chem geek
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Posts posted by chem geek
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Richard,
When using your spreadsheet to evaluate the impact of adding 20 mule team borax, I get different results if I add the borax, then add the acid, vs adding both together. Which is the correct simulation to apply considering I added 76 oz of borax followed immedately by 6 cups of acid?
I fixed the bug regarding the borates "difference" showing up in ammonia and the fixed spreadsheet is in the normal location here. As for adding borax and acid separately vs. at the same time, here's what I get step by step:
pH 7.5
TA 100
Borates 0
I then set the Borates goal to 50 ppm and under the Borates section it says to add 588.7408 ounces weight of Borax (I set the Base to 20 Mule Team Borax earlier). I then enter that in and press the "Calculate pH/TA" button to get the following:
pH 8.93
TA 212.9
Borates 50
Without deleting my entry in the Borates section, I change the Goal pH to 7.5 and click on the "Calculate Acid/Base/TA" button to see 280.7767 for the required amount of acid (the TA in the Goal changes to 105.7). Clicking on "Calculate pH/TA" at this point doesn't change anything so entering a combination of 588.7408 ounces weight of Borax plus 280.7767 fluid ounces of Muriatic Acid results in:
pH 7.5
TA 105.7
Borates 50
If I now do this in two steps, the first step I already showed above (with the pH of 8.93) but if I then click on the new button "Set Initial = Goal and Clear All Inputs" (or manually copy the Goal to the Initial and clear all inputs) and enter 280.7767 for Muriatic Acid and then click on "Calculate pH/TA", I get the same result as above with a pH of 7.5, TA of 105.7 and Borates of 50.
Can you take a look at what you are doing again and see what's different than what I just did? You can also take the latest spreadsheet since the new button does make it easier to do the multi-step analysis.
Now let me do what you did. Starting with pH 7.5, TA 100, Borates 0 (CYA is 30), 76 ounces of Borax gives me:
pH 8.37
TA 114.8
Borates 6.45
If I copy the above from Goal to Initial and clear all inputs (that is, my borates entry) and then enter 6 cups of acid I get:
pH 7.27
TA 96.2
Borates 6.45
If instead I do both steps at once, I get the same as the above (the TA is 96.1 instead of 96.2 if I don't copy over the TDS that changes -- it's a minor effect -- if I copy over everything from Goal to Initial in the two-step method or use the new button to do that, then I get exactly the same result as above).
Richard
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I am looking to get some idea of how corrosive "normal" pool water is. That is normally maintained and chlorinated pool water. What I am considering doing is to connect my geothermal heat pump into a swimming pools filtering loop as it is currently set up as an open loop style using well water. What I am considering doing is to add connections into the filter/pump loop of the pool and to add a fountain head to help displace the heat when the a/c is running in the summer and switch back to the well in the winter. My main concern is how corrosive the water is and will it attack the copper piping.
If your pool is manually chlorinated with chlorine (i.e. not a saltwater pool with a chlorine generator - SWG) and uses Cyanuric Acid (CYA) to protect the chlorine from sunlight, then the pool water is quite a bit less corrosive (by over a factor of 10) than chlorinated tapwater that typically has around 1 ppm FC or so with no CYA. Some municipalities have switched to using monochloramine also at around 1 ppm and would be less corrosive than chlorinated pool water.
By far the more important factor is the pH so just be sure to maintain the pH well above 7.0 (7.5 is typical). The copper piping should do fine with the pool water. Most tap water is closer to 8.0 to intentionally protect against corrosion in copper piping. If your salt level got high, then there would be more cause for concern but even then the corrosion rate is still slow, though not as slow.
If you shock your pool with chlorine, you should bypass your geothermal pump and piping just to be on the safe side though even at shock levels there is about a third of the disinfecting chlorine than in 1 ppm FC with no CYA.
Most pool heaters used to be copper. They have switched to titanium in some heaters mostly to handle the higher salt content in SWG pools. Also, the higher temperatures in the copper heat exchanger increase reaction rates including corrosion.
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I had it backwards. Thanks for catching that error. I've edited the post accordingly. The minimum at 1/14th corresponds roughly to Ben's "Min" column (0.03 ppm disinfecting chlorine) while the 1/9th is a target in between the "Min" and "Max" column (0.05 ppm disinfecting chlorine).
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Right off the bat you should add chlorine. Get some bleach or chlorinating liquid into that pool water right away or else the algae will continue to grow. You may also need to increase alkalinity by adding sodium bicarbonate / baking soda / Alkalinity Up so that your pH is stable (get at least to a TA of 50 -- your fill water might already have high TA so you might not need to add any). With visible algae, it may take a LOT of chlorine -- just keep adding it to keep the chlorine level up around 15 ppm until the chlorine level holds overnight. 2.5 gallons of bleach would raise the FC by 15 ppm in 10,000 gallons so with your water at 3 feet below the top it would take less than half that, so add 1 gallon of 6% bleach right away. If you use 12.5% chlorinating liquid instead, use half the amount as you would for 6% bleach. Always add the bleach slowly, preferably over a return with the pump running. In your case with a partial fill, just slowly distribute it around and then try and mix the water with something, even a brush.
You might need to add some CYA, but I assume that you will likely have some residual CYA left in your pool from before. TO BE SAFE, test the CYA level first. If you don't measure any, then use about 10 ounces weight of Dichlor for your partially filled pool (this will result in a CYA of 13.6 ppm when the pool is filled completely). After you shock the water with chlorine and the chlorine level holds overnight, then see how it is during the day. If it is burning off very quickly, then you may need to add some CYA and the easiest way to do that is with some Dichlor which adds both chlorine and CYA, but of course you won't want to overuse it. For every 1 ppm FC added with Dichlor, you add 0.9 ppm CYA. If you have a good test kit, such as the Taylor K-2006, then you could explicitly test your pool water for CYA to see how much residual was left over from before.
