SWG - ph always normal?

[salinda]

I have a gunite pool and the ph always rises. Maybe it is the gunite....

[PatL34]

You have me confused too as I have a DiamondBrite pool, and am having to maintain the pH using Muriatic acid which is part of the electrolytic process.

To maintain a steady pH, you either have to use acid, or salt to replace the salt that is used in chlorination. Adding salt by itself, should eventually lead to a rise in pH as caustic soda is being formed, and has to be neutralized with Muriatic acid. Using dry acid will also neutralize the caustic soda and lower pH, but raise the TDS.

To have a constant pH, something has to be depleted to keep the Cl level constant. Any upsets will affect the Cl level, requiring extra chlorination. This will affect pH as something has changed regardless of being a vinyl or concrete pool.

I would do a check as to what has to be added constantly to maintain this pH level, because everything I am seeing from this thread does not make sense according to the laws of physics.

Pat

PS: Noticed that Nater is adding salt to maintain the correct level. Ok, that accounts for the change I was expecting, but the pH should have started to go up, as more caustic soda is being created, but no acid is being used to neutralise it to keep the pH constant. Somewhere there is a control or buffer keeping the pH steady.

Oh boy, more research!!

[nater]

Hey Pat, here's some more info on my operation that may glean some clues. By the way, your "SWG Lessons Learned" post should be a sticky, it's full of great info!

Background:
I inherited the pool last April when I bought the house and was thouroughly "pool stored" all last summer. This led to my interest in a SWC as a cost/hassle savings opportunity. Thank God I found this Site over the last winter!!!!

Since I cleaned up the initial mess and balanced the pool in March (when my Autopilot went on-line), my pH had been stable at 7.2 . Last week I added more salt. Prior to that, I had exchanged roughly 8,000 gallons of water to lower my CYA from 150+ to 90-100 (current level) over the course of a few weeks. This water exchanging brought my salt level from roughly 3400 initally down to 2800, hence the roughly 60 pounds of salt I added to get back to 3200. I plan on continuing this over the next month or two unitl I'm in the 60-80 ppm range on CYA.

Also last week I added 4# of baking soda and 4# of Borax to bring my Alk up from 80 to 110, and my pH up from 7.2 to 7.5.

My Fill water (city) tests around 7.2 for pH, with 80ish for Alk.

I run a fountain for fun about 50% of the time (when the solar cover is off). I have seen my alk drop slowly due to this constant aeration, and is what led to my gradual Alk decrease. Perhaps this will not be as evident at the 7.4-7.6 pH range.

My Autopilot is the DG-220 with a 60 series cell, so it runs on Level 1, 12%-15% output to maintain my FC level. Roughly 25V on the display. If I put into boost mode, I'll see H2 bubbles in my return stream, but not typically during normal operation. (I don't use the boost mode anyway as I'll use bleach to shock if needed).

I've never tested my rain water, but I will. It's a very small percentage of total volume anyway, so I don't think it's keeping the pH down.

I've still got the two gallons of Muriatic acid in my garage I'd purchased before my Autopilot came on line, fully expecting to use them by now. I'm not complaining though!

I'm interested to hear if you've got any theories/thoughts to test. I like to experiment with things, and a little broader knowledge of the SWC/pH interaction wouldn't hurt!

[PatL34]

Nater,

I am trying to relocate an article that help me develop the chemistry of the SWC operation. Sorry about the wait. There is a definite correlation between SWC performance and pH, and it is that at 7.2 pH the SWC works best, and at 7.8 pH it is at its worst. That is why I like to work between 7.2 - 7.6 pH.

As I said earlier, I need to find out why a vinyl pool SWC is more stable than a plaster pool SWC, aside from the pH rise, that is expected with a plaster pool. This is still bugging me.

Pat

[nater]

Here's an update. I am now seeig a gradual increase in pH over several days. My Chlorine demand is much higher now that we're swimming alot and the weather has been very warm and sunny. I'd say my pool now more closely resembles the SWC model you're familiar with.

Typically I'll wait for it to creep up to 7.6, then I have to dose with a full gallon of Muriatic acid to get back to 7.2. After that I've been good to go for 1-2 weeks.