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"unexplainable" Chlorine Test Results


DonR6702

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Greetings,

I am a first time poster here, and this post will be a little lengthy as I want to relay all the information I can about this problem.

First, I have a degree in chemistry and worked for 7 years as an analytical chemist, doing primarily water and wastewater analysis, so I do have a little background in the field, though it has been quite a few years since I worked as a chemist. I did many FAS-DPD analyses of water and wastewater for chlorine using laboratory equipment and reagents, but I no longer have access to a real lab. I have a 450 gallon spa that I sanitlze with dichlor and shock with MPS, and I use a Taylor K-2006 test kit for the DPS-FAS analysis of chlorine. I also use LaMotte InstaTest strips sometimes for a quick check of chlorine - those strips also use a DPD method for the chlorine analysis.

Last Sunday I measured free chlorine with the Taylor kit and found only a trace (< 0.5 ppm). It had been a couple of weeks since I shocked, so I added 2 tablespoons of MPS, circulated for 15 minutes, then added 1 teaspoon of dichlor granules, which should have resulted in about 2.5 ppm of free chlorine, and ran the pumps on high for another 15 minutes. Late that evening I checked the free chlorine with the Taylor kit and got a reading of about 4.5 ppm, which was about 2 ppm higher than expected. I checked at that time with a LaMotte test strip and it read between 2 and 3 ppm, which was the expected concentration.

The next morning I checked again and read 4 ppm with the Taylor kit, and a little more than 2 ppm with the LaMotte test strip. Tuesday morning the Taylor kit read 3.5 ppm and the LaMotte strip read between 1 and 2 ppm. This morning (Wednesday) the Taylor kit read 2.5 ppm and the LaMotte strip read only a trace of free chlorine. The spa was used each night by one bather for 15 minutes, and my past experience with this spa led me to believe that the readings I got from the LaMotte strips were reasonable, and that the Taylor kit was consistently reading 2 to 2.5 ppm higher than the expected value.

I called Taylor's customer support line this morning and spoke to one of their tech support reps. He suggested that I make a "standard" chlorine solution with household bleach and analyze it with both the Taylor FAS-DPS reagents and the LaMotte test strip. I did that and both methods read a reasonable free chlorine concentration of 2.5 ppm. Subsequently I checked the spa water again, and both the Taylor kit and the LaMotte strip read only a trace (< 0.5 ppm) of free chlorine.

I don't understand these results, and I am still concerned that one or more of the Taylor reagents are contaminated or otherwise faulty. Does anyone here have another explanation?

Thanks,

Don

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Don,

The MPS will often measure as Combined Chlorine (CC), but at high doses or if the MPS contaminant peroxydisulfate (persulfate) (S2O82-) is present, then this can measure as Free Chlorine (FC). Taylor has the K-2041 MPS interference remover that theoretically removes MPS (but not persulfate) interference, but some people who have tried that have had mixed results. Interestingly, the DPD test should have been as affected as the FAS-DPD test unless the test strip had the interference remover built into it (if it's for spas, it might).

Usually, MPS doesn't last much more than a day at hot spa temperatures. However, in your case it seems the MPS lasted several days. 1 teaspoon of Dichlor in 450 gallons should only result in 1.5 ppm chlorine (remember that the "ppm" in chlorine measurements is relative to chlorine gas with a molecular weight of 70.906 g/mole). Also, 2 tablespoons of MPS (assumed to be relatively pure triple salt of K2SO4•KHSO4•2KHSO5) has the oxidizing equivalent of 1 tablespoon (3 teaspoons) of Dichlor or 4.6 ppm. So the total oxidizer measurement in the spa would really be 6.1 ppm. It appears that the MPS is only partially interfering with the FAS-DPD test (or is otherwise getting partially consumed quickly) and only slightly interfering with the DPD test. This all assumes that additions, volumes, etc. are accurate. I assume that one ounce weight of Dichlor is roughly one ounce volume (2 teaspoons), for example. Your Dichlor (as bulk product) might be more or less dense than that.

I assume you are aware that with Dichlor, for every 10 ppm Free Chlorine (FC) that you add, you also increase Cyanuric Acid (CYA) by 9 ppm.

