Daytime Chlorine Demand Test

Homebrewale

Well-known member
Apr 21, 2020
642
Apex, NC
Is there a test similar to the OCLT except you determine the chlorine demand during the day? I've thought about this. If I can determine the chlorine demand during the day, I could use that information to set my SWCG.

Occasionally, I run the OCLT just to see if everything is running right. I did this again on Sunday night. In the evening, the FC was 10.0 ppm. The next morning it was 9.5 ppm. Since my FC was still plenty high, I decided to keep the SWCG shut off for the day and retest the next evening. My resulting FC was 7.5 ppm. I lost 2 ppm during the day. Interesting enough, my SWCG is set to produce 1.8 ppm per day. My FC for the last few weeks has been steady at around 10 ppm. It seems that I confirmed that my SWCG is set for the right percentage to produce what my pool needs daily.
 

HermanTX

Gold Supporter
May 20, 2020
2,759
Katy TX
Pool Size
14600
Surface
Plaster
Chlorine
Liquid Chlorine
It appears you got it set correctly and correlates to your test. However you loss 2.5ppm overall but not sure if that was a 24 hr period between tests - the one at 10ppm and the one at 7.5ppm. The only other method would be to turn off the SWCG, test before sunrise and then test again at sunset. That would be the PPM loss for that specific day, temperature, cloudiness, etc. So you could do it once a week and then average to obtain a mixture of different "day" parameters.
 

Oly

Gold Supporter
Jun 28, 2017
2,345
Fresno, CA
Pool Size
27000
The precision of the FC test has limits depending on the 10 or 25ml test. Also your SWG may not produce exactly the same amount of FC (unless you are running 24 hours) due to the hours you run it relative to the set cycle time of your unit. It is best to have your FC above your target to provide some buffer for day to day fluctuations including heavy demand. This is most important in the spring as FC demand is steadily increasing.
Once your are close with your SWG adjustment, testing FC every 3 or 4 days will provide a good idea of your output vs your demand trend, but this is a moving target as the year progresses. Once you are dialed in and can confirm you unit is functioning properly each day then testing once a week at a minimum will guide your ideal setting.
 

Homebrewale

Well-known member
Apr 21, 2020
642
Apex, NC
It appears you got it set correctly and correlates to your test. However you loss 2.5ppm overall but not sure if that was a 24 hr period between tests - the one at 10ppm and the one at 7.5ppm. The only other method would be to turn off the SWCG, test before sunrise and then test again at sunset. That would be the PPM loss for that specific day, temperature, cloudiness, etc. So you could do it once a week and then average to obtain a mixture of different "day" parameters.

Yes, overall there was a loss of 2.5 ppm over a 24 hour period. I considered the OCLT test where it went down 0.5 ppm to mean the pool passed the test and there wasn't any algae in the pool. Therefore, all of my loss from sunrise to sunset would be due to UV. The 0.5 ppm loss could have been due to two things. One is that since it takes 20 drops to measure 10 ppm, it could just be testing inaccuracy and there was no loss at all. Or there could be small loss due to some organics in the pool, particularly all the fine mulch that I'm still trying to completely vacuum up that was deposited into the pool by my daughter's dogs that love to swim and then roll in the mulch. Drives me nuts when the dogs step into the pool and brown clouds form in the water around their legs. The dogs spent the whole weekend at my house.

I considered the FC loss during the day to be worst case since the temperature hit 92F and there wasn't a cloud in the sky all day. Worse case for this time of year since each day the amount of daylight is decreasing and the angle of the sun is getting lower. As for your question on period of tests, my tests should be linked through PoolMath in my profile. Basically, I read 10 ppm at 7:30pm, 9.5 ppm at 6:45am and 7.5ppm at 7:30 ppm. So it was 24 hours between first and last test.
 
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Homebrewale

Well-known member
Apr 21, 2020
642
Apex, NC
The precision of the FC test has limits depending on the 10 or 25ml test. Also your SWG may not produce exactly the same amount of FC (unless you are running 24 hours) due to the hours you run it relative to the set cycle time of your unit. It is best to have your FC above your target to provide some buffer for day to day fluctuations including heavy demand. This is most important in the spring as FC demand is steadily increasing.
Once your are close with your SWG adjustment, testing FC every 3 or 4 days will provide a good idea of your output vs your demand trend, but this is a moving target as the year progresses. Once you are dialed in and can confirm you unit is functioning properly each day then testing once a week at a minimum will guide your ideal setting.

I run my SWCG 24 hours a day. About the only time it is dialed in is mid-summer. It just the way my pool operates. In the spring, it's covered by a solar cover. As the days get longer, demand goes up some since the cover only partially blocks the UV. Then sometime in June, the solar cover comes off permanently and FC demand shoots up. It's constant adjustments on the SWCG output. Then in the fall starting right about now, my solar cover goes back on. That drops FC demand quite a bit, along with the shortening days. Now I'm chasing it again as I decrease the SWCG output. One thing I'm not concerned about right now is having a buffer. I've been consistently near 10 ppm this whole swim season. I would really like to be closer to 7-8 ppm but have not been successful. Probably because I like to tweak settings instead of making big moves.
 
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