You didn't say if your pool was vinyl or plaster/gunite. If the latter, then you also need to add calcium chloride. For the alkalinity and calcium, add amounts appropriate for the estimated amount of water you currently have filled. You'll add more later as it fills up more. Also, if your pool is vinyl, then you'll want to be more careful about adding the chlorine with an appropriate amount of CYA so test the CYA level first and use some Dichlor if there isn't measureable CYA. Even if you measure no CYA, do not use more than 80 ounces weight of Dichlor as that would add 30 ppm CYA to the pool. After all of your drain/refill work, you don't want to end up with too much CYA!
As itabb mentioned, you should not use Trichlor tabs/pucks as your sole source of chlorine unless you regularly drain/refill a part of your pool water. Do you have a cartridge filter? If you had a sand filter with regular backwashing then the dilution would slow down the rise in CYA over time and could get to a reasonable balance. You have several choices for ongoing maintenance:
1) Use bleach or chlorinating liquid as your main source of chlorine. This requires daily addition of chlorine unless you have a pool cover in which case you can usually add the chlorine 2-3 times per week.
2) Use Trichlor pucks/tabs plus a weekly dose of PolyQuat 60 algaecide.
3) Use Trichlor pucks/tabs but monitor the CYA level and do a partial drain/refill to keep the CYA lower (30-50 ppm).
4) Use Trichlor pucks/tabs and additional bleach or chlorinating liquid to maintain a higher FC level appropriate for the CYA level.
For options 1, 3 and 4, you should aim for a target FC that is [EDIT] 1/9th [END-EDIT] the amount of CYA (i.e. the ratio of CYA to FC is [EDIT] 8.8 [END-EDIT]). The minimum FC is [EDIT] 1/14th [END-EDIT] the amount of CYA (i.e. the ratio of CYA to FC is [EDIT] 13.7 [END-EDIT]) so you need to add more chlorine to get to a higher FC level to ensure it doesn't drop below this minimum amount by the next time you add more chlorine. For option 2, the algaecide will prevent algae (but is expensive) and your minimum FC level is 3 ppm regardless of CYA level up to around 100 or so. A CYA level above 100 ppm may harm plaster.
Richard
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I clicked the link, opened the spreadsheet, entered 50 as the goal for Borates, and in the Difference column, 50 shows up for both Total Borates and Total Ammonia. I see the formula behind Total Ammonia Difference is identical to the one for borate, so I guess it's just an accident. Since the difference value is not used, I guess it is not a problem.
One other thing, I am thinking when I click Set Initial = Goal, the additions down below should be cleared as well, as if clicking Clear All Inputs. What do you think?
Whoops! That's a bug, but as you point out that Difference column isn't used anywhere so it causes no harm. I'll fix that. For the Set Initial = Goal also doing the clearing of all inputs, that makes sense since the button is really to be used as a "get ready for the next step". I'll make that change and will change the wording on the button accordingly to make clear it is doing two functions. This change should be available in a few minutes. [EDIT] It's done. Same link as before: here. [END-EDIT]
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... mind my asking where I can get this spreadsheet!
It's here. A link to the spreadsheet may also be found on this thread that also shows various graphs and links to tables I've referred to in other posts on this forum. WARNING: The spreadsheet is not for novice users and was created just for my being able to calculate pool water chemistry parameters and eventually dosing of chemicals and their effects in my own pool.
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something else that might be cool. I'm not a good xls programmer, but it would be cool if I hit the initial=goal button you are going to create, the current goal values and the what changed section is coppied coppied to the next worksheet with a datetime to kind of log what is going on. this would copy just prior to setting initial=goal.
That would be cool and probably won't happen because this spreadsheet is way too complicated for most users and is just for my own chemistry calculations. I don't know Excel that well either so don't know how to copy to a new sheet. Of course, that can be done manually with a selection of all (or of the part you want copied), a copy, create a new sheet, and a paste and I'm sure there are some Visual Basic commands to do that, but I don't know them. But then you've got to manage this history of sheets (or column sections in a single sheet if that's how it's done), delete really old ones, etc. The source for the spreadsheet is not hidden and the "protection" can be unlocked since there is no password so anyone can modify it as suits their needs, but I doubt many will be using this. You're the first to show such an interest.
[EDIT] I just updated the spreadsheet to add the new "Set Initial = Goal" button (and code). [END-EDIT]
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The "Calculate Acid/Base/TA" button will always zero out the Buffer and Outgassing of CO2 sections since the purpose of this button is to calculate the acid or base required to adjust pH, not caring about what happens to TA (so TA is instead just calculated and isn't a "Goal"). The "Calculate Acid/Base/Buffer" button will calculate all four pieces: acid or base, buffer or outgassing. It's usually the same numbers that show up in the first blue column, but are slightly tweaked for technical reasons (having to do with these chemical additions affecting TDS and ionic strength).
If I start with your:
pH 7.8
TA 132.8
FC 6.0
CYA 30 (I assume)
CH - doesn't matter
TDS - 614 (though doesn't matter much)
and then skip to your final result of:
pH 7.4
TA 80
FC 21.5
and press the "Calculate Acid/Base/Buffer" button, then it says I need to add 11 ounces of dry acid and aerate 45.3 ppm equivalent TA (the TA drops by more than that due to the pH drop).
