Why is this happening?

[mkfmedic]

When I left to go to work day before yesterday, I was feeling pretty confident about my water chemistry. I have an 11,000 gallon pool/spa with a Jandy SWCG, Del Ozone system and a DE filter. We have two skimmers and a Navigator cruising the bottom.

If it matters, we live about a half a mile from the beach, so we aren't dealing with extreme heat, but it has been in the 80's everday for the last week and the pool has a full southern exposure.

On a previous post, I noted that my chlorine seemed to be low, "all of a sudden," and that I had overflowed the pool losing quite a bit of water. I was thinking that I was missing CYA, but was advised to lower PH and see results. I added approximately 1.15 pints on August 10th, and my water results seemed to come together:

TC - 4
FC - 4
CC - 0
PH - 7.2
Alk - 100

My SWCG is set to 40%.

I came home this morning and my PH is up and chlorine is down again.

TC - 2/3
FC - 2/3
CC - 0
PH - 8.2
Alk - 100

So the drop in chlorine doesn't really appear to be that big a deal, but why would my PH go up that much in a 24 - 48 hour period. Using the Taylor test, it suggested adding another 1.15 pints of muriatic acid which I have done. What els should I be doing and is it normal to have to add acid again so soon?

Any feedback would be greatly appreciated!

Mike

I am still anxiously awaiting my new test kit from this site and am currently unable to measure my CYA with my current test kit.

[duraleigh]

Hi, Mike,

I'm not much help....that's a mystery. The SWG contributes some but that pH rise seems excessive.

My point in posting is to remind you to get it down with muriatic right away. 8.2 is a little too high for any long period of time. I'm sure you knew that but just making sure.

[chem geek]

Even if your CYA was low, this would not account for such a large pH swing. Also, normally a change in pH of this large magnitude would be accompanied by a change in TA as well, but you measured no such change. The only process that increases pH without changing alkalinity is the outgassing of carbon dioxide, but to have the change you saw you would have had to have outgassed about 14% of all of the carbonate in your pool which is a huge amount in such a short period of time. Also, though at 7.2 pH and 100 TA you were likely to outgas CO2 and have a pH rise, this should have slowed down at around 7.5 and not continued on upward unless you had incredibly unusual and efficient aeration.

Now it is possible that somehow your pool had introduced into it the equivalent of 1 pound of caustic soda (lye) or equivalent base and this would increase your pH from 7.2 to 8.2 but should also have increased your TA from 100 to 113. Now the difference between 100 and 110 is just one drop on your TA kit so maybe this is something you didn't notice. So this leaves us with the question of what is introducing so much base into your system. You have both an SWCG and an ozonator, but I can't see how either would normally cause so much base to be introduced (but see below). The SWCG generation does in fact raise the pH when generating chlorine, just as adding liquid chlorine or bleach would do, but then as the chlorine gets used up this would be acidic and restore the pH back to where it was.

There is one "strange" possibility. If for some reason your salt generator had a gaseous leak in it or were operating in a way that the chlorine gas that was generated did not dissolve into the water and instead got outgassed somewhere, then it turns out that if you were to generate 7 ppm of chlorine in the SWCG and outgas the chlorine gas, then this would cause a pH rise from 7.2 to 8.2 in your 11,000 gallon pool and the TA would rise from 100 to 113 just as if caustic soda (lye) were added to your pool. However, I don't know how this could happen in a SWCG.

My hunch is that there is something wrong with your test results (or test kit reagents). The pH test is affected by high levels of chlorine (usually > 10 can cause a problem), but you did not show such high levels. I do not doubt that you had a pH rise, but it just doesn't seem right that the pH jumped from 7.2 to 8.2 in such a short period of time. The only other way this could happen would be if your TA were incredibly low since then small changes in your pool chemistry can lead to large pH swings.

So you have a mystery on your hands and I would first validate the pH reading, possibly using another test -- even an "inaccurate" cheap pH test "strip" would at least give you an idea if you are truly in the right ballpark.

Richard

[waterbear]

What testmethod are you using for chlorine? If it is a DPD test it is possible that your chlorine is actually very high and the test is bleaching out somewhat. A high chlorine level would cause a bogus high pH reading.

Low chlorine readings could also be caused by your ozone production being too high...ozone will destroy chlorine....don't know what effect, if any, it would have on pH.