pH Rise - Borates didn't help

[ptbrunet]

I was hoping adding 50 ppm borates would help with my pH rise but I didn't see any difference. If it matters, my plaster is 11 months old. Here is the rise I saw over the last two weeks:

8/30: FC 5, pH 7.9 // added 1 gal MA
8/31: FC 6, pH 7.1, TA 60, CH 310, CYA 38, NaCl 1200
9/1: FC 6, pH 7.2
9/2: FC 6, pH 7.4
9/3: // not tested
9/4: // not tested
9/5: FC 2, pH 7.6
9/6: // not tested
9/7: FC 4, pH 7.7
9/8, FC 5, pH 7.7, TA 80, CH 310, CYA 37
9/9: // not tested
9/10: FC 4, pH 7.7
9/11: FC 3, pH 7.8
9/12: FC 4, pH 7.8
9/13: FC 5, pH 7.9

My FC was a little erratic as I was learning how to adjust my pump as the pool is now getting more shade and the days are getting shorter.

I notice that the rate of change from 7.1 was faster than the rise approaching 7.8.

I'd like to do something so that I can go at least 16 days without manually adding MA.

 

[JasonLion]

Do you have a waterfall, fountain, spa, deck jets, or anything similar that you use a fair bit?

With borates in the water, you can let the TA go down to 40, which will almost certainly help.

 

[jblizzle]

It is normal for the pH to rise faster at lower levels. Only way to slow it down is to lower the TA. Why did that go up?

Do you have water features?
Why 16 days? That may be unrealistic if you have aeration and not able to lower the TA.

 

[ptbrunet]

I don't use any aeration. I don't know why the TA changed, the last three readings are 80, 60, 80. I assumed the 60 was due to adding 1 gal MA. The 16 day goal is so I can be away from the pool for two weeks.

 

[chem geek]

I don't see the problem here. You shouldn't be lowering the pH as much as you have since as you could clearly see the pH rises more quickly at lower pH and the amount of acid that is needed is much more at lower pH as well. It took one week to go from 7.6 to 7.8 in pH and if you were to acid to lower the pH to only 7.5 or 7.6 instead of 7.2 you would find you would be adding far less acid. Yes, you would add it once a week, but would save a lot on acid. What you noticed in terms of rising faster at lower pH is perfectly explained by the fact that the water is much more over-carbonated at lower pH (see this chart).

The borates do not usually change the amount of acid you need to add over time, but they slow down the rate of pH rise especially at higher pH (which is what you are seeing) so that you don't have to acid as frequently. It looks like you will be able to add acid just once a week. Going from 7.8 to 7.6 will only take 22 fluid ounces (less than 3 cups) of acid per week. Compare that to your 16 cups every 2 weeks (8 cups per week equivalent).

Continue to keep track of your TA over time. That will tell us if the pH rise is mostly from carbon dioxide outgassing or from something else. Let us know the TA of your fill water as well. Your rise in TA is suspicious. Some can come from evaporation and refill, but not likely that much. Some can come from excess lye in bleach or chlorinating liquid, but not likely that much.

By the way, one gallon of full-strength Muriatic Acid (31.45% Hydrochloric Acid) in 15,000 gallons would lower your TA by 30 and the pH (with a TA starting at 90 ppm and 50 ppm Borates) from 7.9 down to 6.73 so I suspect you are using half-strength 15% Hydrochloric Acid, is that correct? That would lower the pH from 7.9 to 7.4 and drop the TA by 15 ppm.

 

[ptbrunet]

Thanks. I added 1 gal as an experiment to see how long I could go without adding more. The MA is 31.45%. I added 1/2 gal this last Friday and pH dropped from 7.9 to 7.6. BTW, with my Taylor Basic kit reading pH is a judgment call so I could be off a bit. But I've always needed more MA than the chart indicates. According to my city water report my fill water TA is 134-236, avg 185. During the period I originally reported I had added some water to compensate for evaporation and forgot I had the water on so I put in about 5x as much fill as I had planned on (7 hours from a hose vs my typical 1.5 hours). I use Clorox 8.25% bleach from Sams or Costco.