From what I could see, you added 7.5 ounces of acid, not 11 ounces total that the spreadsheet predicted would be needed. In addition, when the chlorine gets used up and goes back down to 6, the pH will drop to close to 7 and the TA will drop to around 70.
In one post in this thread you said "CYA should be mostly dissolved by now." Where did that come from? If you added CYA, then that explains why you didn't need to add 11 ounces of dry acid since CYA is acidic. If I add 4.5 ounces of CYA and 7.5 ounces of dry acid, then I can get the pH to be 7.4, but the TA will be 100, not 80. Adding CYA lowers pH with no change in alkalinity (because it is a weak acid that acts as a pH buffer).
So I can't figure out how to exactly get your numbers, but in a spa small amounts of chemicals move things a lot so with measurement error on the tests and chemical amounts its not surprising not to get it exact. The spreadsheet has been right on the money for the lowering TA process in pools, predicting the amount of acid needed to first get to 7.0 (and calculating the corresponding TA at that pH) and then the amount needed to lower the TA to some level.
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Yesterday evening:
500 gallong acrylic spa
FC 2.5
TC 3
PH 7.7
TA 130
Borax 50
TDS 600
temp 90F
Added 8 oz of 6% bleach
spreadsheet says:
FC 10.2
PH 7.84
TA 135
TDS 615
Actual 24 hours later:
FC 6.0
TC 7.0
PH 8+ (4 drops to 7.6, so I guess it's at 8?)
So it appears that over the 24 hours the chlorine has partly been used up, but also there has been CO2 outgassing. So if I move the Goal data (i.e. FC of 10.2, pH of 7.84, etc.) to the Initial and put your actual data (i.e. FC of 6.0, pH of 8.0) into the Goal then I can calculate what may have happened. I enter 4.22 into the chlorine usage "Net Chlorine to Breakpoint", click on "Calculate pH/TA", change the Goal pH to 8.0 and enter 10.0455 in the "Normal Outgassing of CO2 to Air" and then click on "Calculate pH/TA" again.
Or I can do what you did and put in 3.7152 into the chlorine usage and 0.5 into the creation of combined chlorine, click on "Calculate pH/TA", then change the pH to 8.0 and put in 9.7001 into the "Normal Outgassing of CO2 to Air.
Spreadsheet says:Chlorine Usage: 3.7152
Creation of Combined Chlorine: .5
Result:
PH 7.77
TA 132.8
FC 6
TDS 613.6
Therefore, can I assume the rest of the PH is due to aeration, though I have the aerators turned off and the motor only running in standby. I guess there is always some aeration? BTW, no sunlight, covered the whole time.
Well, in such a small volume of water as with a spa almost anything can happen, but in this case since the pH rose then it is likely this is outgassing of carbon dioxide even without aeration. It does seem unusual given that the aerators were off and the spa was covered.
so, I want to burn off some TA and the CCs:move values to initial, set PH to 8.
goal is PH 7, FC 20.
spreadsheet says I need 7.8oz dry acid and 14.5oz Clorox 6%
I added 7.5oz dry acid and 16oz Clorox
Spreadsheet estimates:
PH 6.95
FC 21.43
TS 89.9 (guess from the acid) -- Any way to remove sulfates or just ignore them?
TDS 746.6
Low speed motor is running with aerator open.
Well I can see by the way you are using the spreadsheet in a multi-step process that I should add a button for "Set Initial=Goal". I'll add that feature.
As for acid, dry acid contains sulfates and technically it is indeed adding sulfates to your water. You can either keep track of them or ignore them. They shouldn't be a problem in your spa as you change the water every 3 months anyway. You could use Muriatic Acid instead, but it's awfully hard to measure the relatively small amount that you need (plus its fumes are strong). Another alternative is to use lye / caustic soda / sodium hydroxide, but that is also a very small amount though at least it's a solid (crystals) and won't add sulfates. Dry acid is probably fine, however.
I got 7.1943 ounces of dry acid after clicking on "Calculate Acid/Base/TA", not the 7.8 ounces you got. Not sure what the difference is. I had the copied TA of 133 and an assumed CYA level of 30.
copy goal to initialSo, if the CCs are eliminated and the FC's otherwise hold then I set CC to Breakpoint to .5 (half of the 1CC I have).
Then I'll aerate to 7.8PH and burn off 15FC in sunlight and add 5oz of backing soda. This will result in:
PH 7.5
TA 90
FC 6
TDS 765
Spreadsheet says I'd need about 250 - 300 CA to minimize corrosion. Is corrosion only against gunite or will it damage pipes, heater, and motor as well? And am I understanding how to use this spreadsheet?
I got what you did except without adding the baking soda. I'm not sure where you got that from (if you forced the Reset Goal=Init and then just raised the pH, then there shouldn't be anything showing up for baking soda). After you copied over the Goal to the Initial then they should have been the same (you can use the "Reset Goal=Initial" button to make sure) and if you then just change the Goal pH to 7.8, then pretty much only the "Normal outgassing of CO2 to Air" shows a number (26.6506) which you enter in and can then enter the 15 ppm for normal burn off of chlorine in sunlight and 0.5 ppm CC to breakpoint (I did this after because to do it before you need to do it in two steps with another copy from Goal to Initial and it didn't sound like you did that). Anyway, my final result is close to yours.
