FC jump from adding acid?

[flynwill]

Chemistry question for @JoyfulNoise :

I have noticed that my FC takes a ~1ppm jump the day after adding acid to my SWG pool. I generally test once a day and then add any needed chemicals. The Pebbletec pool is now about 6 months old and always covered except for the 30-60 minutes per day when it is in use, so the chlorine consumption is fairly low but is needing acid additions about twice a week. The additions are typically 32 oz of 14% HCl to the 6,100 gallon pool to drop the PH from 8.0 (and CSI of 0.3) back to 7.3 (CSI -0.3).

So the question is: Does the HCl directly add to the available free chlorine? Or is the efficiency of my SWG being effected by PH? On the latter one possibility that occurred to me is that perhaps a thin layer of calcium scale was forming on the SWG plates during the high side of the PH cycle (when the CSI is positive) and then is dissolved away when the CSI is negative again.

 

[JoyfulNoise]

No, adding muriatic acid will not add to the FC measure.

You are letting your pH swing to wildly and thus your CSI is going far too positive. The pH of your pool water IN the pool may be 8.0 on the high end but the pH of water inside your SWG cell can easily be 10.5 or higher when it is generating chlorine. That will cause scaling to occur and affect the efficiency of the chlorine generation process.

You need to keep your pH under better control. That means reducing the swings by lowering TA and adding acid to keep the pH in a smaller range. When the pH gets above 7.8, you should add only enough acid to drop it down to 7.6. Going all the way down to 7.3 doesn’t help because pH rise is not linear - the lower the pH is, the faster the rate of outgassing of CO2 and the quicker it will rise. Since you have a covered pool, you can easily adjust the TA down to 50-60ppm and then stabilize the pH in the 7.6-7.8 range. That will reduce the frequency of acid additions. As well, you can consider adding 50ppm borates to the water to help buffer the pH rise and slow down the rate of scaling inside the cell.

 

[flynwill]

The TA is currently between 60 and 70. It has been slowly trending lower, it was between 80 and 90 back in March, but haven't seen much change in the last 4-5 weeks. The fill water TA is 120, but being a covered pool the fill water usage is pretty low so I've been assuming that the TA rise is mostly from the still curing plaster. So I'm not sure I can easily adjust it down to the 50-60 ppm range, as I understand it the recommended process for lowering TA is pretty much what I have been doing.

In any case I will try smaller HCl addtions when the PH hits 7.8 and see how that goes.

Also I should add that there are no waterfalls or similar features, the only significant aeration source is the SPA jets, but we're using the spa most days now so that is probably contributing to the PH rise.

 

[TexasEd]

I'm not a chemist but I did have a similar question to the original post. When you add HCl to your pool, my understanding is, the H side reacts with the carbonates in the water to form Carbonic Acid. So where does the Cl side go?

 

[mgtfp]

The H, or more specific H⁺, lowers the pH, which is basically just a measure to count the number of H⁺ in the water (in an inverse way, the more H⁺, the lower the pH) . That will have an influence on all pH dependent equilibriums, e.g the Carbonate equilibriums, but there are others.

The Cl, or more specific Cl^-, or Chloride, adds to the salt level. With an SWG, that Chloride might eventually get upgraded to Chlorine in the form of HOCl, in the same way a Cl^- added via a bag of salt will eventually get upgraded to Chlorine by an SWG. And then turned back to Chloride after it's done its job. But basically, that Cl^- just hangs around, there is no impact on FC levels, Chloride is very different from Chlorine.

 

[flynwill]

You need to keep your pH under better control. That means reducing the swings by lowering TA and adding acid to keep the pH in a smaller range. When the pH gets above 7.8, you should add only enough acid to drop it down to 7.6. Going all the way down to 7.3 doesn’t help because pH rise is not linear - the lower the pH is, the faster the rate of outgassing of CO2 and the quicker it will rise. Since you have a covered pool, you can easily adjust the TA down to 50-60ppm and then stabilize the pH in the 7.6-7.8 range. That will reduce the frequency of acid additions. As well, you can consider adding 50ppm borates to the water to help buffer the pH rise and slow down the rate of scaling inside the cell.

Coming back to this. For the past few weeks I have been adding acid in smaller doses when pH shows above 7.8 as you suggest, and the net result is I'm adding 16 oz of 14.5% HCl every other day vs adding 32 oz twice a week. So using slightly less acid, but at the cost of having to tend the pool every other day -- doesn't seem worth it.

Also the TA is very stable, reliably testing 60 or 70 ppm. This is despite Pool Math's indication that that those HCl additions should be reducing TA by 15-20 ppm every week.

