Decreasing chlorine consumption after converting from SWG to liquid chlorine

al27

Well-known member
Sep 11, 2017
83
TN
Our salt water cell has died so we are using liquid chlorine to chlorinate our pool until we can afford to replace it. We recently completed a SLAM, passed the OCLT, and bumped up the CYA to 55 with liquid CYA. We are losing 4-5 ppm of chlorine a day which is more chlorine loss than I expected with that high of a CYA. Liquid chlorine prices have gone up because of the reported chlorine shortage and it is becoming hard to find, so I need to get chlorine consumption down, if possible. I tested my water and here are the results: FC 6.5, CC .5, PH 7.4, TA 70, CH 200, CYA 55. I did an OCLT last night with chlorine at target level and only lost .5 ppms of chlorine overnight. Water is crystal clear and I don’t see any hidden algae anywhere. Pool is surrounded by trees and I have no pool robot but I’m removing any leaves that blow in at the end of each day, dump out the skimmer once or twice a day, and vacuum once a week. Since our pool has 1900 ppms of salt, I’m wondering if I need to increase CYA higher than 55, like you would for a salt water pool. If increasing the CYA might help, how high should I increase it? I don’t plan on using any chlorine tablets this year, since they are impossible to find, so I’m wondering how high I can go with the CYA without causing any problems. I might buy a replacement salt cell later this year, but I’d really like to just continue using liquid chlorine for awhile since I love that I don’t have to add muriatic acid often like I did with the SWG.
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
27,926
Northern NJ
Pool Size
35000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
Refer to Chlorine / CYA Chart - Trouble Free Pool

Your current CYA is 60. We round up.

At CYA 70 your FC target is 8-10.

At FC 10 and above the pH test begins getting invalid. In addition if you should need to SLAM the pool due to algae you will then need FC of 28 or more.

Overall running CYA above 60 makes a liquid chlorine difficult to manage which is why it is not recommended

Raising your CYA by 10 or 20 is not likely to chnage your chlorine demand significantly.
 
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al27

Well-known member
Sep 11, 2017
83
TN
Do you think the high chlorine demand is caused by the leaves/debris that falls into the pool, even though I'm diligent about removing it at the end of the day and the debris isn't excessive? Or is it normal to lose 4-5 ppms of chlorine with a CYA of 55/60 in the summer when temperatures are in the 80s-90s? I never experienced chlorine loss like this when using the SWG with a CYA of 70/80, but I know that chlorine is slowly added throughout the day with that method instead of just being added once a day.
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
27,926
Northern NJ
Pool Size
35000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
Do you think the high chlorine demand is caused by the leaves/debris that falls into the pool, even though I'm diligent about removing it at the end of the day and the debris isn't excessive?

No.

Or is it normal to lose 4-5 ppms of chlorine with a CYA of 55/60 in the summer when temperatures are in the 80s-90s?

It is on the high side but I wonder about the actual strength of the liquid chlorine you are using.

Exactly how are you calculating your FC loss?

I never experienced chlorine loss like this when using the SWG with a CYA of 70/80, but I know that chlorine is slowly added throughout the day with that method instead of just being added once a day.
All of this is within the margin of the measurement errors.
 

al27

Well-known member
Sep 11, 2017
83
TN
No.



It is on the high side but I wonder about the actual strength of the liquid chlorine you are using.

Exactly how are you calculating your FC loss?


All of this is within the margin of the measurement errors.
I'm using 10% liquid chlorine. I calculated the loss by checking FC in the morning when I get up and checking it again in the evening before I add the chlorine. Even though my pool is surrounded by trees, they provide very little shade so the pool is in full sun most of the day and temperatures have been in the upper 80s/low 90s.
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
27,926
Northern NJ
Pool Size
35000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
From Free Chlorine - Further Reading

Does Chlorine Consumption Change with Water Temperature?​

Warmer water losing more FC due to sunlight than colder water is not true.

Chlorine loss from the UV in sunlight does not depend on temperature because it only depends on the number of photons per area entering into the pool and on the concentration of chlorine. The photons of light are traveling much faster (at the speed of light) than the molecules containing chlorine so the temperature which relates to the speed of those chlorine molecules is irrelevant. From the point of view of the photons, the molecules of chlorine are essentially standing still so the cross-section of those molecules which is the area with which the photon has a quantum probability of reacting with the molecule is independent of the temperature and only related to the concentration of such molecules in the water.

Chlorine consumption that is related to temperature is for chemical reactions with chlorine such as oxidizing pool covers, bather waste, pollen, leaves, algae, etc. And yes, algae grows faster in warmer water but such consumption won't matter if there is sufficient FC/CYA since algae will get killed before it can reproduce so the rate will be based solely on the rate of blown-in algae spores which is usually not measurable (pollen, on the other hand, can be voluminous as can pods and other material dropped from trees).

A pool with a solar cover that is opaque to UV would lower the loss of chlorine from sunlight that is not temperature dependent but would increase the loss of chlorine from oxidizing the cover which is temperature dependent.

There can be a temperature dependence on the subsequent chemical reactions that occur after the photochemical reaction occurs.[5] But for the breakdown of chlorine by UV, it's pretty much all over with such breakdown since the probability of having the OH• and Cl• reform compared to other reactions that lead forming chloride and oxygen gas is low and not temperature dependent. At most, the intermediate concentrations of some intermediate species such as hydrogen peroxide may be higher at lower temperatures, but by that time it's too late and the chlorine is already on it's way to becoming chloride.

There can be a small dependence on temperature in terms of the rate of relaxation from excited states so there is a small but negligible temperature dependence on having the HO-Cl vibrational state be excited and not break apart as often because of low temperature, but it takes a very low temperature before that effect would be seen in practice.
 
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