Unexpected Results

[johnsimion]

Hello, I have been using the TFT-100 kit for a year now and have noticed some results lately that seem weird. For background, we live in Las Vegas, where it rarely rains and untreated incoming water is both very hard and very alkaline. To cope with those issues, I installed a dedicated water softener for the feed line and completely drained and refilled the pool, mostly with soft water and just enough hard water to get the calcium to the minimum acceptable level. As of March, things were nearly perfect: CH 250, TA 90, CYA 56, and Salt 3400. Lately I have been getting results every time like today: CH 375, TA 60, CYA 39, and Salt 4000. These numbers haven't just popped up suddenly, they all show a steady trend.

Fortunately today's results in PoolMath still show the water balanced and everything is normal and pool is crystal clear -- but the situation doesn't make sense to me and I don't like the trend. The CH shouldn't be going up at all. I just checked the output of the water softener and as expected, there is NO calcium in it -- I add 10 drops of R-10 and 3 drops of R-11L and the softener water comes out blue right then and there (red indicates the presence of calcium). So where could the increased CH be coming from? The only other information I can think of is that due to the highly alkaline water, the PH tends to creep upward to 8.2+ and the TA wants to rise to 100+, and thus to counteract this, I have been adding enough acid to keep the PH down to the low 7's and the TA between 60-70. Maybe pool corrosion? However, I have never gotten results from PoolMath showing corrosion of plaster (Pebble-Tec) is likely nor have I detected any noticeable corrosion. In looking at my spreadsheet of past results, however, it does seem like the more I have lowered my TA, the higher my CH has gotten.

I also don't understand Salt could be going up while the CYA is going down. When we used to live in Florida, we'd get heavy rain that would overflow the pool and that would account for lowered salt and CYA levels, but that's hardly the case here. Here we have a lot of evaporation. But you'd think that evaporation would make them both go up, not just Salt.

 

[Texas Splash]

Regarding the rise in your CH, I just have to ask ...... you haven't used any bags of shock or products with Cal-Hypo in it have you?

 

[YippeeSkippy]

As water evaporates it still leaves all the salt and calcium present. So it becomes more concentrated.

 

[pabeader]

Muriatic acid adds chloride, so will show as increase in salt. If I remember my JoyFullNoise and Chem Geek correctly.

 

[JoyfulNoise]

Muriatic acid adds chloride, so will show as increase in salt. If I remember my JoyFullNoise and Chem Geek correctly.

You are correct, Pa. Muriatic acid is hydrochloric acid, aka hydrogen chloride. In my 16,000 gallon pool, every gallon of MA adds approximately 23.2 ppm Cl- (which equates to 37.2ppm NaCl on the K-1766 test) . For any pool, use this very simple formula (assuming 20 Baume MA) -

[EDIT]

I was reminded in a later post by JamesW that my original formula forgot to account for the fact that the Taylor K-1766 measures chloride content as ppm of NaCl not as ppm of Cl- and so one must account for the relative molecular weight of NaCl to HCl. Since one mole of chloride ion (Cl-) from HCl would result in the measurement by the K-1766 as one mole NaCl, then the correct equation would be -

ppm [Cl-] per gallon of MA = 370,880 * (1/{pool volume})

ppm [NaCl] per gallon of MA = 370,880 ppm-HCl * (1-gal/{pool volume}) * (58.44 g-NaCl/mol / 36.46 g-HCl/mol)

[END-EDIT]

As for the CH, your water softener can not COMPLETELY eliminate all of the calcium. You may have measured it just after a regeneration cycle, but there is always some smallish amount of calcium left in the water. And, if I may ask, what happens during a regeneration cycle? Does the autofill still work via a by-pass or is the fill line shut-off during that time? If I'm not mistaken, with residential softeners, when the exchange resin is regenerating, the entire unit is bypassed so that water still flows into the house.

 

[otter86753]

I do not have nor need a water softener, but don't you use salt to soften the water? Does this add salt to your fill water?

 

[pabeader]

No, the salt is only used to recharge the resin bed. Once that is complete the leftover is removed and sent to the drain.