Richard

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Don,

The MPS will often measure as Combined Chlorine (CC), but at high doses or if the MPS contaminant peroxydisulfate (persulfate) (S2O82-) is present, then this can measure as Free Chlorine (FC). Taylor has the K-2041 MPS interference remover that theoretically removes MPS (but not persulfate) interference, but some people who have tried that have had mixed results. Interestingly, the DPD test should have been as affected as the FAS-DPD test unless the test strip had the interference remover built into it (if it's for spas, it might).

Usually, MPS doesn't last much more than a day at hot spa temperatures. However, in your case it seems the MPS lasted several days. 1 teaspoon of Dichlor in 450 gallons should only result in 1.5 ppm chlorine (remember that the "ppm" in chlorine measurements is relative to chlorine gas with a molecular weight of 70.906 g/mole). Also, 2 tablespoons of MPS (assumed to be relatively pure triple salt of K2SO4•KHSO4•2KHSO5) has the oxidizing equivalent of 1 tablespoon (3 teaspoons) of Dichlor or 4.6 ppm. So the total oxidizer measurement in the spa would really be 6.1 ppm. It appears that the MPS is only partially interfering with the FAS-DPD test (or is otherwise getting partially consumed quickly) and only slightly interfering with the DPD test. This all assumes that additions, volumes, etc. are accurate. I assume that one ounce weight of Dichlor is roughly one ounce volume (2 teaspoons), for example. Your Dichlor (as bulk product) might be more or less dense than that.

I assume you are aware that with Dichlor, for every 10 ppm Free Chlorine (FC) that you add, you also increase Cyanuric Acid (CYA) by 9 ppm.

Richard

Hi Richard,

Thanks for the reply. It's good to know about the mixed results with Taylor's deox reagent - the Taylor rep I talked to this morning suggested getting that reagent to eliminate potential interference. It seems to me that MPS will interfere more with an FAS titration analysis than with a test strip or colorimetric analysis since it not only potentially reacts with the DPD reagent, but it can also react with the FAS titrant, oxidizing it before it can reduce the red dye complex.

I too considered that the MPS was sticking around much longer than expected and causing the interference - my spa is not very heavily used and I have been in the habit of using MPS every two or three weeks even though I rarely see more than a slight trace of combined chlorine between MPS doses. But when the free chlorine test suddenly dropped today, in about 5 hours, from 2.5 ppm to less than 0.5 ppm, I pretty much discounted that. This large drop in free chlorine was obviously an analytical phenomenom, as nothing happened to the spa during that 5 hour period, and the test strip method had indicated a believable trace of chlorine first thing this morning, so the level didn't really drop, the FAS-DPD analysis just suddenly started reading correctly.

I understand your point about the molar equivalency. I used a bulk density of 0.8 gm/cc for the dichlor dihydrate, but my error was dividing 1 teaspoon (5 cc) by 0.8 instead of multiplying. And I used 400 gallons (1520 liters) as the fill volume, so 1.5 ppm it is.

I am aware of the CYA issue and I measure CYA occasionally, but with my tub's fairly low usage it shouldn't become a problem. I change the water every three to four months, and I am not likely to add more than 100 ppm of chlorine in that time period.

This afternoon I added another teaspoon of dichlor after finally getting a believable reading with the Taylor kit. I will test tomorrow and every day for a few days and see what happens.

Thanks,

Don

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The saga continues. I tested my spa water again this morning, after adding 1 teaspoon of dichlor yesterday afternoon (about 1.5 ppm). The free chlorine measured only a trace before I added that teaspoon, and the spa was used last night for 15 minutes. This morning the free chlorine as measured with the Taylor FAS-DPD kit was 3 ppm. I checked it with my LaMotte test strips and with an old OTO drop kit, and they both indicated about 1 ppm.

I called Taylor again this morning. They told me they were going to do some investigation and call me back. Late this afternoon they called back and told me they couldn't duplicate the problem I was having using water treated with MPS. They asked me to email to them an image of the label from my MPS, which I did. They are supposed to call me back again tomorrow.

