Disappearing chlorine...

The pool I was working on getting algae free is doing great, I had to tweak a few levels but the Cl is disappearing! The last 2 times I checked the pool with a OTO kit, it barely registered. I use a Taylor FAS-DPD to verify the HTH kit, and there is between .5-1 ppm of free Cl and zero combined. The owner is adding 1/3-1/2 gallon of 6% bleach daily - but I don't think it's enough for a 17K gallon pool. I just added in 1lb of 73% cal hypo and a cup of 54% cal hypo resulting in a free Cl of 7.5 per FAS-DPD test. The CYA level was at 60 last week but we added water in the pool, and now CYA tests at 40.

The pool's in the sun for most of the day, and so I'm wondering what to do either a) more bleach or b) supplement the bleach with a HTH skimmer tab to keep the free Cl a bit more stable.

In a 17,000 gallon pool, each quart of 6% bleach is only going to raise the cl by slightly under 1ppm. So a half gallon is only going to add a little under 2ppm. If he is only adding 1/2 gallon daily, that is not enough. With a cya of 40, the cl needs to stay between 3-6 ALL the time and the shock level is 15. So either he is going to need to use a larger daily dose of bleach or supplement with cal-hypo. I wouldn't suggest using triclor as his CYA is already 40. Don't really want it higher and the trichlor will increase it.

We really could use a complete set of current water testing numbers to better advise. Also what type of pool is this. It would be better to know these things first before recommending the use of cal-hypo. And, please remind us again -- are you in the pool industry or just helping a friend with his pool? (We have so many members and so many different threads that it is hard to remember.)

Thanks.

EDIT -- Since he has let his cl drop so low, I would suggest shocking it up to 15 just in case a bloom might be trying to get started since you can't always see it at first. Probably doesn't but it is cheap insurance just in case. Easier to do this than to clean up algae later.

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Originally Posted by Watermom

In a 17,000 gallon pool, each quart of 6% bleach is only going to raise the cl by slightly under 1ppm. So a half gallon is only going to add a little under 2ppm. If he is only adding 1/2 gallon daily, that is not enough. With a cya of 40, the cl needs to stay between 3-6 ALL the time and the shock level is 15. So either he is going to need to use a larger daily dose of bleach or supplement with cal-hypo. I wouldn't suggest using triclor as his CYA is already 40. Don't really want it higher and the trichlor will increase it.

We really could use a complete set of current water testing numbers to better advise. Also what type of pool is this. It would be better to know these things first before recommending the use of cal-hypo. And, please remind us again -- are you in the pool industry or just helping a friend with his pool? (We have so many members and so many different threads that it is hard to remember.)

Thanks.

EDIT -- Since he has let his cl drop so low, I would suggest shocking it up to 15 just in case a bloom might be trying to get started since you can't always see it at first. Probably doesn't but it is cheap insurance just in case. Easier to do this than to clean up algae later.

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I'm helping a friend with his pool. I'll do a another test tomorrow. The pool is a concrete/plaster pool - so the pool needs Ca.

with a pool that gets all-day sun, a solar cover is very helpful in decreasing the chlorine demand. my cover doesn't do a whole lot to warm the pool in the summer - in fact it warms up more during the day with the cover off - but it probably cuts my chlorine usage in half. (for the record, the solar cover holds in the heat at night far better than it generates heat during the day. which has nothing to do with what you asked...)

anyhow, when i was using 6% bleach in my 24,000 gallon pool, with the solar cover on when the pool wasn't in use, i was putting in 48 oz. each night and that would keep me at a pretty consistent 2 - 2.5 PPM. i had to use twice that over periods when i got too lazy to haul the cover on and off.

The type of cover seems to make a difference with clear covers not doing much to block the UV in sunlight while blue covers seem to block some if it (though not all). So for protecting the chlorine in the water, the blue covers do a better job. Even better for chlorine protection is an opaque cover, but then you don't get much heating from the sun during the day.

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Originally Posted by chem geek

The type of cover seems to make a difference with clear covers not doing much to block the UV in sunlight while blue covers seem to block some if it (though not all). So for protecting the chlorine in the water, the blue covers do a better job.

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Is this an educated guess or a result of experience / experiment? I remember this was discussed last year (different forum) and I decided to replace my blue solar cover with a clear one -- it's on the pool now but not enough time has elapsed for me to be able to draw any meaningful comparisons.