As for the Calcium Hardness (CH, or what you called CA), it is only necessary for plaster/gunite/grout (including any exposed grout in tile if you have any exposed to your spa water). Otherwise you don't need the calcium. Some say that it is needed to protect the heater, but the sources conflict on that and it seems doubtful to me -- pH is far more important for metal corrosion (as is the oxidizer level and chloride level for stainless steel). It wouldn't hurt to add some CH if you fill water is very soft and doesn't have very much, but having 100-200 should be plenty. I would consider that optional, but of course if your spa heater manufacturer warranty requires it, then follow the warranty (don't want anyone to have an excuse).
Though it does appear that you understand how to use the spreadsheet, I hardly ever bother with the combined chlorine part of the calculations because it's so minor. I also don't worry about calculating the outgassing and just use the measured numbers "as is" and then go from there. I just assume any pH rise that didn't come from another source (creation of CC, addition of a base) came from outgassing of carbon dioxide.
You said you wanted to lower your TA, but you didn't follow the procedure for doing that. When you aerate, you don't do so from 7.0 to 7.8. Instead, when the pH gets to 7.2, you add more acid to get the pH back to 7.0. You keep bouncing back and forth between 7.0 and 7.2 with aeration causing the pH to rise and the acid addition causing the pH and TA to drop. You periodically check the TA level and when its where you want it, then you stop adding the acid and just aerate to get the pH to where you want it.
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Huh? I don't see how adding the Borax adds to ammonia. Are you sure you aren't misreading the line in the spreadsheet? It adds to the "Total Borate" line, not to the line below it which is "Total Ammonia". Borax does not add any ammonia. After it dissolves in water, it produces Boric Acid, sodium ion, hydroxyl ion, and water. This is equivalent to adding Boric Acid and lye / caustic soda / sodium hydroxide.
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The CDC recommendations are with NO CYA in the pool. 2 ppm FC with no CYA is equivalent to 20.5 ppm FC with 30 ppm CYA in terms of disinfecting chlorine (hypochlorous acid) level and rates of killing pathogens. So the recommendations given above about shocking are appropriate if you want to recover your pool more quickly (i.e. in the 25 minutes the CDC describes). Now in reality, if you remove any physical material and measure your chlorine levels to find no CCs and have stable FC (if not exposed to sunlight), then the odds are good that even with CYA you will have killed any "easy-to-kill" bacteria in a matter of a few minutes. The CDC recommendation of 2 ppm FC for 25 minutes is to achieve a CT value of 50 WITH NO CYA which is true overkill for easy bacteria (0.04 - 0.08 CT) and is about right for the bacteria Pseudomonas aeruginosa that causes hot tub itch (30-50 CT) and for the protozoan Giardia cyst (42 CT), and is nowhere near able to combat the protozoan Cryptosporidium cyst (7200 CT). It is very unlikely that such cysts (the last two items I listed) are introduced into the pool unless the child is sick with these intestinal illnesses.
If it were my pool, I would shock regardless of the rate of pathogen kill (probably just to an FC amount that is half the CYA amount), just to ensure that I oxidized as many organics as possible and I'd expose the water to sunlight to speed up that process. I'd probably go back in as soon as the FC dropped to 10 ppm from sunlight exposure, but that might take most of the day. If it were a really hot day, I might jump in after an hour even with the shock levels of chlorine. It's not much different than swimming in an indoor pool with no CYA in it.
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Just to reiterate that just because we may now have a "different" explanation for why the high CYA levels may work (i.e. makes chlorine last longer even at proportionately higher levels vs. making the SWG more efficient) that doesn't change the result of higher CYA being more effective for many SWG pools in allowing less chlorine loss from sunlight and allowing lower SWG output and therefore reducing the pH rise. However, by having a more accurate technical explanation, we can more readily answer questions such as whether the higher CYA for SWG makes as much sense if a pool is not exposed to sun as much (due to a cover or due to being indoors). Your pool may fall into this category since it sounds like you keep it covered when not in use. Higher CYA will help when the pool is exposed to sun, but that may not be enough time to be that noticeable -- there's no way to know for sure except actual experiment in your pool.
As waterbear has indicated, we are almost always in agreement, mostly since we both look at the chemistry (or physics) involved but also trust in user's experiences and experiments and then modify the theory accordingly (as needed). As he says, his advice is not different than mine. I understand that your statement to waterbear was mostly just expressing your interest in the technical details.
Richard
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It is reasonable that the high FC at lower pH combined with sun exposure broke down the CCs faster. The disinfecting chlorine level at a pH of 7.3 and FC of 10 is 0.11 (assuming CYA is 50 ppm) while at a pH of 7.8 and FC of 6 it is 0.05 or less than half. By the way, the effect of the pH change is rather small since CYA buffers the disinfecting chlorine so if you had kept the FC at 10 then at pH of 7.8 the disinfecting chlorine is 0.09 so less, but not as much as one might expect (say, compared to not having CYA where disinfecting chlorine would drop from 5.8 to 3.0).
Your experience with pH rise at higher SWG output is well understood at this point. That's just due to more outgassing from the hydrogen gas production in the SWG. The tiny hydrogen bubbles rather efficiently pull out the carbon dioxide out of the water and into the air which causes the pH to rise (with no change in TA).
Since SWG production is dependent on pH, and seemingly rather constant from 7.0 to 7.5 but then drops off from 7.5 to 8.0 (at least with Mark's SWG), it's a tough decision to make as to the proper pH level. At the lower pH of 7.5 one has more efficiency in the SWG so the lower setting (on-time) helps reduce pH rise, but the lower pH itself contributes to pH rise as shown in this chart. Mark reported a 15% reduction in SWG output (FC level) from 7.5 to 8.0, but the difference in outgassing rate at a TA of 100 between a pH of 7.5 and 8.0 is a factor of 5.8 so running at a higher pH is better IF it is needed to prevent a rise in pH. Clearly, a lower TA is better but mostly affects the amount of acid needed (which is proportional to the numbers in the previous chart) while the lessening in pH rise is much less dramatic as seen in this chart (remember these are relative, not absolute, rates of rise).
