Acid Demand

[ltsrt8]

Hey first time poster! I was curious on the acid demand test. Im using a Taylor Deluxe test kit. Now my concern is (I take care of 70 pools a week) how is it that I can have different pools with the same ph but require a different acid demand? So for ex.
Pool A has a PH 8.0 (Acid demand of 3 drops)
CL 5 (yellow oto)
TA 120

Pool B has a PH 8.0 (Acid demand of 4 drops)
CL 5 (yellow oto)
TA 120

What really dictates that acid demand? Any explanation I would greatly appreciate!
Please let me know if there is any other info needed.

 

[JohnT]

Welcome to TFP!

Acid demand is an indication of how hard the pH of the pool will be to lower, and how much acid it is going to take. This is primarily determined by the alkalinity of the water and the borate level if that is being used as a buffer.

 

[Patrick_B]

Welcome to the forum!

You don't need to run one if you use This Calculator

 

[ltsrt8]

thank you all for the replies! Yeah it just confused me how two pools with with the same ph can need different amounts however that think is a bit naive lol. Are those the only two things that effect the acid demand?

 

[JasonLion]

Yes and no. That is a good summary, but if you want to get a bit more technical, the three main factors are carbonate alkalinity, cyanurate alkalinity, and borates. The effect on PH changes of carbonate alkalinity and cyanurate alkalinity are both reasonably well represented by TA, but borates have a noticeable effect above and beyond their contribution to TA. There are also other buffers that can affect things, but none of the others are at all common in swimming pools.

 

[JamesW]

Also note that none of the tests are exact. One drop difference is not that much. If there were three or more drops difference, then that might be reason to be concerned.

 

[Richard320]

I would suspect maybe the FC is greater than 5, which could skew the pH readings. I know anything from 4 to 7 looks like 5 to me. And even up to 10 or 11 isn't significantly different looking. You're also at the end of the pH scale, so one pool could be much higher than 8.0 pH.

 

[ltsrt8]

Thank you for the responses. I feel more comfortable now that I have a better understanding of the acid demand and what can attribute to its testing results.
I am not sure if i should start a new post for my next question and/or advice about the acid demand but ill go ahead and ask it and if the mods need me to make a new post I will. So the company I work for has changed the way we were doing chemicals about a month ago. We now keep are TA on the high end of the scale (depending on the stabilizer level). So one week they had us bring all the TA up once they told us the new way of doing chemicals to all are pools. I realize by doing this I am going to come back to a High PH the following week. Its been 2 or 3 weeks with some of my pools were I've only had to add acid. So i guess what i need advice on or an answer to is my PH aren't hitting 7.4-7.6 range even tho I am adding what the acid demand says. I also dilute the acid before adding it. We use muriatic acid 31.45%

 

[JasonLion]

When TA is high and there is a source of aeration, for example a SWG, the PH will be constantly going up. So one possibility is that the PH goes up between when you add the acid and when you next test the PH.

There is also always some imprecision in the acid demand test results, though it shouldn't be all that large.

 

[ltsrt8]

Is aeration something that is happening when the pool is running and if so does flow rate of the pool dictate how much aeration is going on?

 

[Richard320]

Is aeration something that is happening when the pool is running and if so does flow rate of the pool dictate how much aeration is going on?

aeration happens whenever the surface of the water is disturbed, such as by swimming. Also return nozzles that point upwards, waterfalls, spa jets, and slides.

 

[ltsrt8]

I believe that explains why my PH is rarely a 7.4-7.6 the following week. Thank you all for the responses.