Can't keep TA high enough

[eemueller]

I haven't been having any trouble with my water chemistry, until recently. In the last couple of weeks, I can't keep the TA at 70. It wants to drop. I will add baking soda to drive it up, get the pH back to 7.5, and then within a few days the TA is headed down again. It stabilizes around 30. I'm not sure why this has started to happen, or how to stop it. Thanks for any advise.
Eric

 

[jblizzle]

How are you adding chlorine to your pool?

 

[JamesW]

The acid pump is the likely cause. What pH do you have the pH controller set for?

What is the TA of the fill water?

Note: It's a salt system.

 

[eemueller]

It is a salt pool. I keep the pH controller set at 7.5 and it hardly ever varies. The pH of the fill water is about 7.7-7.8. I've used my Taylor test kit to confirm what the CAT controller is showing for pH, and it checks out fine. I'm stumped.

 

[jblizzle]

How much acid are you generally adding? You may find that the pH is more stable with less acid (and thus a more stable TA) if you set it up higher like 7.7

 

[eemueller]

What's so weird is this just started in the last couple of weeks. All summer, it was fine. Can lower nighttime temps have anything to do with it? We're not really swimming anymore, and the water temp is down to around 70.

 

[JamesW]

If you're getting extra aeration, then the pH will go up due to carbon dioxide off gassing. Then, the acid pump turns on and the pH and TA go down. This is the basis of the lowering TA (Total Alkalinity) procedure.

Then when you add bicarb to raise the TA, the process starts all over. It's an endless cycle.

The way to break the cycle is to maintain a lower TA (50 to 60)
and a higher pH (7.7 to 7.8) to lower the amount of carbon dioxide in the water.

You should also maintain the CYA (Cyanuric Acid) at 60 to 80 ppm, and the CH (Calcium Hardness) high enough to reduce the risk of dissolving calcium carbonate out of the plaster.

Another possibility is a leak and the auto fill introducing a lot of fresh water. You should verify if the auto fillers are adding a lot of water.

Adding Borates can help, but I would wait to see what you can resolve without them first.

Can you post a full set of readings, and the TA of your fill water?