Meanwhile, I prepared a solution of MPS using bottled drinking water, and containing about 50 ppm of the MPS triple salt, which is about the same concentration as two tablespoons in 400 gallons of water. I tested the drinking water before adding the MPS, and it came out negative for free chlorine, as expected. As usual, though (at least with my Taylor kit), letting the sample containing DPD sit for a couple of minutes caused a slight pink color to develop, which disappeared with one drop of the titrant. I then tested the drinking water to which I had added 50 ppm of MPS. When I added the DPD powder, a pink color started developing almost immediately. Within 15 seconds it resembled spa water containing about 0.5 ppm free chlorine. I titrated after about one minute and the free chlorine measurement was 9.5 ppm. I chedked the same water with the LaMotte test strip and with my old OTO test kit, and both resulted in no measurable free chlorine. I then took another sample of the MPS-treated bottled water, added DPD, and started titrating after about 10 seconds, when the pink color was faint. Even though the pink color never got very dark, it still took 6 drops (3 ppm) before it became colorless, and within a few seconds after that it was turning pink again. I let it sit for 30 minutes, and it is now quite dark, resembling spa water with 3 or 4 ppm of free chlorine.

I still don't know if the problem is with my reagents or with all of the Taylor DPD kits. The Taylor rep told me that it takes "a lot" of MPS to interfere with the free chlorine test, but didn't specify how much is "a lot". At any rate, they told me that waiting 24 hours after adding a normal dose of MPS should be enough time before performing the free chlorine test.

I have a magnetic stirrer, and I am going to prepare another 50 ppm MPS solution and let it stir vigorously overnight. Tomorrow I will test it and see what happens. I can't keep this up forever though, because I'm running out of titrant in my Taylor kit, and I won't buy any more until/unless they can give me a satisfactory solution. If anyone else can help with thoughts, ideas or experience please do.

Thanks,

Don

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Don,

You've pretty much been told the same thing I was about how high levels of MPS could break through to FC but that normally it measures as CC and that after 24 hours there shouldn't be any breakthrough. However, your experience is clearly different than that. What is also interesting is that you are finding that the OTO and test strips are roughly consistent in measuring the true free chlorine level while the FAS-DPD kit seems to be more interfered by MPS.

50 ppm MPS of the triple salt is roughly equivalent to 5.2 ppm FC in oxidizing power and I wouldn't think that extraordinarily high, though it's not low either. It will be interesting to see what Taylor says about all of this. It would certainly through a wrench into trying to measure chlorine when also using MPS for shocking. I knew there could be interference, but figured that a weekly MPS shock wouldn't be a problem if people wanted to do that since I thought it would dissipate within a day. It sounds like that isn't the case.

By the way, in terms of the total amount of oxidizer needed for bather waste, a rule-of-thumb is that it takes 3-1/2 teaspoons of Dichlor or 5 fluid ounces of 6% bleach or 7 teaspoons of MPS for every person-hour of soaking at hot (100-104F) temps. Many people don't use enough chlorine or oxidizer for their bather load.

Richard

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Hi Richard,

Yes, it will be interesting to see what Taylor says. I'm not sure what I expect them to say or do about this, but I will find out. I bought the Taylor kit after using test strips and cheap kits with the o-toluidine chlorine drop test successfully for several years. I like to keep a close eye on the chemical balance, and I was never satisfied that I was getting good results with the methods I used, especially for alkalinity and hardness. I found out about the Taylor K-2006 kit, which does indeed use very good alkalinity and hardness methods, an easily readable pH comparator, and as an extra added benefit the ability to get accurate combined chlorine results - or so I thought. I'm very happy with the kit except for the chlorine issues, which apparently have to do with MPS.

To be fair about it, I was pleased at first with the chlorine test too, and that was soon after I had drained and refilled the spa. After a couple of months I started seeing some questionable results on free chlorine, and I called Taylor and basically got nowhere, but didn't really have time to pursue it. Then I drained and refilled the spa again and all went well for a while, until recently. So I think that there may be issues in addition to the recent use of MPS, perhaps related to total dissolved solids, buildup of cyanurates, sulfates, etc. Water chemistry is a pretty complex subject, and spa chemistry is closer to wastewater chemistry, an even more complex subject, especially if it's been a while since you've drained and refilled.

