Why so much acid demand?

davethomaspilot

Bronze Supporter
Aug 30, 2015
95
Apex,NC
I have a three + year old, 17K gallon pool. Pentair Intellichem is used for dispensing acid and chlorine.

Target ph is 7.3 to 7.4. TA is about 100. Calcium is 300.

I love the Intellichem--hassle free, perfect ph (and chlorination). But, I'm still using about two gallons of 100% muriatic acid each week. That's less than when the pool was first built, but still seems exceesive?

I've checked the ph sensor against Taylor reagent tests--it's always spot on.

Is this much acid demand to be expected? If not, any ideas on why it's so high?

Thanks!
 

davethomaspilot

Bronze Supporter
Aug 30, 2015
95
Apex,NC
I use undiluted Jasco Green Muriatic Acid. I assumed it was 100%, but a quick google search shows this:

10° baume (15.725%)

So, I guess far from 100%. So, given the difference in baume, seems my usage is consistent with yours? (I'd believe I use 1.5 gallon versus 2 per week).
 

PoolGate

Gold Supporter
TFP Guide
Jun 7, 2017
4,817
Damascus, MD
I use undiluted Jasco Green Muriatic Acid. I assumed it was 100%, but a quick google search shows this:

10° baume (15.725%)

So, I guess far from 100%. So, given the difference in baume, seems my usage is consistent with yours? (I'd believe I use 1.5 gallon versus 2 per week).
Yeah it is close is that a new pool or at least new plaster?
 

davethomaspilot

Bronze Supporter
Aug 30, 2015
95
Apex,NC
If I let the TA drop (it eventually will), I need less acid, but the ph starts fluctuating wildly. So, I've been keeping TA a little on the high side, but it's expensive to keep buying those cases of acid.
 

Msch99

Gold Supporter
Bronze Supporter
Jun 11, 2018
900
Verona, MO
I am not in your area with your fill water, but I think raising TA and dropping PH becomes a see saw chemical addition? Many drop their TA down to 50 or 60 before the PH rise stabilizes.
 
  • Like
Reactions: davethomaspilot

davethomaspilot

Bronze Supporter
Aug 30, 2015
95
Apex,NC
TA goes down when acid is added to lower the ph. When I notice ph starting to fluctuate too much (I can see it plotted versus time with the Pentair ScreenLogic App), I know it's time to add baking soda to raise TA again.

I can let it continue to drop and add only enough baking soda to bring it to around 60 and assess the new acid demand and ph volatility.
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
17,213
Tucson, AZ
Your target pH is too low. Most terrestrial water bodies on the surface of planet Earth with any appreciable carbonate alkalinity in them will want to rise in pH to around 8.0-8.2 due the exchange of CO2 gas between the water and atmosphere. You are adding acid at a rate that is proportional to the release of that CO2 and so you will always have falling TA. Then you correct by adding baking soda. Then the process starts all over again. It’s not so much a yo-yo or see-saw as it is a slow wave.

Set your pH target to 7.8 (which I’m sure the IntelliChem probably won’t like because they follow pool industry recommendations) and get your TA closer to 60-80ppm. Once that happens, you will likely use a lot less acid.
 

davethomaspilot

Bronze Supporter
Aug 30, 2015
95
Apex,NC
Unfortunately, Intellichem will lock out chlorine dispensing at ph >= 7.8.

I could still set it at 7.7, but then, shortly after running out of acid, chlorine stops dispensing. If I target 7.6 or lower, I have a day or two before chlorine lockout.

And, yes, Intellichem says "scaling" if ph is over 7.6. I've been keeping at 7.3 - 7.4 to see the "ideal, ideal". But, I can go back to 7.6 (as your recommended to me several years ago on this forum!), to save some money on acid.

I wish the Intellichem control firmware had an adjustable chlorine lockout threshold. But, aAt some point, I'll just use an rpi to roll my own controller instead of using the Intellichem controller. I can already control pumps and temps and read ph and ORP using just the rpi. Maybe later releases of the software I use already have a way to dispense acid and chlorine. If not, it seems really easy to add code for that along with a GUI (I do things like that often).

