Relation between alkalinity and pH

[basjke]

I have a saltwater pool with a Zodiac chlorinator with pH module for a few years already, and the pH was pretty stable at 7.6 with an alkalinity of about 40 ppm.

Then about a month ago I made the mistake to read somewhere that alkalinity should be around 110 ppm, so I added baking soda.

Soon I discovered that my pH would rise quite fast, at about .4-.5 points a day, so I started to add HCI to lower the alkalinity again.

In the mean time alkalinity is back around the 40 ppm level, but my pH still keeps rising at about .2 points a day.

Am I correct that to stabilize the pH level I should bring the alkalinity level further down, or is there a danger adding more HCI until I reach that point?

Should I look at another cause, however to my knowledge nothing has changed other than that I added baking soda.

My CYA levels are kept below 50 ppm on purpose since my free chlorine levels test above 5 ppm. I'm using Aquachek silver 7 in 1 strips to check my chemical levels, which I understand may not be 100% accurate, but the pH level is read by the Zodiac pH module of course

 

[JasonLion]

Welcome to TFP!

TA has two separate effects. First, higher TA levels make the PH more difficult to change with chemical additions. Second, higher TA puts upward pressure on PH, which will raise the PH in the presence of aeration. So, yes lowering the TA a little bit more should eventually stop the PH drift, unless there has been a significant increase in aeration.

We don't recommend lowering TA below about 60 unless you are using borates. Low TA without borates leaves the PH able to swing around too easily in response to any chemical additions.

I don't really follow your logic on CYA. We recommend CYA be between 70 and 80 with any salt system. Compared to where you are now that will lessen the PH drift and increase the lifetime of the SWG (salt water generator) cell. Also, the unit should be adjustable so that you can set the FC level anywhere you want it.

 

[basjke]

Welcome to TFP!

TA has two separate effects. First, higher TA levels make the PH more difficult to change with chemical additions. Second, higher TA puts upward pressure on PH, which will raise the PH in the presence of aeration. So, yes lowering the TA a little bit more should eventually stop the PH drift, unless there has been a significant increase in aeration.

We don't recommend lowering TA below about 60 unless you are using borates. Low TA without borates leaves the PH able to swing around too easily in response to any chemical additions.

I don't really follow your logic on CYA. We recommend CYA be between 70 and 80 with any salt system. Compared to where you are now that will lessen the PH drift and increase the lifetime of the SWG (salt water generator) cell. Also, the unit should be adjustable so that you can set the FC level anywhere you want it.

Thanks for the reply.

I'm located in Thailand, so my pool has a lot of sun all year round. I keep the CYA down for safety reasons, and actually also because CYA is very expensive in Thailand for unknown reasons. My SWG Zodiac Tri large is set at 60% currently with 2 x 2h a day filtration on a 55.000 liter pool, but I'm considering to lower it to 50%.

Maybe I should add some Trichlor and watch the effect, since the CYA level may have decreased also as we just finished the rainy season.