So we'll see ... on the brighter side, it's about time to be draining and refilling again, so perhaps I'll get back to a more predicatable regime soon.

Your figures on chlorine usage are right on according to my experience. With my spa, it's usually just my wife and I, and more often than not just me, for 15 minutes on most nights when the weather allows. A teaspoon of dichlor does the trick on the nights when it's just me for 15 minutes. My water has always been crystal clear since I switched from biguanide to chlorine. Right now though I'm willing to sacrifice water quality for the benefit of finding the answer to this issue, since it's almost time to change the water anyway. I'll let it go without adding chlorine until the Taylor kit says to add it, or until I know why not.

Thanks,

Don

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Well, I have some resolution on this.

Taylor called back today and the tech support rep said that he was able to duplicate the problem by adding MPS to a water sample. He said it took "quite a bit" of MPS to bleed through into the free chlorine test - I asked how much is "quite a bit" and he said around 20 ppm as potassium monopersulfate. That's about how much you get when you add two tablespoons of non-chlorine shock containing 45% MPS (the normal formulation) to a 400 gallon spa.

He agreed that the MPS seems to interfere with the DPD-FAS method in two different ways - by producing a faint pink color with the DPD reagent and then by reacting with the FAS titrant, causing more drops to be needed before the pink color is discharged.

Taylor's solution is to use their deox reagent, R-0867, before titrating with FAS. The tech support rep said he was able to eliminate the inteference this way. How much of the deox reagent you need depends on how much MPS is in the water, of course, and he said a 20 ppm dose could dissipate in hours or could take days, depending on the water's oxygen demand.

I guess the moral is, if you use the DPD-FAS method (and to a lesser extent the DPD color matching method), you shouldn't add MPS unless you really need it, and then no more than you really need. Or switch to shocking with chlorine.

I'm happy with that answer, and I will continue to use the Taylor K-2006 kit. I will buy a small bottle of the R-0867 deox reagent, but likely will switch from MPS to chlorine for shocking in the near future.

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Don,

Thank you so much for getting to the bottom of this. Though I knew about interference of MPS with the chlorine tests, I didn't know that it had a greater effect on FAS-DPD due to its partial oxidation of the FAS titrating drops in addition to its effects on the DPD color dye. Usually, using chlorine to shock works well if one is using the Dichlor-then-bleach method since shocking with bleach won't increase the CYA. On the other hand, using sufficient chlorine daily can prevent one needing to shock at all, similar to pools that properly maintain an FC/CYA ratio.

Just keep in mind that if you are using Dichlor-only and shock with Dichlor that your CYA will rise much faster. You mention you have low bather load and change the tub when the cumulative FC would be around 100 ppm. That might be OK, but some rough tracking of hot tub itch/rash/lung incidents I made (see this post) seemed to indicate a greater risk after 1-2 months of regular Dichlor use, but that was in tubs used daily so with around 100 ppm CYA accumulating per month (at roughly 4 ppm FC in 350 gallons daily chlorine usage). So I've been recommending around 20-30 ppm CYA if one uses the Dichlor-then-bleach method just to be safe (and based on some rough CT calculations for the hot tub itch bacteria from some conservative sources).

Richard

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Don,

Thank you so much for getting to the bottom of this. Though I knew about interference of MPS with the chlorine tests, I didn't know that it had a greater effect on FAS-DPD due to its partial oxidation of the FAS titrating drops in addition to its effects on the DPD color dye.

Hi Richard,

Actually, this doesn't surprise me at all - it's what I suspected all along, though it doesn't account for the production of red color when the DPD is added. In fact, the Dupont Oxone technical published by DuPont, which you can find on the internet, states that you can quantify Oxone in aqueous solutions by neutralizing it with a known quantity of ferrous ammonium sulfate (in excess), then back-titrating with potassium permanganate. This method is described as suitable for concentrations of Oxone higher than about 20 ppm, and indicates that Oxone reacts stoichiometrically with FAS at those concentrations.

As far as the red color developed with DPD, I suspect that MPS either reacts directly to form the red dye (not very likely according to the info I can find on DPD) or reacts with other substances in the water to produce compounds that form the red dye with DPD. MPS does in fact have the ability to oxidize chloride to free chlorine, though it's a fairly weak reaction. We know of course that MPS readily oxidizes iodide and bromide, so if any of those have crept into your spa that would account for the red color.

