Liquid Chlorine Usage Not Consistent

[Ssouthar]

Hey folks, I hope all of you are doing well! I've been super happy with our pool which came with our house purchased in Sept of '19. It had a lot of problems but I sorted it all out and it's been nearly perfect for the past year. I use the fantastic Pool Math app to keep track of everything. One thing I've struggled with is just getting my head around chlorine and the affects of loosing strength due to age. I've read everything I can find and I'm still missing how I'm supposed to calculate how much more LC I should be using as it gets older. I have a pretty good reliable source at a local grocery chain very close to my house. They store it indoors vs the large box hardware store all store theirs out side in the sun. That's great but they don't have a very fast turn around so, their supply is often quite old - upwards of 340 days. Because of that, I've been buying from Leslie's Pool Supply when they have it in stock. Recently, the local place had much newer stuff that was only 3 months old so I snagged a few cases. Here's what I'm dealing with now - I typically put 32 oz into a 20K gal pool. My CYA is stable in the low 40s. I keep the PH around 7.6 on average. Very recently, I'm noticing that the chlorine is not very stable but it's not predictable in how it's behaving. For example, I had an FC of 4.0 and put 64oz in (twice the normal). The next morning it was still 4.0. I put in 32oz and the next morning it dropped to 3.4. So I put in 96oz yesterday and today it's only recovered back to 4.0. Last year around this time as an example, I had FC of 3.4 and put in 32oz of LC and the following morning, FC was 7.2! Now one thing that has changed was I ripped out the Renaissance UV/Ozone unit because it sucked. Yes I know, you all told me. <grin> You were right. Could that be the difference? I'm just at a loss as to what is going on here.

 

[nobody291]

I live in Socal, so your experience might be bit different up north, but I think your chlorine loss is about what I would expect. My pool is about the same size as yours, and before my SWG conversion, I would add almost a gallon of 10% chlorine a day (that was fresh, almost always less than 3 months old) with a CYA of 40. The actual loss each day will obviously vary based on cloud cover, pool use, and time of year. I liked to keep a comfortable margin above the minimum, so some of my loss was because I intentionally kept my FC on the higher side. Unless you're using 25mL samples, your FC measurements suggest you might be using a pool store to determine FC. You should consider using the FC test kit suggested on this site. If you're interested in chlorine loss of potency with age, there are threads that discuss that in great detail on this site.

 

[Kellyp]

Do you test the chlorine 30 mins after adding to ensure that it is increasing the amount you expected? If you are, and it is, you may want to consider doing an OCLT to rule out algae that isn't visible yet eating your chlorine.

 

[Ssouthar]

I live in Socal, so your experience might be bit different up north, but I think your chlorine loss is about what I would expect. My pool is about the same size as yours, and before my SWG conversion, I would add almost a gallon of 10% chlorine a day (that was fresh, almost always less than 3 months old) with a CYA of 40. The actual loss each day will obviously vary based on cloud cover, pool use, and time of year. I liked to keep a comfortable margin above the minimum, so some of my loss was because I intentionally kept my FC on the higher side. Unless you're using 25mL samples, your FC measurements suggest you might be using a pool store to determine FC. You should consider using the FC test kit suggested on this site. If you're interested in chlorine loss of potency with age, there are threads that discuss that in great detail on this site.

Thanks nobody291! Yes I'm using the Taylor test kit and test every morning. It's just that I'm needing so much more than last summer but I'm suspecting it was the Ozone unit which helped with the chlorine usage but that is removed now.

 

[mknauss]

the Ozone unit which helped with the chlorine usage

The Ozone unit consumed FC.
So the increase is either from environmental factors (more sun, more swimmers, etc) or the chemistry in the pool is different.

 

[proavia]

Post a full set of current test results from your testkit.
How old are yout kit reagents?

FC
CC
pH
TA
CH
CYA
Water temperature

Follow the FC/CYA Levels
Use the PoolMath app
Also, consider doing an Overnight Chlorine Loss Test
If you don't pass the OCLT, you will need to SLAM Process

 

[Ssouthar]

Do you test the chlorine 30 mins after adding to ensure that it is increasing the amount you expected? If you are, and it is, you may want to consider doing an OCLT to rule out algae that isn't visible yet eating your chlorine.

Thanks Kellyp!
Yes I tested it yesterday afternoon to check that and 32oz at 10% kicked up the level from 4 to 5 which is pretty close to what I would expect in a 20,000 gal pool. The size of the pool is a best guess anyway. That's a good thougt on the OCLT so I will pursue doing that.

Post a full set of current test results from your testkit.
How old are yout kit reagents?