It is normally recommended by the industry that no CYA is to be used in indoor pools since there is no sunlight. However, that is, in my opinion, incorrect advice since the disinfecting chlorine levels will be far too high in such pools. That leads to much faster corrosion, ruining of swimsuits (my wife can attest to that during the winter when she uses an indoor pool vs. the summer in our own pool with no problems), faster creation of disinfection by-products, etc. I believe that some CYA, even 10-20 ppm, would be very beneficial in indoor pools. For indoor pools using an SWG, it is virtually imperative that CYA be used or else corrosion of stainless steel is quite rapid -- one user saw such corrosion in less than a year. Yet no one in the industry is recommending that. This is all part of the "CYA doesn't matter" philosophy pushed by manufacturers of CYA-based products (Trichlor, Dichlor) and not only causes many pools to get algae when the CYA gets high and the FC isn't raised accordingly, but it prevents sound reasoning from being applied to indoor pools causing greater rates of asthma and respiratory illness in competitive swimmers and small children -- all because the industry refuses to acknowledge the science behind the chlorine/CYA relationship ("it doesn't apply to REAL pools" is their standard line and they refer to a study which I discuss in this thread). I quite frankly don't know how these people sleep at night. Perhaps they only see their position simply generating profits from pools getting algae (and subsequent sale of algaecide; or preventing algae through regular sale of algaecide), but the negative impacts of the position are far worse, in my opinion. I have contacted some of these manufacturers and will continue to do so in the hope such positions will change (eventually).
Richard
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Any change in pH due to acid or base will also change TA. So a rise in pH from Borax will also raise the TA. In addition, Borax raises the TA by a small and usually not measurable additional amount. It certainly raises the TA by far less (about half) the amount that using pH Up (sodium carbonate / soda ash / washing soda) does and Borax adds no carbonates. A more pure base for raising pH is sodium hydroxide / lye / caustic soda but that is not normally used (though certainly can be).
For drop tests where color changes, the "end" is the last drop that causes a color change. If you add another drop and there is no change, then the previous drop is the "last" one to be counted. So yes, if you add a drop and it turns pink and does not go back to green, then you are close and that *may* be the last drop. If you add another drop and the color does not change, then it was indeed the last drop, but if the color changes some more (gets noticeably more red), then this drop should also be counted (and another drop should not cause any more color change). The test is only accurate to within 1 drop anyway so consistency is more important.
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You also asked this on this thread on poolcenter.com. In case no one answers you there, here is my answer.
It depends on your TA level. If your TA is at 100, then in your 10,500 gallon pool it would take 73.7 ounces weight (about 8.8 cups) or nearly one box of 20 Mule Team Borax (one box is 76 ounces weight).
If your TA is already high and you want to lower it, then just aerate the water to raise the pH since that will do so without any change in Total Alkalinity (TA).
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Experiments by Mark (mas985) starting on the second page of this thread seem to indicate that my theory about higher CYA levels making the SWG more efficient is wrong. Instead, it appears that the industry's charts on CYA retention are wrong and that CYA protects chlorine more than would be predicted by a blend of half-lives based on the different chemical species (chlorine unbound to CYA vs. chlorine bound to CYA). So if Mark's results are correct and occur in most pools, then a higher CYA level with a correspondingly higher FC level would in fact be better for lowering absolute chlorine loss from the sun and allow one to lower SWG output since the loss from sunlight would be dramatically reduced even accounting for the higher FC level.
Now in your situation, if your pool is covered when not in use and does not get a lot of sunlight, then the higher CYA level is not as worth it. Maintaining a higher FC level at a higher CYA level in your case probably wouldn't be wasteful since the FC loss should be similar in absolute amount (so similar SWG on-time) but it would be unnecessary. You could run at lower CYA and FC so long as your FC was enough to handle local consumption and imperfect circulation (usually 1-2 ppm minimum). The 3 ppm FC is already pretty low so having that at a lower CYA just gives you some extra safety against algae growth, including yellow/mustard algae.
Richard
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Yes, the aeration is more effective at lower pH because at lower pH there is more literal dissolved carbon dioxide (the specific species -- not all carbonates) in the water -- that is, some of the bicarbonate is shifted towards carbonic acid and carbon dioxide when the pH is lower. That puts the pool even more out of equilibrium with the carbon dioxide in the air so the rate of outgassing is increased. The relative rate of outgassing can be seen in this chart and the numbers came from the "[H2CO3] (actual-normal)/normal" line way down the spreadsheet at around line #364. A number of 0 means equilibrium; negative numbers mean more carbon dioxide in the air than in the water; a number of 1 means twice as much carbon dioxide in the water as in the air and is used as the base rate for all other rates. So a number of 5 means the outgassing is 5 times faster than at 1. You can readily see from the chart that lower pH very rapidly increases the outgassing rate. It's just more efficient, but you can certainly lower TA at higher pH -- it just takes longer or requires more aeration to get the same speed of reduction.