I've always noticed I used less chlorine with the cover on. I don't know how much is due to UV blocking and how much is due to inhibiting evaporation. I don't KNOW that clear covers don't block UV. Clearly auto glass does--ask anyone with photochromatic eyeglasses--they only change in direct sunlight (the old glass lenses changed in the car--the new plastic ones don't). Blue PROBABLY blocks UV better, but, again, I don't know.

Clear allows more light to reach the water, heating more, but the greenhouse effect of both is important, as is the insulation qualities.

But I've also found that cheaper covers fall apart after 3 seasons, but the expensive ones always go a 3 full years, and the warranty NEVER covers what fails--just the seams....So I now go with the cheaper ones, which are also easier to carry on and off when the season begins and ends.

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Originally Posted by polyvue

Is this an educated guess or a result of experience / experiment?

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Multiple reports from users of clear and blue covers. I have a mostly opaque cover myself so no surprise that it cuts down on daily chlorine use. There were at least two separate pool owners who had tried both types of cover -- clear and blue -- and both found that clear did not significantly lower chlorine demand but that the blue one did. Other people using only one type of cover also reported not much change with the cover on vs. off with clear, but noticing a difference with the blue cover.

See this post for one example (you were on that thread so might remember this post). This post is another. This post is another. It is consistently reported that the covers that let more light in, so are more clear, heat the pool more during the day (which makes sense). So the tradeoff is between daytime heating vs. chlorine loss. The ideal cover would block UV but let infrared and visible light through, but that doesn't seem to exist.

I had initially hoped that the clear cover would block UV but let infrared or visible through, but that apparently is not the case. Plastic doesn't behave the same as regular glass which does block a lot of UV (quartz glass lets UV through).

I haven't had time to fool with the pool - but the pump impeller was trashed. $140 of parts from Leslie's(only place locally that had Hayward parts) and 2 packs of HTH later the pool's somewhat back in shape - algae is starting to rear its ugly head but the pool was scrubbed down yesterday.

Levels:
Free Cl: 2ppm, I had it at 3.5 yesterday
TA: 120ppm
CH: 350ppm
pH: 7.5
CYA: 65ppm

I told my old boss to dump in 84oz of bleach per the TFP calc. His next purchase will be one of those blue solar covers - but I'm trying to get him to invest in an Liquidator. :D

CH:350?, CYA:65? FC: 2?You should now ONLY be using bleach or liquid chlorine.

HTH (cal-hypo powder) will rapidly push your CH over 400--the max recommended for plaster/concrete pools.

Tri-Chlor tabs and Di-Chlor powder will push your CYA up which is ALREADY very high at 65.

FC is far too low and NOW needs to be between 5 and 10ppm just for maintenance.

You are still within normal, recommended ranges for everything (but your low FC). But I would STRONGLY recommend you stick with bleach/LC given your CH and CYA numbers.

And to add to what Carl said above, if algae is trying to start again, you need to shock the pool up to 20. Then, try and hold it there for a few days until you can go overnight with no more than 1ppm cl loss. Test and dose with bleach at least morning and evening when the sun is off the pool, and if someone can test and dose a time or two during the day, all the better.

Also, I'm noticing some reprocity error with the CYA tests - when I do them, I drop in the CYA/melamine reagent mix until I cannot see the black dot - between 40-50. When me and my friend did it in the sun, he dosed it in the tube until the dot just disappears between 60-70ppm. Could age and vision have anything to do with this?

So you have a 10 ppm discrepancy. This is within the precision of the test. Split the difference and call it a day. It was from the different lighting conditions since I assume you did not do the test in full sunlight when you got the 40-50 ppm.
BTW, I prefer to do this test in open shade and not full sun. It makes it easier to determine when the dot disappears because there is no glare on the surface of the sample.

Taylor recommends that you stand outside with your back to the sun so that you shade the tube in front of you looking down into the tube. If you have the sun shining directly on the tube, you'll get a reading that is too low. If you do the test with insufficient lighting, you'll get a reading that is too high. It's a finicky test, especially compared with the titration tests.

But, even more importantly is to do the test the same way every time. This allows you to track changes and mark progress. You learn over time if you are over- or under-representing the results and compensate.