But right now, too much on my platter to start another project. Probably next winter.

Also, at one point you indicated that a fixed dose of chlorine each day from a steiner pump might be all that was needed, rather than relying on an ORP probe (of course calibrated against reagent measured FC and TC).

When I'm out of town for two or more weeks, I set up the maximum chloring dosage on the Intellichem so I don't deplete all the chlorine before I return. That means the ORP doesn't reach target level, but since I don't use and CYA ,it takes very little chlorine to do the job.

Worked fine (no green pool) on three separate, three week occasions.


Thanks for the reply!
 

davethomaspilot

Bronze Supporter
Aug 30, 2015
95
Apex,NC
After I posted the reply to JoyFulNoise, I checked the Intellichem manual for the ph threshold for chlorine lockout. I found this:


Preferences (1/2)
To access the delays menu: Press MENU / Configuration Preferences 
Press / to adjust the following settings.
Preferences
Lockout: 7.8
pH Lockout: is the pH reading at which IntelliChem will no longer dose sanitizer (Chlorine/
Bromine). Above pH of 7.8 it is generally understood that the ORP reading is suppressed
and therefore will not be used to administer additional sanitizer. Just bringing the pH level
down will raise the available chlorine and the effective ORP. At this trip point, an alarm
message will be shown on the screen and ORP doses will cease.
Range is 7.7 to 8.2, default is 7.8.

Whoo hoo! Off to try it now and target 7.8 ph.

7.8 is actually low enough to prevent scaling?
 

PoolGate

Gold Supporter
TFP Guide
Jun 7, 2017
4,817
Damascus, MD
After I posted the reply to JoyFulNoise, I checked the Intellichem manual for the ph threshold for chlorine lockout. I found this:


Preferences (1/2)
To access the delays menu: Press MENU / Configuration Preferences 
Press / to adjust the following settings.
Preferences
Lockout: 7.8
pH Lockout: is the pH reading at which IntelliChem will no longer dose sanitizer (Chlorine/
Bromine). Above pH of 7.8 it is generally understood that the ORP reading is suppressed
and therefore will not be used to administer additional sanitizer. Just bringing the pH level
down will raise the available chlorine and the effective ORP. At this trip point, an alarm
message will be shown on the screen and ORP doses will cease.
Range is 7.7 to 8.2, default is 7.8.

Whoo hoo! Off to try it now and target 7.8 ph.

7.8 is actually low enough to prevent scaling?
Scaling is a function of your total CSI. Keep that slightly negative and you'll be good.
 

davethomaspilot

Bronze Supporter
Aug 30, 2015
95
Apex,NC
Yes, I can change the ph at which chlorine dispense lockout occurs. I changed it to 8.0.

But, it seems the maximum ph I can target is 7.6. So, I can try that.

I'm a bit skeptical that 7.8 is low enough to prevent scaling? Surely Pentair wouldn't limit the maximum ph setpoint to something below which is perefectly ok?
 

davethomaspilot

Bronze Supporter
Aug 30, 2015
95
Apex,NC
I think the Intellichem "scaling" versus "corrosive" takes calcium hardness, TA, and temperature into effect when displaying scaling, normal, ideal, or corrosive. The TA and calcium hardness are inputs that you enter manually.
 

davethomaspilot

Bronze Supporter
Aug 30, 2015
95
Apex,NC
I think the Intellichem "scaling" versus "corrosive" takes calcium hardness, TA, and temperature into effect when displaying scaling, normal, ideal, or corrosive. The TA and calcium hardness are inputs that you enter manually.
Probably uses the "Langelier Index". There's a caculator at this Pentailr URL:


It shows 7.8 as slightly scaling at typical pool water temp, and my current CH, and TA.

intellichem.png
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
17,213
Tucson, AZ
Lower your TA. 80ppm is more than enough.

You don’t have an SWG so cell scaling is a non-issue. TFP uses CSI which is more accurate and based on the thermochemistry of calcite formation. The LSI is an older index that is based on empirically derived values.