You'll also note that for normal pH and TA ranges in all pools, there is ALWAYS outgassing of carbon dioxide since the pools are intentionally overcarbonated. It's just that outgassing is a slow process since only a portion of the pool water is in contact with the air (only the surface) and that surface tension inhibits the outgassing which is a physical process so still air will also slow that down. Splashing, aeration (forcing air bubbles into the water) and wind speed up the movement of carbon dioxide from the water into the air. SWG systems do a lot of aeration since the hydrogen gas bubbles they produce contain no carbon dioxide and are small bubbles with high surface area to volume ratios so can pull a lot of carbon dioxide out of the water quickly.
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Oh, now I see what you are trying to do and yes, the "Combined Chlorine to Breakpoint" can tell you the pH effect for what happens when your 0.5 ppm goes away from superchlorination in the SWG or from sunlight. Just enter in 0.25 (I tell you why it's half below) in the light orange area and click on "Calculate pH/TA" to see the effect.
In a 10,000 gallon pool, adding 1 gallon of 6% bleach will raise the pH from 7.5 to 7.79 (and the FC would increase by 6.17). I calculated this by putting in 16 cups into the "Adding chlorine with NaOCl" light orange area. If two-thirds of this chlorine (i.e. 6.17 * 2/3 = 4.1148) were to combine with ammonia or organics, then the pH would further increase from 7.79 to 7.83. I calculated this by adding 4.1148 into the light orange field in the "Creation of Combined Chlorine" section. When such combined chlorine (CC) is broken down, it takes one chlorine for every two CC so half the amount of chlorine or 2.0574 ppm and this decreases the pH from 7.83 to 7.50 back to where we started (it's actually 7.501 so slightly more basic to the small amount of extra lye left in bleach to add stability). I could equivalently just put in 6.1722 in the light orange in the "Net Chlorine to Breakpoint" section to see the net result of these two steps. The reason for the two-thirds of total chlorine in the creation of CCs step is arbritrary since it's whatever amount of CCs you want to produce -- I used two-thirds since the breakdown of CCs needs half as much and I wanted the sum to be the initial total of 6.17. In your case, you would just use 0.5 ppm for the creation of combined chlorine and 0.25 ppm for Combined Chlorine to Breakpoint.
In your case for the breakpoint of 0.5 ppm CCs, you would enter in half that, or 0.25 ppm, into the light orange field in the "Combined Chlorine to Breakpoint" section. The reason for half is that it takes half the CC amount of additional chlorine to achieve breakpoint. The reason for the 10x rule for raising FC to "break" CC is based on ppm values of ammonia measured as ppm Nitrogen because it takes 3 chlorine molecules to fully oxidize 2 ammonia molecules so in a molar basis the ratio is really 1.5 to 1 but chlorine is measured in ppm relative to chlorine gas which is 70.906 g/mole while Nitrogen is 14.0067 g/mole and 1.5 * (70.906 / 14.0067) = 7.6 which is where the 10x comes from (the "optimal" ratio of chlorine to ammonia is around 8-10 to force the right reactions to occur). So you might ask the question, "so is the 10x rule when using CC measurements based on chlorine, not nitrogen, wrong?" and the answer is yes. If you are measuring CCs already, then it only takes half that amount of chlorine to fully oxidize (i.e. achieve breakpoint). I'm guessing that the 10x rule perpetuated because in the presence of CYA, the disinfecting chlorine level is so low that the breakpoint reaction would be too slow if you didn't increase the FC level by quite a bit. At 30 ppm CYA, the disinfecting chlorine level is about 1/30th what it would be without CYA (at a pH near 7.5) so using 10x the CC level would be more than 20 times what is needed so would bring reaction rates closer to what it would be if one used a 1.5x multiple without CYA. Sorry this stuff is so confusing. It took me a while to get this ppm, Nitrogen, Chlorine, etc. figured out.
Anyway, 0.25 ppm of chlorine to break 0.5 ppm CCs only lowers the pH from 7.5 to 7.47 so isn't even measurable. So I don't think you will see much of a pH effect from the breakpoint of your CCs. So if you are seeing a downward trend in pH, then it's probably from something else. If the overall FC was dropping, then that would be significant (say, from burning off from sunlight faster than your SWG replenishes it) but you said the FC was holding.
As for chlorine tests, dilution before using the DPD test shouldn't cause a problem. There shouldn't be any bleaching effect if you dilute the pool water (to below 5 ppm FC) before doing the test. You essentially produce lower FC water by dilution (assuming dilution is done with water that does not contain chlorine nor monochloramine -- i.e. NOT tap water, but filtered or distilled water). Nevertheless, the DPD test is a visual color test so isn't very accurate and when combined with the errors in dilution measurements, just gets worse. You're seeing a bigger effect than I'd expect, but the point is the same -- an OTO or DPD test is only good for a rough check of having chlorine, and is not accurate for knowing if you have enough if you are on the edge (if you are well over your target, then such tests should be fine).
As for the FAS-DPD powder, I found that there are variances in the test that are more a function of time than anything else. I've tried different powder amounts (1, 2 and 4 scoops) but through repeated measurements the only thing that was consistent was that the chlorine level would drop over time. I'm not sure whether this is from oxidizing contaminants in the vials (even though they were rinsed thoroughly) or from outgassing of chlorine or what, but it was definitely repeatable and this was from a single large sample jar (so avoids the variation from where I sampled in the pool). I think that the 0.5 ppm test is fine if you normally have high FC levels (probably because of high CYA levels and no SWG). I personally use the 0.2 ppm test because I usually keep my chlorine level near 3 ppm, but for regular measurements the 10 ml sample with 0.5 ppm accuracy is fine for people who manually dose chlorine. With your fine tuning your SWG which should get to a reasonable steady-state level of FC (unless your bather load varies or you have storms, etc.), you can use the 25 ml sample with 0.2 ppm accuracy, but it's really up to you.
