Should I add 2.1 ounces or 32 ounces of washing soda? PoolMath contradicts itself

[midtempo]

Last year my pool's alkalinity was in the 150-175 range and CYA was around 70, which sound high. Yet my pH was very stable and I did little or no aerating and certainly never had to add acid. I've always used the Taylor chemicals to balance my pool throughout the season.

This year is different. I started off with the recommended alkalinity of 100. My pH has continually been decreasing and I've had to aerate many times when my pH has been getting down to 7.2 to 7.3. I'll get the pH to 7.6 and a week later (or less) I'll have to aerate again. My CYA has been in the 30-50 range but I recently increased it to 50. I usually have a solar cover on my pool since it is in part-shade. I would like to stabilize my pH and it seems that I should vastly increase my alkalinity to fix these problems.

I have never used washing soda before, but it seems that it will have the dual benefit of raising my pH and alkalinity at the same time. However, the PoolMath calculator gives contradictory advice about adding it. Here is a screenshot of all my chemical levels (please note that I could not paste it here because its size is too large):

https://image.ibb.co/fudNt9/PoolMath.gif

Translation: FC=8.5, pH=7.4, TA=70, CH=90, CYA=50, Salt=unknown, Borate=0, Temp=80. Suggested FC min=4 max=8. I have no way of measuring borates or salt, but I never add borates and we have soft city water around here. Pool water has been partially replaced several times, so I'm not worried about salt.

In the image, I circled two areas that gave me contradictory information about adding washing soda. In the top circle, it advises me to increase the pH from 7.4 to 7.6 I should "add 2.1 oz by weight...of washing soda." But in the bottom circle, after adding in my own number, it advises me that "adding 32 oz of washing soda will raise pH by 2.24 and raise TA by 73." If I change the bottom figure to add 2.1 ounces of washing soda, it says that doing so "will raise the pH by 0.15 and raise TA by 4.8." These statements both can't all be true. You can't add 2.1 ounces of washing soda to raise the pH by 2.0, or instead add 32 ounces of the same substance to raise the pH by 2.24. That's mathematically impossible.

So which is it? Should I add 2.1 ounces or 32 ounces of this stuff? And is this a known issue with PoolMath?

 

[AimeeH]

I think you may be confusing washing soda with baking soda?

 

[midtempo]

No I am not. I am aware that adding baking soda will fix my TA problem, but then I'll have to spend extra time aerating. I suppose that's not too big a deal, but look at the picture. It specifically says that adding 2.01 ounces of washing soda will increase pH from 7.4 to 7.6; then on the bottom it advises that adding 32 ounces of washing soda will increase pH by 2.24 and raise TA by 73.

 

[mknauss]

Read the note below the Effects Of Adding Chemicals. The pH calculation is not accurate in the range you are using.

I would suggest to NOT use soda ash. It can change your pH and TA far more than you can imagine.

Use baking soda to raise your TA if you want. Your pH is fine, it is in the 7's.

 

[AimeeH]

Midtempo

I did look at your picture. Closely. That bottom section you referenced is where you can calculate the effects of adding different chemicals and not poolmath advice. So that is where I'm confused about your question and thought you might be confusing the two. I was wondering why if it says to add 2.01 oz of washing soda to raise PH but then your plugging in the consequences of adding 32 ounces instead?

 

[Richard320]

I want to know how your pH (and TA) keep dropping. The CYA addition might have lowered things, but only once, not continuously. So either you're adding a lot of low pH water or you're adding a lot of acid, either by acid or by trichlor or dichlor. Before you think about fixing the problem, you ought to figure out why you're having it.

And to reiterate what Marty osted: read the disclaimer right there beneath Effects of Adding Chemicals

Note: pH calculations are not exact. These numbers are only suggestive of the relative magnitude of the pH change you can expect. Small changes, +-0.4, with pH between 7.2-7.8, TA around 80-120, and Borate near zero will be approximately correct. The further you go from those ranges the less these pH changes will correspond to reality.

 

[midtempo]

I guess I don't have a good answer about the contradiction in the PoolMath page. I'll just aerate to 7.6 then using baking soda to raise TA to 140 like I had last year. I also don't have a good answer on why the pH just continually decreases, but from what I've read, a high TA can guard against that.