Usually, using chlorine to shock works well if one is using the Dichlor-then-bleach method since shocking with bleach won't increase the CYA. On the other hand, using sufficient chlorine daily can prevent one needing to shock at all, similar to pools that properly maintain an FC/CYA ratio.

Just keep in mind that if you are using Dichlor-only and shock with Dichlor that your CYA will rise much faster. You mention you have low bather load and change the tub when the cumulative FC would be around 100 ppm. That might be OK, but some rough tracking of hot tub itch/rash/lung incidents I made (see this post) seemed to indicate a greater risk after 1-2 months of regular Dichlor use, but that was in tubs used daily so with around 100 ppm CYA accumulating per month (at roughly 4 ppm FC in 350 gallons daily chlorine usage). So I've been recommending around 20-30 ppm CYA if one uses the Dichlor-then-bleach method just to be safe (and based on some rough CT calculations for the hot tub itch bacteria from some conservative sources).

Richard

Thanks for the info. I just found this forum a few days ago and all I can say is "WOW!". There's a large amount of very valuable information here, keep up the great work!

Thanks,

Don

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Don,

I hadn't seen the FAS/MPS reaction info, but then again I didn't bother to search for it either (here, for example). Thank you for doing so.

Since you have a chemistry background, you might be interested in other info on pool/spa water chemistry that I mostly put into The Deep End at Trouble Free Pool. Just look for threads started by "chem geek". The core info for the chlorine/CYA relationship starts with the definitive 1974 paper in this link with most discussion about this in this thread. I have a spreadsheet that does a lot of the chemistry calculations here. A simplified version of these formulas was used for The Pool Calculator created by Jason Linhart.

I sometimes make mistakes and though I often see them later and correct them, it's always better to have a fresh pair of eyes reviewing one's work. If you see anything that doesn't make sense or is incorrect or needs further clarification, please let me know (that is, if you have the time -- I know there's a lot of info and it's an accumulation over the years). I am not a chemist, I do not have a PhD, do not work in the pool/spa industry nor in anything having to do with chemistry, though I did major in physics/chemistry years ago at Berkeley.

Thanks,

Richard

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Hi Richard,

Ah, it looks like The Deep End is another good resource, thanks for sharing it.

I don't have a PhD either, just a BS, but I did work as an analytical chemist for several years. I have sort of stayed in the field, going to work for an analytical instrument manufacturer many years ago, and I still work there but now I develop software.

Wow, that spreadsheet has a lot of info! I'll be glad to have a look at it.

Thanks,

Don

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Don,

Welcome to the forum. Thanks for the info about MPS. That's good to know. The only time I use MPS is when I have high bather loads. It really helps to maintain FC levels. However, with normal loads I just use Chlorine (Dichlor/Bleach). Also, I don't like to use too much MPS because it's acidic and lowers the TA/pH. But if I know I'm going to be using it, I'll just keep an eye on TA/pH, and raise if needed.

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Don,

Welcome to the forum. Thanks for the info about MPS. That's good to know. The only time I use MPS is when I have high bather loads. It really helps to maintain FC levels. However, with normal loads I just use Chlorine (Dichlor/Bleach). Also, I don't like to use too much MPS because it's acidic and lowers the TA/pH. But if I know I'm going to be using it, I'll just keep an eye on TA/pH, and raise if needed.

Hi Nitro,

Yes, that's what I do with the TA and pH. I find that I need to add some bicarbonate (maybe 2 oz or so) every month as the TA and pH creep down. I raise the alkalinity, and the pH follows. I practically never need to use a stronger base to get the pH up.

I'm about to switch to the dichlor/bleach method describe on this forum for maintaining sanitizer, and to stop using MPS except perhaps rarely, so I will probably need to add some acid occasionally to bring the pH down. I have a bottle of concentrated potassium bisulfate solution, which should work to lower the pH/alkanity, and some sodium carbonate if I need to go the other way.

Regards,

Don

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Don,

I hadn't seen the FAS/MPS reaction info, but then again I didn't bother to search for it either (here, for example). Thank you for doing so.