Richard
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The spreadsheet won't predict the amount of CCs. The purpose of the two sections under "CHLORINE USAGE" called "Creation of Combined Chlorine" and "Combined Chlorine to Breakpoint" which together are the same as "Net Chlorine to Breakpoint" are to see the effects on pH and TA when chlorine gets used up in these ways. This has nothing to do with borates.
So if you want to know what happens to pH and TA when you add chlorine to the pool, then you first set your "Free Chlorine" number under the "Goal" to be what you want compared to the "Initial" so that the "Difference" should show the incremental amount you want. Then in the blue section under "ADDED CHLORINE" you should see the amounts of different types of chlorine needed to achieve that FC level. Also, in the blue section under "CHLORINE USAGE" you will see a negative number of the FC level. Type in the amount of chlorine in the appropriate place (based on the type of chlorine you are adding) in the light-orange field ("Inputs" column) in the ADDED CHLORINE section. Then type in the FC difference as a positive number under the CHLORINE USAGE section. Normally, you just put this full amount under the "Net Chlorine to Breakpoint" section since that's what normally happens, but in your case you could put all but 0.5 ppm there and put the other 0.5 ppm under "Creation of Combined Chlorine". Then click on the "Calculate pH/TA" button and you will see the net effect on pH and TA which should show that in this case the addition and usage of a hypochlorite source of chlorine is not pH neutral and has the pH rise because the creation of combined chlorine increases pH instead of the normal decrease in pH from the more typical usage of chlorine to breakpoint.
Technically, what I described above is really two steps I did at the same time. First, one just adds chlorine as with ADDED CHLORINE amounts and then can click on "Calculate pH/TA" to see the effect on pH and TA. Then, you can copy the Goal values to the Initial values (especially for pH and TA and FC) and then set a new Goal value with a lower FC. You then enter that in the CHLORINE USAGE section appropriately (split among categories that you want) and then click on "Calculate pH/TA".
So I'm not sure what you are trying to figure out with the spreadsheet, but it won't tell you why your eyes are stinging or why you have combined chlorine. I'm pretty sure the stinging is from the CCs and not the Borates -- could also be the pH if that wasn't near 7.5.
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For every pound of 20 Mule Team Borax added for borates, it takes 7.6 fluid ounces (about a cup) of Muriatic Acid (31.45% Hydrochloric Acid) to maintain the pH. It would take 10.3 ounces weight of Dry Acid (93.2% Sodium Bisulfate) to do the same thing. The latter adds sulfates to the water while the former does not and high sulfate levels may degrade plaster (the jury is still out on that one). Unless there's a reason the Pool Guy has that I don't know about, using Muriatic Acid is more pure as it does not add sulfates. It is a harsher acid in its applications as the fumes are nasty, but if you are careful then you should be fine. I usually just position myself upwind and hold my breath while opening and pouring.
BleachCalc is wrong about how much Borax it takes to raise the Borates level (this is a known problem and is due to an incorrect assumption of how borate levels are measured -- the "ppm" is for Boron, not for Boric Acid or tetraborate). For your 15,000 gallon pool, to raise the Borates from 0 to 50 ppm it would take 883 ounces or 55.2 pounds (11.7 boxes of 20 Mule Team Borax) plus 421 fluid ounces (52.6 cups or 3.3 gallons) of Muriatic Acid to maintain the pH.
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Jerry,
Your comments regarding maximum chlorine levels are appropriate for water that does not contain any CYA in it or for water that is consumed by drinking it (i.e. tap water). It does not apply for pools that use CYA since it is the disinfecting chlorine (hypochlorous acid) level that is the disinfectant, inhibits and kills algae, and is the oxidizer that damages skin, etc. See this link for more technical info on the CYA/chlorine relationship including a spreadsheet that calculates the disinfecting chlorine level. Essentially CYA combines with most of the chlorine to form what are called chlorinated isocyanurates and these are not effective sanitizers nor prevent algae nor irritate the skin. They can be best seen as a chlorine "buffer" since they release chlorine as hypochlorous acid as it gets used up (i.e. it's an equilibrium with a relatively fast half-life of 0.25 to 4 seconds -- i.e. half of the buffer can be released to chlorine in seconds).
The common standards for pools do not take into account the combination of CYA and chlorine levels and that is wrong. By trying to oversimplify by just setting ranges for each parameter separately, pools with high CYA (around 100) and recommended FC levels of 2-3 ppm can get algae and there are numerous reports of this on multiple pool forums. Maintaining higher FC levels at higher CYA levels as shown in this chart prevents algae (at a minimum of the "0.03" column for disinfecting chlorine level for manually dosed pools -- SWG pools seem to be able to get away with the "0.02" column most of the time). Obviously one shouldn't follow the chart for low CYA levels since one still needs a buffer of chlorine in the pool as it gets used up locally, so a minimum of 2 ppm FC regardless of the chart value is appropriate (unless alternative systems are used to handle some of the chlorine demand, mostly from oxidizing organics, and to inhibit algae so metal systems or ozonators, etc.).
Spas are a different animal due the bacteria that causes hot tub itch that requires higher disinfecting chlorine levels to kill, but I've talked about that in other posts on this forum. This post is an early one on this forum that talks about this including some other references to the CYA/chlorine relationship. I started getting involved with this about 2 years ago when I had problems with my own pool when the CYA levels went way up due to using Trichlor tablets and having a cartridge filter (so no backwashing).