 

[JoyfulNoise]

I do not have nor need a water softener, but don't you use salt to soften the water? Does this add salt to your fill water?

Water softeners contain cationic exchange resins (sulfonate polymers). They exchange two sodium cations (Na+) for one calcium cation (Ca++). There is no chloride (Cl-) added during the ion exchange process. When the exchange resin becomes saturated with calcium, a saturated solution of sodium chloride is washed through it to remove the calcium (regeneration).


Sent from my iPhone using Tapatalk

 

[PoolChickTX]

What brand of softener do you have?

 

[johnsimion]

Texas Splash asked whether I had used any bags of shock or products with Cal-Hypo. No, I have never had the need.

YippeeSkippy said: "As water evaporates it still leaves all the salt and calcium present. So it becomes more concentrated." I agree, except for two things that are still odd here. First is that the pool has an automatic filler that should be replacing the water as fast as it evaporates, and the fill water is soft and therefore lacks either salt or calcium. Second is the fact that my CYA is dropping at the same time the salt is rising. If the salt is becoming more concentrated, so should the CYA.

Joyful Noise suggested that the chloride from the muriatic acid may be contributing to the salt reading. Didn't think about that one, and it may well be the answer, but it would be surprising that a few gallons of MA could increase my salt level by 800 ppm in a few months. It also brings up another question. Does my Taylor salt test pick up **hydrogen** chloride from the MA, or just the **sodium** chloride from the salt that I added to the pool to run the SWG in the first place? If it does, how would I know when to be concerned about excessive salt in the pool?

Joyful Noise also said, "Does the autofill still work via a by-pass or is the fill line shut-off during that time? If I'm not mistaken, with residential softeners, when the exchange resin is regenerating, the entire unit is bypassed so that water still flows into the house." I am sure the entire unit is bypassed while it's regenerating, but it only regenerates every now and then and when it does, it only takes an hour or so. It's hard to believe the water going into the pool through the automatic fill during that one hour could have a very significant impact on the amount of CH in the pool. And, the amount of CH in the pool was steady for several months; the rise only has occurred since June.

Otter86753 inquired about whether salt makes its way from the softener into the pool. I agree with pabeader and Joyful Noise that softeners do not add salt to the water that is softened. The salt simply cleans out the unit and gets flushed down the drain. This is a common misconception about soft water.

PoolChickTX asked what brand of softener I am using. It is a WaterBoss from Lowe's or Home Depot (forget which one). This one sits outside and is dedicated for the pool -- it's separate from the other water softener in the garage that is used internally for the house.

 

[JoyfulNoise]

Based on your 19,000 gallon pool, every gallon of muriatic acid adds ~ 19.5ppm chloride (Cl-). So you can figure out how much chloride you've added if you know, approximately, how many gallons of acid you've used since the SWG was started.

The Taylor K-1766 measures total chloride concentration, [Cl-]. Therefore it measures all forms of chloride - NaCl, CaCl2, HCl, etc.

CYA does naturally decrease over time from splash out, filter backwashing and chlorine oxidation. The typical rate of CYA loss is about 3-5ppm per month based on FC level and the amount of actual water loss (i.e., NOT evaporation). If you find an excessive rate of CYA loss, then that points to a possible water leak BUT you would notice ALL chemical levels lowering. Remember, your CYA test is only good to about 10-15ppm, so keep that in mind when testing.

 

[JamesW]

Note that the 1766 measures chloride but it reports in units of NaCl. Therefore, every gallon of acid added will increase the test result by 30 ppm in a 19,000 gallon pool.

 

[JoyfulNoise]

Note that the 1766 measures chloride but it reports in units of NaCl. Therefore, every gallon of acid added will increase the test result by 30 ppm in a 19,000 gallon pool.

Thanks, James! I forgot to correct for the relative molecular weight differences.