 

[zea3]

The Effects of Adding Chemicals section does not recommend a level of anything to add. What is does is use the size of your pool and the test results from the Now column to calculate a "what if" situation. At some point you entered 32 ounces of washing soda in the Effects of Adding Chemicals section. It doesn't default to a 0 setting after you close Poolmath, so if you did not enter 32 ounces of washing soda in the Effects of Adding Chemicals section when you entered the round of test results you quoted in the first post, you had entered it there at some point in the past.

The effect of adding 32 ounces of washing soda to a 3100 pool would be to increase the pH by 2.24 and raise the TA by 73 ppm. If you change the 32 ounces of washing soda to 2.1 ounces, the effect is that pH would increase by 0.15 and TA would increase by 4.8 ppm

 

[DuaeGuttae]

You can't add 2.1 ounces of washing soda to raise the pH by 2.0, or instead add 32 ounces of the same substance to raise the pH by 2.24. That's mathematically impossible.

So which is it? Should I add 2.1 ounces or 32 ounces of this stuff? And is this a known issue with PoolMath?

If you added 32 oz of washing soda and raised your pH by 2.24, you’d end up with a pool pH of 9.64. 2.1 oz to raise the pH by .15 takes you to 7.55, rounds up to 7.6. I think it’s just those pesky decimals that like to dance in front of fatigued eyes. 2.1 is the right one, but I think Marty’s suggestion to use baking soda was wise. I also thought Richard’s suggestion of figuring out why your pH is falling is an excellent one. How do you chlorinate the pool?

 

[jmbuys]

don't see a contradiction...if you add 2.1 it raises pH 0.15 which is what you are looking for but if you add 32 it raises it 16x more which is 2.24 which takes you into the high 9's for pH which you never want but in turns of the math is correct

 

[ka8yiu]

In the image, I circled two areas that gave me contradictory information about adding washing soda. In the top circle, it advises me to increase the pH from 7.4 to 7.6 I should "add 2.1 oz by weight...of washing soda." But in the bottom circle, after adding in my own number, it advises me that "adding 32 oz of washing soda will raise pH by 2.24 and raise TA by 73." If I change the bottom figure to add 2.1 ounces of washing soda, it says that doing so "will raise the pH by 0.15 and raise TA by 4.8." These statements both can't all be true. You can't add 2.1 ounces of washing soda to raise the pH by 2.0, or instead add 32 ounces of the same substance to raise the pH by 2.24. That's mathematically impossible.

This looks like a simple misplaced decimal. 2.1 oz will raise it "0.15" in the bottom, or "0.2" in the top, not "2.0" as quoted above. Yes? 0.15 and 0.2 are probably well within an error range of this calculator. Am I seeing it wrong?

 

[midtempo]

There seems to be some confusion about how I am using the PoolMath app. In order to more explicitly point out the contradiction, I have enclosed a new screnshot of the PoolMath app. Notice how in the top red circle recommends that I "add 2.1 oz by weight...of washing soda" as an alternative to raise the pH from 7.4 to 7.6; but in the bottom red circle it says that "adding 2.1 oz of washing soda will raise pH by 0.15 and raise TA by 4.8." My goal is to raise the TA by about 70 and to raise the pH by a bit. The question is, will adding 2.1 ounces of washing soda raise the pH by a paltry 0.15 or will it raise the pH by a bigger 2.0? And how much will 2.1 ounces of washing soda raise the TA -- by a paltry 4.8 or by something else?

Keep in mind that 2.1 ounces of washing soda is just two tablespoons, whereas 32 ounces is around a quart. I'm inclined to believe the 32 ounces for raising TA by 73 and pH by 2 is correct because it sounds more reasonable and, for raising alkalinity, is more in line with the amount of baking soda that would used. But I'm not willing to do a simple chemistry experiment to find out right now. I do not own any washing soda yet or hydrochloric acid to reverse the effects of a pH that is too high. I've never had to use acid. I only use bleach to chlorinate, although I experimented with using trichlor tabs for about two weeks before being dissatisfied. It's obvious around here that washing soda is not a popular pool additive.

 

[AimeeH]

Midtempo,

I'm not sure I'm understanding your issue. You have circled in red, the very bottom of the poolmath page the section of the app that is there to let you figure out the effects of adding different chemicals. That isn't a suggestion from poolmath to you? That section is there for you to make calculations.