Your water would have to have a CSI >> +0.6 for months on end for scaling to be an issue. As long as your CSI is anywhere between -0.3 and +0.3 your pool will be fine.
 

davethomaspilot

Bronze Supporter
Aug 30, 2015
95
Apex,NC
There's no point in trying to lower TA to save acid, is there? (Since the only way I know to lower TA is by adding acid).

Instead, I just live with the higher acid demand until TA eventually reaches about 80 ppm and when adding baking soda, target the lower TA.
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
17,213
Tucson, AZ
There's no point in trying to lower TA to save acid, is there? (Since the only way I know to lower TA is by adding acid).

Instead, I just live with the higher acid demand until TA eventually reaches about 80 ppm and when adding baking soda, target the lower TA.
Either way, it doesn’t matter. You’ll be consuming the same amount of acid if you lower the TA all at once or if you let it drift down and hold it there. Up to you.
 

davethomaspilot

Bronze Supporter
Aug 30, 2015
95
Apex,NC
Thanks! I'll just wait until ph changes seem erratic, measure TA and add just enough for 80 ppm.

Why did you say
You don’t have an SWG so cell scaling is a non-issue
I thought scaling meant results in white deposits at waterline of pool, coating of heat exchanger coils, and other pool equipment, not just SWG.

Also, isn't chlorine less effective as a sanitizer as ph increases? Or is the impact insignificant at ph = 7.8 versus 7.2?

Thanks!
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
17,213
Tucson, AZ
As I said, your water's CSI has to be way out of balance for long periods of time for scaling to become a problem. Saturation balance simply indicates the potential for scaling or etching, it says nothing about the rate at which those phenomenon will happen. SWG's are the most susceptible to scaling because they generate chlorine electrochemically and a consequence of the fast pH rise inside the cell near the anode plates causes calcium to want to precipitate faster there. Heaters are only susceptible to scale if the water saturation is too positive and they are run at high temperatures for long periods of time or shutdown prematurely before allowing time for the heat exchanger to cool off. Plaster scaling is only seen when the CSI > +0.6 and even then it is very slow. Typically the saturation needs to be closer to +1.0 for there to be any noticeable deposits.

There is no such thing as "effectiveness" when talking about chlorine and pH. That is an industry term that has no scientific grounding and it's designed to scare people into buying products or treating their pools in a specific way. Active chlorine concentrations, ie, the concentrations of hypochlorous acid (HOCl) and hypochlorite anion (OCl-), do vary with pH. At a pH of 7.5, the amounts of HOCl and OCl- are roughly equal. In a pool with no cyanuric acid, the variation can be quite extreme between 7.2 and 7.8. In a pool with cyanuric acid, the amount of variation is quite small on an absolute scale and there is no need to worry about pH and active chlorine concentration. As long as TFP recommended levels are followed, there is ample amounts of sanitizer in the water at all pH values from 7.0 to 8.0. The only time pH makes a difference is when one needs to SLAM a green pool as it is better to drop the pH first to 7.2 and then start the SLAM. That produces the highest concentration of HOCl in the water and alleviates some of the pH rise issues when adding large quantities of liquid chlorine to a pool all at once.

If you are operating your pool with no CYA stabilizer and relying on ORP control, then your HOCl and OCl- concentrations are going to vary a lot with pH. You may still have enough sanitizer in your water to combat disease transmission and algae growth but the harshness of that sanitizer, as defined by the HOCl concentration, on clothes, swim suits and hair will be a lot higher. With a pH of 7.5, 0 ppm CYA and 1ppm FC, HOCl concentration is 0.484ppm. When the pH rises to 7.8, the HOCl drops to 0.320ppm. With 30ppm CYA in the water those values are 0.013ppm and 0.011ppm respectively. 0.011ppm HOCl corresponds to ~ 650mV of ORP which is the minimum for sanitation (pathogen control). TFP typically recommends 2-3ppm FC with 30ppm CYA which raises the HOCl above 0.03ppm or 3X the minimum value needed for sanitation. However, one can easily see that having some CYA in the water improves the water quality by not having so much HOCl present which is very irritating to eyes, mucosa, swim suits, hair, etc.