioner) to the pool? If you don't have any CYA in the water, then chlorine will break down very quickly in sunlight. You want at least 30 ppm CYA in the water, possibly more, if it's exposed to direct sunlight. If you didn't have CYA in the water when you shocked your pool and you used Cal-Hypo for shock instead of Dichlor, then half of the chlorine will get broken down by the UV in sunlight every 35 minutes.

If you plan to use Trichlor tabs as your main source of chlorine, then just keep in mind that for every 10 ppm FC, these also increases CYA by 6 ppm. To prevent algae, you will have to either raise your FC level as the CYA rises or use a supplemental algaecide (PolyQuat 60) or a phosphate remover or will have to do a partial drain/refill every now and then to dilute the water to keep the CYA in check (unless your pool is small, you backwash the filter weekly, and you have a short swim season or have regular rain overflow for water dilution).

Richard

[Rednekk]

Wow, thank you for the quick response!

The test kit I am using turns yellow in the presence of chlorine, so that would be an OTO kit, correct?

I'm 99% sure we were given the Cal Hypo shock to use. We did not add anything other than the shock, algaecide 50, and chlorine tablets.

The pool was filled with well water, so there should not have been any initial chlorine present there.

The pool is currently uncovered and in direct sunlight.

[chem geek]

Yes, the test you are using is OTO so does not get bleached out. Since you didn't add any Cyanuric Acid (aka conditioner or stabilizer), your chlorine is likely getting broken down very quickly by sunlight. I'll bet if you added the Cal-Hypo in the evening after the sun went down, then you'd still have chlorine in the morning before the sun comes up, but as the sun hits the pool the chlorine will drop quickly (losing half the FC every 35 minutes in direct noontime sun).

Anyway, the Trichlor tabs will increase the CYA level, though they are slow to dissolve. If you want to add CYA more quickly, you can either get pure CYA or get Dichlor, the latter being a fast-dissolving combination of chlorine and CYA (for every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm).

I suggest you read the Pool School so that you learn about how to better manage your pool.

Richard

[rbdeli]

Last year, I was having this problem. Even though my water was clear, my free chlorine readings were 0. One day, the water became cloudy and green overnight.

It ended up being an ammonia problem. Those ammonia test kits are so cheap and easy to use, it wouldn't hurt to have one and try it whenever a peculiar FC problem exists.

Just my two cents based on my own experience last year.

Rob

Wow, thank you for the quick response!

The test kit I am using turns yellow in the presence of chlorine, so that would be an OTO kit, correct?

I'm 99% sure we were given the Cal Hypo shock to use. We did not add anything other than the shock, algaecide 50, and chlorine tablets.

The pool was filled with well water, so there should not have been any initial chlorine present there.

The pool is currently uncovered and in direct sunlight.

[Rednekk]

Thanks again, Richard! I'm going to grab some stabilizer and put it in tonight. Any tips on how much, or should I just go by the direction on the container?

Thank you for the link as well. Looks like a ton of info to digest. I'm sure it will be very helpful.

Frank

[chem geek]

Frank,

Use The Pool Calculator to figure dosing and add some chlorine to get to 10 ppm FC in the early evening after the sun has gone down. Then measure 1 hour later and see if you get a reading. If your chlorine is near zero, then Rob may be correct and you could have an ammonia problem, but this test will tell you that. If you do measure chlorine, then try measuring again the next morning. If you still measure about the same amount of chlorine, then the problem was just that you didn't have Cyanuric Acid (CYA) in the water. You could use Dichlor for chlorine and cumulatively add around 35 ppm FC as that will add around 31-32 ppm CYA which should be good to get you started though. If you continue to use Trihclor tabs, then the CYA will climb over time unless you have a very large amount of water dilution.

Richard

[rbdeli]

I'm still wondering what in the world could have caused the ammonia.

I'm wondering if it could have been lawn fertilizer blowing in the pool from the neighbor's lawn or trees.

Doesn't lawn, tree and plant food contain some ammonia?

Rob

Frank,

Use The Pool Calculator to figure dosing and add some chlorine to get to 10 ppm FC in the early evening after the sun has gone down. Then measure 1 hour later and see if you get a reading. If your chlorine is near zero, then Rob may be correct and you could have an ammonia problem, but this test will tell you that. If you do measure chlorine, then try measuring again the next morning. If you still measure about the same amount of chlorine, then the problem was just that you didn't have Cyanuric Acid (CYA) in the water. You could use Dichlor for chlorine and cumulatively add around 35 ppm FC as that will add around 31-32 ppm CYA which should be good to get you started though. If you continue to use Trihclor tabs, then the CYA will climb over time unless you have a very large amount of water dilution.

Richard

[Rednekk]

Update...

I added the stabilizer over the weekend, and shocked the pool last night. I checked the FC about 2 hours later, and it was at the high end of the tester. I checked it again this morning and it's about the same. They're not calling for much sunshine this week, so I'm assuming that the level may take awhile to come down to proper levels.

[chem geek]

I'm still wondering what in the world could have caused the ammonia.

I'm wondering if it could have been lawn fertilizer blowing in the pool from the neighbor's lawn or trees.

Doesn't lawn, tree and plant food contain some ammonia?

Rob

Fertilizer contains nitrates and phosphates which are algae food so if the FC/CYA ratio is too low then fertilizer will give a boost to algae growth and you can get an algae bloom. It also encourages bacterial growth if the chlorine gets close to zero.

It's soil bacteria that can get into the pool and if allowed to grow due to zero chlorine then they can convert Cyanuric Acid (CYA) into ammonia as described in this post. This can happen if one lets a pool go over the winter without maintaining a chlorine level. The symptoms are a large drop in CYA and an insatiable chlorine demand.

An ammonia test can be used for confirmation, but such a test gets interfered with monochloramine so one must also do a Combined Chlorine (CC) test (with a FAS-DPD chlorine test for accuracy) and subtract one-fifth of this amount from the ammonia result to know the true amount of ammonia. Also, there may be other intermediates that will consume chlorine and not measure in these tests so these tests give you an idea of the MINIMUM amount of chlorine needed before chlorine starts to register. A bucket test can give you an idea of the actual amount of chlorine that will be needed.

Richard

[Rednekk]

Update...

4 days since I shocked the pool, and the FC is still over 3.0.

[chem geek]

Update...

4 days since I shocked the pool, and the FC is still over 3.0.

That's great. So the chlorine is holding well. Now read up at the Pool School to learn how to maintain your pool and maintain the appropriate FC relative to your CYA level so that you can prevent algae growth. If you don't want to regularly add chlorinating liquid or bleach and instead want to use Trihclor pucks/tabs, then you'll need to use a weekly algaecide (PolyQuat 60) or a phosphate remover at extra cost.