 

[AimeeH]

There seems to be some confusion about how I am using the PoolMath app. In order to more explicitly point out the contradiction, I have enclosed a new screnshot of the PoolMath app. Notice how in the top red circle recommends that I "add 2.1 oz by weight...of washing soda" as an alternative to raise the pH from 7.4 to 7.6; but in the bottom red circle it says that "adding 2.1 oz of washing soda will raise pH by 0.15 and raise TA by 4.8." My goal is to raise the TA by about 70 and to raise the pH by a bit. The question is, will adding 2.1 ounces of washing soda raise the pH by a paltry 0.15 or will it raise the pH by a bigger 2.0? And how much will 2.1 ounces of washing soda raise the TA -- by a paltry 4.8 or by something else?

Keep in mind that 2.1 ounces of washing soda is just two tablespoons, whereas 32 ounces is around a quart. I'm inclined to believe the 32 ounces for raising TA by 73 and pH by 2 is correct because it sounds more reasonable and, for raising alkalinity, is more in line with the amount of baking soda that would used. But I'm not willing to do a simple chemistry experiment to find out right now. I do not own any washing soda yet or hydrochloric acid to reverse the effects of a pH that is too high. I've never had to use acid. I only use bleach to chlorinate, although I experimented with using trichlor tabs for about two weeks before being dissatisfied. It's obvious around here that washing soda is not a popular pool additive.

TA is the number you need to be most concerned with and you'd be wise to use baking soda, not washing soda to raise that. PH will drift up with aeration and higher TA.

 

[jblizzle]

I am confused too. If you are wanting to add 70ppm of TA (which seems like way too much IMO) then why are you focusing on raising the pH a little? The pH will come up when you raise this TA.

Why is your TA and pH crashing in the first place? Are you using trichlor tablets and or dichlor shock packets?

Although I now see that you claim to just be using bleach. What is the TA and pH of your tap water?

 

[midtempo]

Why is your TA and pH crashing in the first place? Are you using trichlor tablets and or dichlor shock packets?

I've aerated so much that it's reduced the TA over time to 70 or below. I'm ready to replace it with baking soda but this time bring the TA to 140 instead of 100. I'm hoping this will fix pH continually drifting down.

Although I now see that you claim to just be using bleach. What is the TA and pH of your tap water?

Strangely, I never measured it before. The pH of the city water is 7.3, which is pretty good. Rain water -- I'm not sure yet but it wouldn't surprise me if it's below 6.

 

[AimeeH]

Midtempo,

Aerating does not reduce TA. Only adding some form of acid will reduce TA or adding water that is very low TA. I know I have very VERY low TA from my tap that I top off with which keeps the TA low in my pool.

You need to just focus on raising TA a bit-- I would NOT shoot for above 120 until you can see the effects on your PH.

Aerating will raise you PH but not affect your TA

TA measures your waters ability to buffer PH changes. So low TA will tend to cause PH to drop more quickly. High TA will cause PH to raise more rapidly. Once you find a sweet spot, PH should become more stable.

 

[jblizzle]

So you have a lot of aeration intentionally? Spa? Water features?
And then are you trying to keep your pH too low and thus driving down the TA with all the acid?

I still feel like there is a fundamental misunderstanding here.

 

[midtempo]

TA measures your waters ability to buffer PH changes. So low TA will tend to cause PH to drop more quickly. High TA will cause PH to raise more rapidly. Once you find a sweet spot, PH should become more stable.

Exactly! It seems that the recommended TA of 80-120 is too low for my yard. It wouldn't surprise me that, had I talked to a local pool store, they could have told me that Greater Boston pools need a high TA. Last year I think my TA was at 170, but the chemistry was stable. I certainly never added acid in my life. I will start off the TA at 140 and see how it goes. I already finished aerating and dumped in 64 ounces of baking soda. Thankfully, baking soda is the cheapest chemical I would ever need to buy.

So you have a lot of aeration intentionally? Spa? Water features?
And then are you trying to keep your pH too low and thus driving down the TA with all the acid?

No, I was intentionally turning the my return jet upward to create a fountain when I realized that my pH was getting too low repeatedly. Since my pool is in part-shade, I keep it covered with a solar cover unless it's in the midst of a heat wave. Thus turning the jets up and down is a chore, since I must also remove the cover and replace it. Additional water testing is always a chore too, of course. Here's hoping the pH remains stable with the additional baking soda.

 

[jblizzle]

Sounds like you apparently have a unique pool where the pH drops on its own without the use of acidic chlorine or adding acid. Very rare if true, does not make sense.

Acid rain?