ph keeps falling

[chem geek]

ljh,

With a vinyl pool, your CH is fine and I was looking for CH that was too high anyway so your pool's CH level doesn't explain the pH drop.

As for a pool's natural tendency towards a pH, that usually refers to a directional push based on chemical additions plus outgassing of carbon dioxide. So with Trichlor pools, the tendency is for lower pH and generally a base (Borax or pH Up / Washing Soda / Sodium Carbonate) needs to be added. For hypochlorite pools (bleach, chlorinating liquid) the tendency is neutral or rising pH depending on the aeration. For SWG pools, the tendency is usually rising pH due to aeration from the SWG. The degree varies based on the amount of aeration, but the direction is usually pretty consistent. For these latter pools with a tendency to rise in pH, the rise slows down at higher pH so appears to slow or stop at a higher pH (i.e. to reach a natural pH), but this is consistent with outgassing based on this table (moving from left to right).

The TA drop would be consistent with some sort of acid getting introduced into your pool combined with some degree of outgassing (assuming the pH was the same each time you measured the TA -- if the pH was lower, then a somewhat lower TA would be expected, though not a 20-30 drop). I wouldn't think DE would do that.

With a solar cover on, you are keeping the outgassing to a minimum so there is probably very little upward push on pH so that is understandable. The mystery is what is causing the pH to drift downward. 4 pounds of Borax at a TA of 100 (CYA of 30) should cause the pH to go from 7.2 to 7.4, not 7.6, but that could just be measurement error on both ends of the pH measurement. At a TA of 80, 4 pounds goes from 7.2 to 7.5 which is closer to what you are seeing.

Going from 7.6 to 7.2 in a week is like having the equivalent of 40 ounces of Muriatic Acid added to your pool. It is just very, very odd.

Increasing your TA will reduce the amount of pH drop per week, but won't change the amount of Borax you need to add. The carbonate buffer gets stronger at lower pH so dropping from 7.6 to 7.2 in one week would go to just below 7.0 the following week (i.e. it wouldn't go to 6.8). Nevertheless, with your vinyl liner you don't want the pH to get too low (i.e. below 7.0) so you do have to stay on top of this each week.

If you do aeration, then you can keep the pH more stable, BUT you'll have to add Baking Soda periodically to maintain the TA. If you aerated enough to compensate for the equivalent of 40 fluid ounces of acid per week, you would lose about 7 ppm TA every week and need to outgas slightly less and add 36 ounces weight of Baking Soda per week.

I think we need to figure out the source of the "acid". By any chance, this isn't a new liner, is it? Though we know that new plaster cures and causes a significant rise in pH, I've never heard of new vinyl doing the opposite, but I'm open for any suggestion at this point.

You say you check the chlorine daily, but what sort of chlorine consumption do you have at your 3-5 ppm FC level? With a cover and the lower temps, you shouldn't be having much consumption at all -- if it's more than 0.5 ppm FC or so then something is consuming chlorine and maybe that something is producing acid (though that's a heck of a lot of acid). Maybe the cover, since it's a solar cover, is letting UV rays through and that would lead to higher chlorine consumption. My cover is fairly opaque (it's an electric safety cover) so my chlorine consumption is very, very low when it's kept covered.

Richard

[ljh]

added baking soda today - will check the numbers tomorrow morning. the liner is 4-5 years old so I don't think that is it. I usually lose 2-3 ppm chlorine a day, even with the cover on. Depends on how sunny it is and how warm the water is- I have the clear bubble wrap type liner. The water is clear and there is never a lot of CC when I check - usually .5. I thought I was consuming a lot of chlorine because of the ph drift or low CYA....or is a that normal? I usually add the chlorine at night when the sun goes down so the chlorine gets to do some work (very light bather load, by the way, just me and a few friends, once or twice a week, not any splash out to speak of) and I check in the morning before I go to work to make sure it's between 3-5, and then I check when I get home at night to see how much to add that night. If it's cool or cloudy I may skip that day - and so far it's never been below 2-3, (Had algae the first summer - I am never fighting that battle again!) Does that seem normal or too much chlorine loss per day?

[chem geek]

It sounds like the cover lets UV through -- at least some. That would explain the variation on sunny vs. cloudy days. See if you can check the loss overnight. So add chlorine at night with the pump running, wait at least 30 minutes or up to an hour, then check FC levels (you can then turn off the pump if you don't normally run it at night). In the morning, turn on the pump if it hasn't already started, wait at least 15 minutes, then check the FC level (early in the morning before the sun gets too high in the sky. What is the FC drop? That will tell you if you've got something consuming chlorine other than sunlight.

The loss is normal if the cover lets UV through so let's assume that's the case. The overnight (non-sun) loss is really the key so let's see what that is before jumping to conclusions.

The pH drop is still a mystery. Though you have little splash-out, check the pH of the fill water just in case (maybe there's a leak in the pool -- doubtful though).

Richard

[ljh]

Alk is now up to 100, will try testing for CL at night and again in morning and post results, thanks for the suggestion

[ljh]

last night at 10:30 pm CL was 4, this morning at 6:30 am it was 3.5 with .5 CC. So, should I shock, add algaeside, filter more? I've added 8 lbs of baking soda to move the TA from 80 to 100 and I've added 4 lbs of borax to move the ph from 7.2 to slightly under 7.5 (the color isn't quite 7.5 but definately not 7.2)

[chem geek]

I would shock the pool. At the CYA level of 30 ppm you reported, I would shock to at least 12 ppm FC (Ben's chart says 15 ppm so you can do that instead -- 30 ppm CYA is at the low end of the range) and hold it there until the following three things occur: 1) water is crystal clear (it may already be), 2) FC does not drop overnight or does so minimally (0.5 ppm is on the edge), 3) you do not measure any significant CCs (< 0.5 ppm). So your measurement overnight is on the edge so shocking would be reasonable to do.

[ljh]

okay, will do, I have company coming for the 4th so probably a good idea anyway. The water is crystal clear, but I want to make sure it stays that way. The temp will be cool this week (low 70's day/ 50's night) the water temp has dropped to 76. even with the cover on all day and night.