 

[chem geek]

As of March, things were nearly perfect: CH 250, TA 90, CYA 56, and Salt 3400. Lately I have been getting results every time like today: CH 375, TA 60, CYA 39, and Salt 4000.
:
The CH shouldn't be going up at all. I just checked the output of the water softener and as expected, there is NO calcium in it -- I add 10 drops of R-10 and 3 drops of R-11L and the softener water comes out blue right then and there (red indicates the presence of calcium). So where could the increased CH be coming from? The only other information I can think of is that due to the highly alkaline water, the PH tends to creep upward to 8.2+ and the TA wants to rise to 100+, and thus to counteract this, I have been adding enough acid to keep the PH down to the low 7's and the TA between 60-70. Maybe pool corrosion? However, I have never gotten results from PoolMath showing corrosion of plaster (Pebble-Tec) is likely nor have I detected any noticeable corrosion. In looking at my spreadsheet of past results, however, it does seem like the more I have lowered my TA, the higher my CH has gotten.

I also don't understand Salt could be going up while the CYA is going down. When we used to live in Florida, we'd get heavy rain that would overflow the pool and that would account for lowered salt and CYA levels, but that's hardly the case here. Here we have a lot of evaporation. But you'd think that evaporation would make them both go up, not just Salt.

Evaporation and refill adds whatever is in the fill water even if it's a low value. You use a water softener so that removes calcium and replaces it with sodium, but that won't show up in the salt test which tests only for chloride. The sources of chloride are the chlorine you add to the pool and acid (if hydrochloric acid) you add to the pool.

If you've been adding 2 ppm FC per day with chlorinating liquid or bleach then this would increase your measured salt level by about 100 ppm per month. From March until now is 7 months so 700 ppm while you've measured a 600 ppm increase. You've also added acid to the pool where every gallon of full-strength Muriatic Acid (31.45% Hydrochloric Acid) adds 31 ppm salt. So the increase in salt level is understandable. As for the CYA level, it will only decrease over time as chlorine slowly oxidizes it. 56 ppm to 39 ppm is 17 ppm which over 7 months is a loss of 2.4 ppm FC per month which is well within normal CYA loss (and you aren't really measuring CYA that accurately anyway).

So that leaves the CH level and its increase. With your pH in the low 7's your CSI from your starting to current numbers would be (using 85ºF temperature and pH 7.2) between -0.6 and -0.7. Just because PoolMath doesn't say that plaster is harmed until the CSI gets very low make that true. ANY low CSI is detrimental to plaster. It's only a matter of degree. A CSI of -0.3 has HALF the needed amount of calcium or carbonate needed to saturate the water to prevent plaster degradation. A CSI of -0.6 has only one-fourth the needed amount. Much worse, however, is a low CSI that is also at a low pH since lower pH accelerates the dissolving of calcium carbonate when the CSI is negative (when the CSI is zero or positive, low pH doesn't dissolve plaster).

I know there are those on this forum that don't believe in the CSI, but the fact is that keeping a low CSI especially with low pH over an extended period of time is a bad thing to do. As for you not noticing the degradation, a rise of 125 ppm in your 19,000 gallon pool is 9 kg (about 20 pounds) of calcium carbonate which is 3320 cm³ of volume. If I assume your pool is 18'x32'x4.5' average then an even distribution represents a thickness of only 0.036 mm though even if only 20% of the pool surface were calcium carbonate this is still only 0.18 mm so probably not noticeable with Pebble-Tek. If this were to continue on for years, however, you may start noticing the effects.

If you wanted to keep your TA lower and were doing so by having the pH be lower (to force more carbon dioxide outgassing) then you could have your CH level be higher to compensate. However, instead of doing that why not try targeting a more moderate pH of 7.5. Is the issue that the TA rises too quickly from evaporation and refill if you maintain only a pH of 7.5?

 

[BuckeyeChris]

To the OP: your pool is just like mine here in west phoenix. I have a softener, but the CH still goes up because there is still 30 ppm or so in the softened water, much more towards the end of that recharge cycle. It will slowly accumulate until you do a backwash. Mine is at 800ppm

My salt level has been going up, but that's because I use Liquid chlorine. I'm at about 4200 also.

My CYA drifts down by about 5 ppm per month. No real explanation other than slowly broken down from the chlorine.