I have checked my fill water usage a few times (by shutting off the auto fill for a week and carefully observing the water level) and that is ~1% off the pool volume per week. (The pool has an auto-cover and is covered for all but about an hour per day) Fill water TA is 120 ppm, so I would expect to be gaining 1.2ppm/week from the fill.

I assume the rest of the TA rise has to be coming from my still curing plaster?

CH has also been rising faster than I would expect from the fill water. The fill water CH is high (200) so I would expect it to be adding about 2ppm/week but the actual rise is closer to 10ppm/week. Would that be blamed on curing plaster also?

Using Pool Math to determine CSI as a function of pH for my pool's current conditions ( FC 5.0, TA 70, CH 575, CYA 30, Salt 3200, Temp 90):
pH 7.2 -> CSI -0.27
pH 7.4 -> CSI -0.08
pH 7.6 -> CSI 0.12
pH 7.8 -> CSI 0.31

 

[proavia]

During the cover open time, how long is the "Riverflow swim current" on?
This is possibly adding a lot of aeration - causing the pH to rise quicker than if it was off.

Post a pic of the pool with cover open and "riverflow swim current" on.

 

[flynwill]

I can post a picture, or link a short movie later today. The Riverflow run time is maybe 10 minutes per day and does very little aeration by itself, it's moving water not air. The slashing of swimming in it is probably comparable aeration to someone swimming in a normal lap pool. The SPA on the other hand does mix a lot of air with the water, and we have that going 20-30 minutes every day.

 

[flynwill]

Here is a link to a movie I made shortly after completion:
Pool Demo

 

[proavia]

Thanks for the video. Nice looking pool/spa!

Both of those together are causing the pH rise do to aeration.
Being a relatively new surface is adding to the pH rise.

The water breaking the surface with the riverflow on is also causing aeration.

 

[JoyfulNoise]

What are output settings and runtimes on the SWG?

Also, lower the TA to 50ppm. I suggest you use 31.45% MA, it’s more cost effective. The 15% “green” stuff typically is priced the same as the 20 baume acid but only gives you half the effect.

 

[flynwill]

SWG is set to 18% and running 10 hrs/day. It took me a while to tune that in, but with that setting the FC level has been very stable between 4 and 6.

Regarding the Acid, yes I realize that the 20 baume stuff is more cost effective, but that requires a trip to the pool store vs Lowes. That's quite a bit further away, and not in a direction I would combine with any other errand. (Lowes and Home Depot in CA don't carry the 20 Baume stuff.) Also the weaker concentration makes it easier to measure since the quantities are larger. I made a measuring stick so I can mark off the disposable jugs in 16 oz increments.

Are you saying that adding a given amount of 31% will be more effective at reducing TA than 2x the amount of 14%?

 

[JoyfulNoise]

I said it was “cost effective” because most hardware stores charge the same amount for 15% or 31% which means you’re paying more for less. But if you prefer to use the 15% as a convenience, then so be it. Acid is acid, so you just have to use more of the 15% to get the same effect.

Why are you running your FC so high? Your CYA is 30ppm. If you intend to keep it there then you don’t need much more than 2ppm FC in the pool. You can probably cut the pump run time down a few hours. Your SWG is adding about 1.8ppm FC per day. But your pool is covered and fairly warm so most of that is likely thermal loss of FC unless there’s lots of bather usage as well. You might try cutting down on the SWG run time a bit and see if that helps with the pH.

In theory, the plaster should not be emitting anything, calcium or alkalinity. But we often see reports of newly plastered pools taking time to settle down in acid demand.

@onBalance - Amy thoughts on new plaster and acid demand?

 

[flynwill]

Acid is acid,

Indeed that's what I thought. I have time this holiday weekend I was thinking I might try more aggressively lowering TA by adding HCl to lower pH to 7.2 then run the SPA in spillover mode (which will generate tons of aeration) until the pH comes back to 7.8 or so.

I also still wonder if perhaps the CA test is reading falsely high due the stuff the Pool builder added on startup (see picture below).

As for FC level, I did run it at about 2 for a while. But my understanding from is that 2 is the minimum and that I should target at least 4. But you are right the bather load is very light (just my wife and myself for ~30 minutes daily) I also recently did do the experiment of turning the SWG off entirely for a day with no obvious difference in the rate of pH rise.

I'm keeping the CYA ~30 because with an always covered pool there seemed little to be gained by keeping it any higher than needed to buffer the Cl for the people. I've been adding 10ppm of CYA when I am able to see the dot again at the 20 line on the test. (Which seems to be every 6-8 weeks).

