Increased acid use?

[JamesW]

Multiply the TA (Carbonate Alkalinity) by the percentage to get the (relative) CO2 level.

pH........% CO2

7.0.........18.3
7.1.........15.1
7.2.........12.4
7.3.........10.1
7.4.........8.2
7.5..........6.6
7.6..........5.3
7.7..........4.3
7.8.........3.4
7.9.........2.7
8.0.........2.2

For example, if the pH is 7.2 and the TA is 120, then the relative CO2 level is .124 x 120 = 14.88

If the pH is 7.8 and the TA is 60, then the CO2 is 0.034 x 60 = 2.04

14.88/2.04 = 7.3 times as much pH rise due to CO2 offgassing and 7.3 times more acid use.

If the pH is 7.0 and the TA is 120, then the relative CO2 level is .183 x 120 = 21.96.

21.96/2.04 = 10.76 times more acid use.

This explains why a low pH and high TA causes so much acid use.

It also requires the constant addition of baking soda to keep raising the TA after the acid has lowered it below the target range.

For a pH of 7.2 and a TA of 120 the best option would be to use CO2 as a pH reducer because you are just adding the same amount of CO2 as was lost to offgassing.

However, this is a huge waste of money.

y = 100-(100/(1+10^(6.35– x))) and y =100/(1+10^(6.35– x)), x from 4 to 9 - Wolfram|Alpha

Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.

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[JamesW]

 

[JPMorgan]

It's been about 10 days now that I have been maintaining lower TA (60 ppm) and higher pH (7.6) and chemical usage seems to have decreased substantially.
Thanks for the help with this.

 

[JPMorgan]

Was told today by the guy who services our pools that we should maintain alkalinity at 80 ppm. Otherwise we risk damaging the heater. Any validity to that? I have been maintaining TA at around 60-70 and pH at 7.5 - 7.6 based on the advice I received here. Just want to confirm that this has no negative effect on heater or other equipment. Thanks!

 

[PoolStored]

Can you post your full set of results including temperature? FC, CC, pH, TA, CH, CYA, Temp.

 

[JPMorgan]

Can you post your full set of results including temperature? FC, CC, pH, TA, CH, CYA, Temp.

Heading down to take new readings this evening. Will post shortly.

 

[JPMorgan]

Can you post your full set of results including temperature? FC, CC, pH, TA, CH, CYA, Temp.

FC 2.0
CC 0
pH 7.6 (pool service guy had 7.4; controller reads 7.6)
TA 70
CH 300 (pool service guy had 200)
CYA 30
Temp 87
(ORP 780)

 

[PoolStored]

Your CSI is practically Zero. A TA of 80 is not going to make a material difference.

What damage does he indicate will happen?

 

[JPMorgan]

So.... CSI is the critical thing to keep an eye on, not just alkalinity. He didn't specify what damage, but I assume he meant corrosion of internal heater parts.
I suspect this is just the guideline he has been given by his company and he may not actually understand CSI calculations. He actually is saying that TA of 80 ppm is a minimum. They like it around 100 ppm, but we were using way to much acid and sodium bicarbonate trying to maintain that level. Keeping TA at 60-70 and pH around 7.6 has worked well and reduced our chemical usage substantially. I know how to use Pool Math to calculate CSI, so if that is the only issue, then I'm not worried. As long long as I'm within CSI of +/-.6 there shouldn't be any concern about the heater or other equipment, right?

 

[PoolStored]

One has to be exceedingly careful when throwing around terms like saturation indices and corrosion. There are so many caveats and assumptions that need to be carefully considered before using them as a guide. Historically, saturation and corrosion indices were developed by chemical engineers simply as a means of shorthand so that water managers could have an easy-to-understand number to help drive decision making. These indices are EMPIRICALLY derived and very few of them rest on first-principals derivation. The CSI/LSI is also solely an index used to predict how calcium carbonate scale will react in water ... it says NOTHING about metal corrosion. In water boilers, steam pipes and cooling towers, calcium scale is a big issue for both equipment upkeep as well as the PREVENTION of corrosive liquids and gases from contacting metal surfaces. That is all these indices are good for. I have no idea what he is using as a foundation for "damage." Does he mean scale? Does he me corrosion?

The Raypak heater have a copper or cupro-nickel core. They can, in the presence of calcium, form a light scale that protects the copper. You have 300ppm of CH and should be fine. Low pH is a bigger killer of heaters.

Calcium Hardness - Further Reading

www.troublefreepool.com

There are other empirically derived indices that are used to PREDICT when corrosion MIGHT be possible. The Pukorius Index (PI) and Ryznar Index (RI) are used in this regard.