All that said, my CSI is like 0.02 and the water is perfect except I don't rake the bottom enough

 

[chem geek]

A rise of 125 ppm over 7 months would only be 18 ppm per month but even with 17" per month evaporation (that's normal July pan evaporation in Las Vegas) this would be 9.9 feet of evaporation over 7 months so with 4.5 foot average pool depth that's 2.2 times pool volume so the fill water would have to be 125/2.2 = 57 ppm CH to possibly explain the result based on fill water and that assumes the full evaporation rate over the entire 7 months which is unlikely.

 

[johnsimion]

Of the sources Chem Geek mentions, only the muriatic acid applies. I exclusively use a SWG and have never added chlorinating liquid or bleach since I got the SWG. As stated I do use muriatic acid but it's a weaker variety and I doubt if I've used 10 gallons since March. Haven't kept track but I only remember buying acid once or twice. Of course even adding 29 ppm of chloride x 10 gallons, accounts for a lot of the salt level rise. Since I have finally gotten the TA level down, I will start letting the PH rise back up to the 7.5 range and hopefully use less acid in the process. As for the CH rise, I guess it must just be residual CH in the soft water from the fill line as well as a little bit of the pool being eaten up, which is a side effect of me adding too much acid to get the TA level down. I have been kind of paranoid about the CH level because the water here is so darned good at leaving a calcium ring, and I have all this beautiful natural stone on the entire east side that doesn't take well to media blasting -- not to mention the expense. Regardless, I will let the CSI level rise a bit next time. As for the CYA, I didn't realize that this would degrade over time but now that I know, what's happening certainly makes sense.

I have to partially drain the pool again over the winter to do some tile work, so that will be another opportunity to even things out.

Thanks to everyone for the help.

 

[Swampwoman]

^This might be a stupid (but possibly related) question for ChemGeek or joyful noise: Is it possible in a pool environment that softened water, which carries sodium but not chloride, combines with fresh or spent chlorine (or chloramines) that produces a reading of salt, either falsely, or accurately?

John, by way of background, ChemGeek has previously explained to me that softened water has sodium, but not sodium chloride, coming out.

However, I did a salt test of my pool water (3500 ppm) AND my softened tap water at the pool builder's store when discussing a switch to swg. They tested 450 ppm salt in my softened water. I then repeated said tests at home with salt strips and got the same result. My softener system is dual...one runs while the other regenerates. I had switched to this system this year because on a single system with frequent top ups, I'd end up with raw well water sometimes by tapping out my capacity before the next regeneration cycle. But my water is definitely more saline now since switching...by taste anyway.

Pa Beader has elsewhere has shared with me that there is a way to adjust your rinse cycle time so that salt is not coming out from the softened taps. (At the time, I mistook sodium chloride to mean sodium, and had read that sodium is in fact in all softened water, and that there is a formula to calculate same which happens to calculate to my reading of 450.)

From all this in my own exploration of softener, well, and increasing salt, I'm wondering if in YOUR case:

A) Re calcium, you've occasionally "tapped out" your capacity...so that at times your auto system is running raw well water into the pool

And
B) whether its possible that your softener needs an adjustment in terms of rinse cycles if salt (as opposed to sodium without the chloride) is coming out. In which case your salt level might rise with substantial evaporation and no freshwater (rain) dilution, even though any salt in softened water would be substantially less than pool water. Or if the sodium that come out with softened water somehow combines with chlorides to produce falsely high readings or salt itself

I've been trying to work out if such scenarios might occur prior to switching to swg ... Eg If I'd need to dilute salt somehow over time even though I don't want to use raw well water...but pool is closed for the season so I haven't persued the investigation much further

 

[domct203]

Hello, I have been using the TFT-100 kit for a year now and have noticed some results lately that seem weird. For background, we live in Las Vegas, where it rarely rains and untreated incoming water is both very hard and very alkaline. To cope with those issues, I installed a dedicated water softener for the feed line and completely drained and refilled the pool, mostly with soft water and just enough hard water to get the calcium to the minimum acceptable level. As of March, things were nearly perfect: CH 250, TA 90, CYA 56, and Salt 3400. Lately I have been getting results every time like today: CH 375, TA 60, CYA 39, and Salt 4000. These numbers haven't just popped up suddenly, they all show a steady trend.

