Chlorine levels not rising to calculated levels

GlendalePool

Member
Sep 24, 2019
8
Glendale,AZ
Hi all,

My first pool and TFP been helping alot. I have the PoolMath app. Details of my pool and equipment in signature.

Briefly: my pool is clear and I'm happy with the clarity. Algae not present. But it is taking me a lot of chlorine daily to keep it looking good.

My CYA level is 55. Yesterday AM Cl was 6 and 4 pp at dusk. Based on PoolMath, I put in the recommended amount during SLAM to bring my Cl to 24. I put the amount in at dusk and at dawn the level was only 19.

I'm writing now because this has happened every day: rise does not match calculation.

The Cl was Home Depot 10%. But even when I use the 12.5% from the local pool store (bottle exchange program) the rise in daily Cl does not match the PoolMath Calculation.

For my 12K gallon pool, I'm adding 64 oz of 12.5% Cl per day. This seems like a lot to me. The dawn levels are never matching the levels that PoolMath gives me.

I can understand the Home depot Cl may be weak, but I would expect the pool store version to be closer to advertised. Is there something I'm missing?
 

mas985

TFP Expert
LifeTime Supporter
May 3, 2007
13,700
Pleasanton, CA
Are you measuring FC shortly after dosing (i.e. within 60 min)?

Both sun and algae reduce FC levels so if you wait too long, it won't be related to what you added. However, once the algae is defeated, overnight CL loss should be minimal but it may take a while.
 

GlendalePool

Member
Sep 24, 2019
8
Glendale,AZ
Are you measuring FC shortly after dosing (i.e. within 60 min)?

Both sun and algae reduce FC levels so if you wait too long, it won't be related to what you added. However, once the algae is defeated, overnight CL loss should be minimal but it may take a while.
I am dosing only at dark and I'm checking the next AM before sun up.

Another example: if my PM Cl is 4 and I want the next AM to be 7, pool math tells me to add 37oz 12.5%. If I add that amount, the AM level is only 5.5. I'm adding 64 oz to get to that level. It's been frustrating to go through a gallon every 2 days for a pool that's not very big.

I just don't want to be missing anything obvious.

Also I've cleaned my filter cartridges 3x since March and also cleaned out the pool light niche as well. Use the pool robot every 1-2 days.

I guess bottom line: I can understand Cl drop during the day from the AZ sun. I can't understand Cl consumption over night to the degree I'm seeing.
 
Last edited:

mas985

TFP Expert
LifeTime Supporter
May 3, 2007
13,700
Pleasanton, CA
I would measure at night when you dose and also in the morning. The reason the FC levels are lower in the morning may be because the algae is still alive using up the FC. You have to continue SLAM levels until the loss overnight is negligible.
 

cme4oil

Bronze Supporter
Sep 19, 2019
45
Phoenix Arizona
Hi there

I am in Peoria so not far from you. Generally you won't lose much chlorine overnight but you won't know if you don't check the levels about 15 - 30 minutes after dosing at night. So, dose at night, check your FC / CC and the check it in the morning. If you're losing that much overnight you might have an algae issue.

When you follow the slam, make sure to check your FC levels several times a day and add chlorine to keep at the SLAM level until your OCLT is .5 or less.

Tons of great people here as well as all the information you coul possibly want.

I am dosing only at dark and I'm checking the next AM before sun up.

Another example: if my PM Cl is 4 and I want the next AM to be 7, pool math tells me to add 37oz 12.5%. If I add that amount, the AM level is only 5.5. I'm adding 64 oz to get to that level. It's been frustrating to go through a gallon every 2 days for a pool that's not very big.

I just don't want to be missing anything obvious.

Also I've cleaned my filter cartridges 3x since March and also cleaned out the pool light niche as well. Use the pool robot every 1-2 days.

I guess bottom line: I can understand Cl drop during the day from the AZ sun. I can't understand Cl consumption over night to the degree I'm seeing.
 

GlendalePool

Member
Sep 24, 2019
8
Glendale,AZ
Hi there

I am in Peoria so not far from you. Generally you won't lose much chlorine overnight but you won't know if you don't check the levels about 15 - 30 minutes after dosing at night. So, dose at night, check your FC / CC and the check it in the morning. If you're losing that much overnight you might have an algae issue.

When you follow the slam, make sure to check your FC levels several times a day and add chlorine to keep at the SLAM level until your OCLT is .5 or less.

Tons of great people here as well as all the information you coul possibly want.
thank you for the follow up. That’s the confusing part. The fluid is always clear when I add the five drops of R-0003 in the AM test after the FC test. if it was being consumed at the rate because of algae I would expect there to be some color on that total chlorine test. But there’s not.

I just wonder if I’m getting some weak chlorine that’s not acting at the advertised strength
 

BadDogPSD

Gold Supporter
May 24, 2020
112
Reno, NV
Only way to know for sure is to do the OCLT.
Test in the evening after sunset. Add the recommended amount liquid chlorine while the pump is running.
Retest after 30-60 minutes. Did the amount of chlorine you added raise your FC as expected? If not, your pool may have more volume than you thought.
If yes, restest in the morning before sunrise. If your morning test is more than 1ppm less than your evening retest, you still have algae growing.