My wife uses a local indoor pool during the winter and that pool has around 2-3 ppm FC with no CYA. Her swimsuits degrade over a single season and her hair dries out considerably. During the summer (actually, around 7 months) in our own outdoor pool with 3 ppm FC and 20-30 ppm CYA she experiences no degradation of swimsuits and her hair does not dry out (in both winter and summer she does take a shower to at least rinse off after swimming). The difference is that the indoor pool has over 30 times as much disinfecting chlorine since it has no CYA which means the chlorine reacts to oxidize organics in rubber and dye in swimsuits 30 times faster as well as proteins and other organics in hair.
Richard
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The chart at the bottom of this link shows the half-life of 12.5% chlorine kept below 80F to be over 3 months. Higher temps and higher concentrations degrade the chlorine more quickly. I have found that the 12.5% chlorinating liquid I get from my local pool store seems to be 12.5% or so (sometimes a tad higher) when I get it and it seems to still be that way or close to 12% after a month or even 6 weeks, but I store it in a pool shed with decent venting and our temps aren't usually in the high 80's very often (and even less frequently in the low 90's). I have no doubt that it degrades, but this is one of those things where you just have to see for yourself since the quality of the chlorine (i.e. not having metal contaminants) makes a big difference.
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If your pool is in full sun, then probably every day unless you have a pool cover in which case it will likely be 2-3 times per week. Just make sure you maintain a minimum FC level appropriate to your CYA level. At 90 ppm CYA, you need at least 7 ppm FC in your pool at all times to ensure your pool does not get algae.
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Because the SWG super-chlorinates a portion of the water flowing through the cell and that this dosing is done frequently throughout the day, many SWG users are able to run with only 3 ppm FC at 70-80 ppm CYA with no problems so that is the usual minimum (the manufacturers say 1-3, but at 1-2 ppm quite a few users have had problems with algae). And yes, that is a disinfecting chlorine level of around 0.015 which is below the minimum of 0.03 and the normal target of 0.05 when manually dosing a pool with chlorine. A very small number of users found they needed to keep their chlorine level for their SWG at the minimum 0.03 disinfecting chlorine level which would be 6 ppm FC but that was mostly for keeping away mustard/yellow algae.
[EDIT]
My hunch, and this is just a hunch, is that having 3 ppm FC with 80 ppm CYA will work in an SWG pool if your periodically (weekly?) brush your pool surfaces to prevent any algae buildup in biofilms. Most green algae is free-floating so will eventually get zapped in the SWG, but if any get stuck on a wall or in a corner, then only the FC level in the main body of pool water will keep it from growing and 3 FC with 80 ppm CYA is below what normally is enough to prevent algae (but this depends on how "ideal" the algae growth conditions are which probably explains why a few pools still had a problem at this level).
[END-EDIT]
The spreadsheet color coding was for a nominal "target", not a minimum. It is "green" when at the target. It is "yellow" when below that target (I didn't have a separate color code for the "minimum" since Excel only lets you have 3 conditional colors). It is "red" when below the target but above the minimum possibly needed for disinfection (which is lower than that needed for algae prevention).
So while I would say that a non-SWG user would need to keep a minimum of 6 ppm FC so a typical "target" of 9 ppm FC (to allow for some drop), an SWG pool such as yours could get away with a lower FC level. You may find that it isn't as hard as you think to keep an FC level higher than 3 ppm anyway since the high CYA level seems to protect the chlorine from sunlight more. At any rate, I wouldn't go lower than 3 ppm FC, but with the SWG you could likely go lower than even the normal "minimum" of 6.
[EDIT]
There are other options that can help prevent algae and let you use a lower FC level and this includes using 50 ppm Borates in your pool. This has the additional advantage of acting as an additional pH buffer that is well suited to the rising pH that is typical in most SWG pools and seems to help with that problem (probably by killing algae so chlorine doesn't have to so you can lower your SWG output which lowers hydrogen gas bubble production which lowers aeration which lowers carbon dioxide outgassing which reduces the pH rise -- WHEW!). waterbear can tell you more about that as it has been very successful in his pool as well as others.
[END-EDIT]
Richard
Acceptable Alkalinity Level In Intex Pop Up Pool
in Swimming Pool Water Chemistry
Posted
The process of lowering the TA can be accelerated by aerating the water at a low pH. It is not necessary to lower your TA for a vinyl pool unless you are experiencing a frequent rise in pH which requires frequent addition of acid. Lowering TA simply accelerates the natural process of outgassing of carbon dioxide from the pool -- pools are intentionally over-carbonated to provide a pH buffer. The process for lowering the TA is as follows.
ACTIVITY .......... pH .... TA ... In your case (assuming 6.8 is the lowest measurement on the pH test kit)
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Acid ..................... - ........ - ... Add enough acid to bring pH down to 7.0 (if it's already there, then just skip to the next step, aeration)
Aeration ............. + ....... 0 ... Aerate until pH rises to 7.2
Acid ..................... - ........ - ... Add enough acid to bring pH down from 7.2 to 7.0 (you may continue to aerate while you do this)
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Aeration & Acid .. 0 ....... - ... Continue this combination (cycling of the two above) until TA is at the target you want
then AFTER you have reached your target TA,
Aeration .............. + ....... 0 ... Aerate until the pH rises to your target pH (say, 7.5).
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Net of Above ....... 0 ........ -
Note that there is NO addition of base (Borax or otherwise) in the above procedure. Aeration would include running your SWG, running your waterfall, adding any fountains or other aeration features, getting an air compressor with a nozzle that produces tiny bubbles and putting that in the deep end of the pool, etc.
Hope that helps,
Richard