Fortunately today's results in PoolMath still show the water balanced and everything is normal and pool is crystal clear -- but the situation doesn't make sense to me and I don't like the trend. The CH shouldn't be going up at all. I just checked the output of the water softener and as expected, there is NO calcium in it -- I add 10 drops of R-10 and 3 drops of R-11L and the softener water comes out blue right then and there (red indicates the presence of calcium). So where could the increased CH be coming from? The only other information I can think of is that due to the highly alkaline water, the PH tends to creep upward to 8.2+ and the TA wants to rise to 100+, and thus to counteract this, I have been adding enough acid to keep the PH down to the low 7's and the TA between 60-70. Maybe pool corrosion? However, I have never gotten results from PoolMath showing corrosion of plaster (Pebble-Tec) is likely nor have I detected any noticeable corrosion. In looking at my spreadsheet of past results, however, it does seem like the more I have lowered my TA, the higher my CH has gotten.

I also don't understand Salt could be going up while the CYA is going down. When we used to live in Florida, we'd get heavy rain that would overflow the pool and that would account for lowered salt and CYA levels, but that's hardly the case here. Here we have a lot of evaporation. But you'd think that evaporation would make them both go up, not just Salt.

Out of curiosity, how are you getting those CYA numbers? The CYA test in the TF-100 is graduated by 10's.

Dom

 

[JoyfulNoise]

^This might be a stupid (but possibly related) question for ChemGeek or joyful noise: Is it possible in a pool environment that softened water, which carries sodium but not chloride, combines with fresh or spent chlorine (or chloramines) that produces a reading of salt, either falsely, or accurately?

Short answer is, no it does not.

Perhaps it might help to explain the argentometric chloride test (also known as Mohr's Method)-

There are two reagents in the test - potassium chromate (K2CrO4), which is the yellow indicator dye and aqueous silver nitrate (AgNO3), which is the titrant. You take a sample of pool water that containes an unknown number of chloride ions (Cl-) and add one drop of the potassium chromate indicator. Potassium chromate does not react with the chloride ions in any way and is a spectator in this first part of the test.

You then add, drop-wise, the silver nitrate reagent. Each drop of silver nitrate is designed by it's concentration and water sample size to react with 200ppm (in NaCl units) of chloride (Cl-) ion in the following reaction (I'm splitting up the reactants into ion pairs) -

Ag[+] + NO3[-] + {X}[+] + Cl[-] -----> AgCl(solid) + {X}NO3(aqueous)

A confusing equation so let me make a couple of notes - first, the {X} represents any counter-ion to the chloride, eg, Na+, K+, Mg++, Ca++, etc, it makes absolutely no difference what those counter ions are except for the fact that doubly-charged ions (Ca++ or Mg++) would consume twice as many Ag and Cl ions. Another note to make is that the silver chloride, once formed, is no longer aqueous but precipitates out as the milky white curdled substance you see in the view tube. Since sodium chloride is the dominant species in salted pool water, an exemplary reaction would be this -

AgNO3(aq) + NaCl(aq) -----> AgCl(s) + NaNO3(aq)

Once the silver nitrate titrant has reacted with all of the chloride ions (Cl-) in solution, there is a second reaction that occurs between the potassium chromate indicator and the silver nitrate as follows -

K2CrO4 + 2*AgNO3 -----> Ag2CrO4 + 2*KNO3

The silver reacts with the chromate to form silver chromate which is brick/salmon red in color. That is why you conclude the test as soon as you see the red color form as that indicates that all of the chloride is gone. Taylor likely adds some pH buffers to one of the solutions in order to keep the pH near neutral for all of the reactions to occur properly, but that that's the basic test process.

So you can see form the above that it makes absolutely no difference what the counter ion is to the chloride in your pool water and it makes no difference where the chloride came from. The test reacts with all of the chloride in the water and is ignorant of the counter ion species. Chloride is only in the water from sources add - liquid chlorine, pool salt and muriatic acid.