I think the longer term plan is that I'll hire the RO truck some time this fall and get the CH back down into the 250-300 range. Then once the water is balanced again try adding boric acid. Maybe that will stabilize the pH in the 7.6-7.8 range which with the lower CH will keep CSI at an acceptable level.

 

[JoyfulNoise]

The Beautec stuff is just HEDP sequestrant, it doesn’t affect CYA testing at all. Chlorine will eventually oxidize it to orthophosphate. You might check your phosphate levels at some point as they could be high. Not that it matters much but it would be something to eliminate eventually so as to not give algae any extra reasons to grow (phosphates are algae food). Being in Los Angeles, it’s likely your municipal supplier is adding phosphates as well.

You can certainly run at lower CYA levels if you want but your FC loss is going to be higher. Based on your generation rate, you’re losing 1.8ppm per day (call it 2ppm). That’s the low end but I can tell you from experience that my uncovered pool with higher CYA (80ppm) has had historical FC loss rates lower than 1pm per day. The stabilizing effect of CYA can not be overstated enough. It’s NOT just “sunscreen” as all the industry people like to simplistically push. It literally binds to the active chlorine and reduces all of the reaction rates. That helps in stabilizing the FC levels a lot. And when you’re dealing with a low bather load residential pool that has no algae in it, almost all of the FC loss is attributed to the chlorine reacting with things in contact with the water like the cover, etc.

Again, it’s up you but you might consider increasing CYA once you get your water better balanced and the pH drift is acceptable to you.

 

[onBalance]

New pool plaster finishes, including pebble, will generally increase the pH, TA, and CH, and therefore, increase acid demand. The greatest effect is during the first month, then less so for six months to a year, and then very little after one year. However, poor quality plaster will greatly increase that effect and for much longer than normal.
How? Poor quality pool plaster releases greater amounts of calcium hydroxide (and calcium chloride if added) thereby resulting in pH, TA, and CH increases.

 

[mgtfp]

Your pool seems to be a nice study case. Going back to your initial question about the observed chlorine jump after adding acid.

As pointed out, acid doesn't directly add FC. But at higher pH, a higher percentage of FC is in the form of OCl^-, which degrades faster under UV light, resulting in a higher chlorine loss. With a mostly covered pool, that effect shouldn't be too noticeable, but on the other hand, you are keeping FC quite high for a SWG-pool with CYA 30.

The SWG FC/CYA chart doesn't cover CYA 30 because TFP recommends higher CYA with SWGs. But if you choose to run a lower CYA level (which could be justified in a mostly covered pool), then you should target the liquid chlorine min-level rather than the liquid chlorine target-level (have a look at footnote 3 in chem geek's chart that is linked in the acknowledgements under the chart).

At FC 5-6, you are three times higher than your target. At the SWG target-FC, the pH effect on UV-losses shouldn't be very pronounced, especially not on a mostly covered pool. If you really see a significant effect of pH on chlorine consumption, then you should run a lower FC. Or increase your CYA. If you feel more comfortable at higher FC (below SLAM), then there is nothing wrong with that, but you'll have to put up with higher absolute chlorine consumption.

 

[flynwill]

The Beautec stuff is just HEDP sequestrant, it doesn’t affect CYA testing at all. Chlorine will eventually oxidize it to orthophosphate.

Well explains the instructions that the pool would need more of the stuff in 6 months. (Which I ignored) The picture is the jug it came in but I later learned that the contents were a premixed cocktail of which the Beautec stuff was just one ingredient. The pool builder also added a bottle of "Jacks Magic" sequesterant as soon as the pool was filled.

Again, it’s up you but you might consider increasing CYA once you get your water better balanced and the pH drift is acceptable to you.

Indeed now that I know CYA is nowhere near as permanent as the Pool School texts imply I may give that a try.
Meanwhile I have shut off the SWG for the moment, will let the FC drift down to 2 before restarting.
I'm also the acid-aeration routine today to see if I can get the TA to drop to 50. I'll report back with the results.

 

[JoyfulNoise]

…. I later learned that the contents were a premixed cocktail of which the Beautec stuff was just one ingredient. The pool builder also added a bottle of "Jacks Magic" sequesterant as soon as the pool was filled.

Almost all pool care sequestering agents are HEDP. The fact that they are “mixing a cocktail” is just silly … anyway, there’s no need to use a sequestrant regularly unless there are on-going issues with metal staining. The start up sequestrant addition is something the NPC recommends to help keep any potential calcium scaling to a minimum when the plaster is initially curing after application.

 

[Kellyp]

FYI, Lowe's quit selling kemtec full strength muriatic acid here in GA, but check the paint ailse. I was able to find kleen strip full strength MA right next to the "green" version.