I guess the solution to going away for two weeks is either to have someone come over and dump in a half gallon at an appropriate time. That would probably be the easiest solution. Or I could add another pump to handle the MA - or is there a simpler solution for a one time automation?

 

[chem geek]

Your high TA fill water will make it necessary for you to be adding acid even if you got your TA lower since it will creep up with evaporation and refill.

So something does not make sense with adding half a gallon (8 cups) of acid to 15,000 gallons. That will drop the TA by 16 ppm, but if the TA started at 80 ppm and had 50 ppm Borates, the pH should drop from 7.9 to 7.26. I don't have an explanation for what you are seeing.

 

[ptbrunet]

The next time I add 1/2 gal MA I'll record all the values before and after and report back.

 

[ptbrunet]

Here are my numbers before and after the most recent add:
9/21 (PM): FC 5, pH 7.9, TA 70, CH 300, CYA 27 // added 1/2 gal MA
9/22 (AM): pH 7.4, TA 55

My borate test strip showed closer to 80 than 50 but I followed the process so I'm guessing it's closer to 50.

The above numbers are close to what chem geek predicted (pH drop of .6, TA drop of 16). My starting pH actually might have been 8.0; the test kit isn't that precise; so the pH drop might have actually been .6).

I recorded TA at 55 because the test solution was slightly red at 5 drops and a deeper red at 6 drops.

I need to get my TA back to 70. Will it come up over the next week with the pH rise? Do I need to get it up to 85-90 so that every time I add 1/2 gal MA the TA will stay above 70.

p.s. I also need to put some pucks in and raise my CYA.

 

[chem geek]

Unless the pH rise is due to something getting added to the pool that is alkaline, such as plaster dissolving calcium carbonate or a defect leaching calcium hydroxide, or you have a lot of evaporation and refill with TA in the fill water, then the TA should not rise. Carbon dioxide outgassing has the pH rise with no change in TA.

If you add TA, do not add enough to go to 80 ppm. A lower TA will outgas less so just get to 70 ppm and see how that goes.

So you aren't doing what I suggested which was not to lower the pH by so much. Don't go below 7.5 and instead perhaps just add enough acid to go to 7.6. Then when it gets to 7.8 or higher, add acid. You may notice that you are adding less quantity of acid this way by not trying to lower the pH as much and not trying to have the TA be as high.

 

[ptbrunet]

Thanks CG. Like the addition of 1 gal, the addition of .5 gal was another experiment, i.e. how far can I stretch out the time between additions in case I have to be away from the pool for a while.

However, I also need to come up with a plan that will work for most of the time when I am home so I appreciate your input. The next time I add MA I will only add enough to drop to 7.6 and report back my numbers before and after.

Two related questions:
1) In the 11 months I've had the pool I've only needed to backwash once and I have not yet cleaned it. I plan on cleaning it next month. Could there be anything in the filter, e.g. the sediment from the new plaster, that could be causing my pH rise?
2) If it turns out that this pool will always have a pH rise that causes me to add MA every week or every several days are there any automation options besides possibly using a tank and pump? And is a tank and pump suitable for MA? The Stenner tank I use now for bleach is not air tight at the fill cap so I'm concerned with the MA gasses corroding things around it, e.g. the MA pump, the chlorine pump, the control panel, and two nearby air conditioners.

 

[chem geek]

I doubt there is anything in the filter having the pH rise. If anything, biofilms and organics that chlorine can oxidize will tend to have the pH go lower.

There are definitely peristaltic pump and tank options for dosing acid. Search this site for acid dosing. There are some considerations with the type of tubing that is used and to avoid fuming you may want to use half-strength Muriatic Acid. Also, acid fumes are the most detrimental to metal nearby so the acid tank needs to be in a well-ventilated area.

 

[ptbrunet]

Thanks CG, It's outside so that's good. I'll look for the discussion about MA dosing a little later in the week.

 

[JamesW]

I think that if you stop raising the TA, your pH will stop rising. I would keep the pH at about 7.7 to 7.8. As long as you don't lower the pH below 7.7, your TA won't go too low. You might want to raise the CH a little to help offset a lower TA.