pH Keeps Increasing

[Masta Jer1]

Ok… So, this is my first forum attempt. Im not exactly sure if Im doing this properly but here goes.

I currently have a 20,000gal. spa that i am maintaining. Chlorine at approximately 3.0-3.5, Alkalinity at approximately 160ppm and PH at approximately 7.3, and the temperature at about 94 degrees thats just the basics. We have a CO tank to automatically keep the PH stabilized, and a chlorinator to keep the Chlorine automatically stabilized all by computerized system that is manually set to desired levels of each chemical.

Now, I now I need to lower the TA to approximately 80-100, and Im completely aware of how. But, I have 2 questions that I may need answers to.
1. What causes the PH to increase drastically causing the CO tank to deplete more than usual? And how do I correct it.

2. Our calcium Hardness is always really high. How do we prevent it from increasing so much when using calcium hypo-chloride tablets, and without constantly draining the pool water? Is there a tablet that doesn't have calcium in it???

 

[chem geek]

Welcome to TFP!

I would call that a therapy pool and not a spa since it probably doesn't have seats, aeration jets, and the ability to go to 100+ºF temperatures.

Is this indoors or outdoors? Is there any Cyanuric Acid (CYA) in the water? What is your Calcium Hardness (CH) level? By "chlorinator", is it a saltwater chlorine generator or is it chlorinating liquid in a tank fed by a peristaltic pump? It sounds like it might be a Cal-Hypo tablet inline chlorinator.

By the way, you mean a CO₂ tank for carbon dioxide, not CO for carbon monoxide (which is an odorless, poisonous gas). The reason that the pH increases more dramatically in your situation is the combination of a high TA with a low pH. This increases the amount of carbon dioxide in the pool so it will therefore outgas faster. You should be operating at a far lower TA level so should not be adding baking soda to try and maintain the level that high. You should also target a higher pH level (after you've lowered your TA). If you have not been adding any baking soda and you have been using only CO₂ for pH control, then the TA rose because of the excess lye in chlorinating liquid or excess calcium hydroxide and calcium carbonate in Cal-Hypo. You need to use at least some acid for that and cannot use only CO₂. See the presentation Reduce Facilities Chemical Costs By Proper Management of Total Alkalinity and pH.

You should not be using Cal-Hypo tablets if you want to prevent Calcium Hardness (CH) from rising. For every 10 ppm Free Chlorine (FC) added by Cal-Hypo, it also increases CH by at least 7 ppm.

 

[Masta Jer1]

You are correct. It is a therapy pool with no seating except the seats that are manually rolled into the pool, but there are jets along the perimeter of the pool, and the temperature can go as high as 120 degrees if we wanted it to. But we keep the temperature between 93-97 degrees based on what the users prefer.

It is an indoor pool, there is no Cyanuric Acid, calcium level at this moment is about 700. We normally keep it between 200-450… But with the Cal-Hypo Tablets that we use with the chlorinator that we use as i vaguely specified in the original message, the calcium is constantly increases.

I do apologies for the mistake of saying CO when it is obviously CO2. My mistake….. But, Even with the chemical levels at the proper readings the PH still increases a lot. I do not use baking soda at all to lower the TA, I only use acid to lower the TA.

As for Lye, I truly know nothing about that at all….
Also, if not Cal-Hypo tablets what would be a better choice? In my opinion I like liquid chlorine. What do you think is the best choice?

 

[Kiss4aFrog]

Welcome to the forum

Using liquid chlorine with a peristaltic pump would be the way to go. How often do you need to dump water to control the calcium hardness issue ?

 

[chem geek]

Yes, use chlorinating liquid or bleach (i.e. sodium hypochlorite) as that will eliminate the rise in Calcium Hardness (CH).

To minimize the rate of pH rise, lower your TA level to 70 ppm. Also, target a higher pH level of around 7.7. The lower TA and higher pH will result in slower CO₂ outgassing so you won't need to add as much to control the pH. As for acid, you'll only need to add a small amount to counter the TA rise you'll get from the chlorinating liquid or bleach.

If you have 3-3.5 ppm FC with no CYA in the water, then that will be very harsh on swimmer's swimsuits, skin, and hair and will create disinfection by-products faster as well. Unless your local regulations prohibit you from using CYA indoors, you should use a small amount of around 20 ppm and target your FC at 4 ppm. If you cannot use CYA, then you should have a lower FC target -- 1 ppm if that is allowed and if you are able to maintain that level consistently.