FC
CC
pH
TA
CH
CYA
Water temperature

Follow the FC/CYA Levels
Use the PoolMath app
Also, consider doing an Overnight Chlorine Loss Test
If you don't pass the OCLT, you will need to SLAM Process

Thanks proavia!
FC 4.6
CC 0
pH 7.6
TA 70
CH 440
CYA 42
Water temperature 79
CSI 0.02
I do use the Taylor kit and all regeants are up to date. I'm going to do the OCLT tonight. The water has been absolutely crystal for the past year with zero hickups. I brush it once a week and never a hint of anything coming off the walls like I had when we first bought the house. They had been using pucks with one of those inline injectors and CYA was up over 100 when I took it over. It was in pretty bad shape. I did a 3/4 draining and started with LC after refilling and have had zero issues.

 

[proavia]

How did you get a CYA of 42? That reading isn't possible on the test vial as the scale is logarithmic and not linear. Anything over 40 up to 50 is reported as 50. And you dose FC using CYA 50 according to the FC/CYA Levels.

Suggest you use a 10ml sample for testing FC - using one scoop of powder - to save on reagents. Count the drops, divide by 2 and that is your FC. Use the smaller sample size for the CH test also - each drop is 25ppm. That accuracy is close enough for normal frequent testing.

Let us know how the Overnight Chlorine Loss Test works out.

 

[Ssouthar]

How did you get a CYA of 42? That reading isn't possible on the test vial as the scale is logarithmic and not linear. Anything over 40 up to 50 is reported as 50. And you dose FC using CYA 50 according to the FC/CYA Levels.

Suggest you use a 10ml sample for testing FC - using one scoop of powder - to save on reagents. Count the drops, divide by 2 and that is your FC. Use the smaller sample size for the CH test also - each drop is 25ppm. That accuracy is close enough for normal frequent testing.

Let us know how the Overnight Chlorine Loss Test works out.

Ah! Well that's the wonderful thing about this forum - you learn stuff. <grin> Yep I didn't know the CYA test was not linear and just figured it was just above the 40 line so that must be about 42. I don't mind using the larger amount of regents as it's now my workflow and I like the greater accuracy. Plus, I track it all using the app and my TA, CH and CYA numbers drift extremely slowly. I can get away with testing those only once a month and any change in those is very small. If I ever notice a large swing I'll increase testing of course.
The OCLT came back with zero loss. I brought it up to 6.2 last night and it was 6.2 this morning. Combined was 0 in both cases. I have a couple new cases of pool store purchased chlorine I'm about to break into so I'll see if I see any difference in behavior as that chlorine is much fresher.

 

[proavia]

At CYA 50, your target FC is 6-8 and your absolute minimum is 4.
Never let FC get below 4. Target at the high end of the range. Most pools lose between 2 and 4 ppm FC per day. It is also okay to dose a little "hot" - more than 8. FC 4 (for CYA 50) is a cliff you never want to fall off of.

Given that you were at 6.2 last night and this morning, chances are good that you will drop below the minimum of 4 during the day today, unless you add chlorine soon.

 

[Palpatine]

My pool is probably a little larger than yours, but I'm in SoCal and the sun has been blazing lately. My pool also gets full sun all day. Currently my CYA is 50 and I lose about 4ppm a day (give or take). I'm dosing with 12.5% chlorine that I buy from the pool store that only the tradespeople can shop at. Yes, I'm that cool. Or, just lucky that my friend has an account there and lets me use it.

 

[Rich807]

My pool is about the exact size as yours & is getting brutal sun / heat right now for a big chunk of the day (its been above 110 degrees for day stretches lately) . My CYA is 50 & I'm dosing around 39 ozs of liquid pool chlorine (walmart brand) a day.

ETA: Running a baseline of 8ppm & lately when I test at end of each day the FC is around 6.5. CC is normally .5.

 

[MITBeta]

You can always test your chlorine supply by diluting 10,000:1. Put 1ml of bleach into 99ml of distilled water, stir that up, and take a 1ml sample of that and put it into another 99ml of distilled water and mix well. Now test the result like normal. You should measure the % of the bleach you're using as ppm.

 

[Ssouthar]

At CYA 50, your target FC is 6-8 and your absolute minimum is 4.
Never let FC get below 4. Target at the high end of the range. Most pools lose between 2 and 4 ppm FC per day. It is also okay to dose a little "hot" - more than 8. FC 4 (for CYA 50) is a cliff you never want to fall off of.

Given that you were at 6.2 last night and this morning, chances are good that you will drop below the minimum of 4 during the day today, unless you add chlorine soon.

Yep, that occurred to me and I added more that morning. My normal routine is test and adjust in the morning. It's never under 4 and I just did my monthly full test this morning. CYA drifted down under 40 so that's my number now. Alkalinity also dropped from 70 to 60 but hardness is still stable at 440.