Since you have a chemistry background, you might be interested in other info on pool/spa water chemistry that I mostly put into The Deep End at Trouble Free Pool. Just look for threads started by "chem geek". The core info for the chlorine/CYA relationship starts with the definitive 1974 paper in this link with most discussion about this in this thread. I have a spreadsheet that does a lot of the chemistry calculations here. A simplified version of these formulas was used for The Pool Calculator created by Jason Linhart.

I sometimes make mistakes and though I often see them later and correct them, it's always better to have a fresh pair of eyes reviewing one's work. If you see anything that doesn't make sense or is incorrect or needs further clarification, please let me know (that is, if you have the time -- I know there's a lot of info and it's an accumulation over the years). I am not a chemist, I do not have a PhD, do not work in the pool/spa industry nor in anything having to do with chemistry, though I did major in physics/chemistry years ago at Berkeley.

Thanks,

Richard

Hi Richard,

That's a very interesting article by O'Brien on the chlorine/cyanurate chemistry. I didn't realize that cyanurates could have such a large impact on available chlorine. After I drain and refill (this weekend if the weather allows) I am going to switch to the bleach regime you have described, after getting the CyA up to around 30 ppm with dichlor.

Your spreadsheet is amazing. It will take quite some time to go over everything in it. I did find a little glitch though. I entered 80 for alkalinity and 100 for the target alkalinity, and the spreadsheet calculated 1.86 oz of bicarbonate needed, which is about right. Then I switched the units to metric, and the bicarbonate needed changed to 13.2 grams. Now 1.86 oz is about 52.7 grams, or about 4 times what the spreadsheet calculates. I didn't try to trace down the error but it looks like a simple error in a conversion factor.

Regards,

Don

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Don,

Thanks for the compliment. The spreadsheet was done over an extended period of time and enhanced and expanded a little bit at a time.

What you describe is not an error. When you change the Units of Measure from U.S. Imperial to Metric, then you also need to change your Gallons entry from 400 to 1514 Liters and you need to change the Temperature from Fahrenheit (say, 104F) to Celsius (say, 40C).

Richard

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Hi Nitro,

Yes, that's what I do with the TA and pH. I find that I need to add some bicarbonate (maybe 2 oz or so) every month as the TA and pH creep down. I raise the alkalinity, and the pH follows. I practically never need to use a stronger base to get the pH up.

I'm about to switch to the dichlor/bleach method describe on this forum for maintaining sanitizer, and to stop using MPS except perhaps rarely, so I will probably need to add some acid occasionally to bring the pH down. I have a bottle of concentrated potassium bisulfate solution, which should work to lower the pH/alkanity, and some sodium carbonate if I need to go the other way.

Regards,

Don

I think you'll find the Dichlor/Bleach method much better than using Dichlor alone. Your water will last twice as long, and stay almost as fresh as when you first fill it. I just changed my water after 6 months, and I can barely tell the difference.

Regarding TA/pH: Once you switch to Bleach after raising CYA to ~30 ppm using Dichlor, lower your TA to 50-60 ppm. That will help keep pH rise down. Also, I HIGHLY recommend adding borates (at least 50 ppm) to your tub. That will also keep pH rise down, not to mention make the water feel nice and silky. You can add it using 20 Mule Team (Borax) and acid, or by using a product called Gentle Spa.

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Richard, I think Jason's Pool Calculator has you beat on this one... if I switch it from U.S. to Metric, it automatically converts the gallons and temp for me.

--paulr

That's a nice feature. My spreadsheet isn't intended for novice users and people aren't usually switching back and forth between U.S. and Metric -- they usually pick one and stick with it. Though I know how to attach a macro to a button, I'm not even sure how to attach one to a pull-down menu selection (in Excel) and it would take a macro to "update" a type-in field (as opposed to a calculated field that could simply look at the pull-down menu setting). I probably won't be changing this.

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Don,

Thanks for the compliment. The spreadsheet was done over an extended period of time and enhanced and expanded a little bit at a time.

What you describe is not an error. When you change the Units of Measure from U.S. Imperial to Metric, then you also need to change your Gallons entry from 400 to 1514 Liters and you need to change the Temperature from Fahrenheit (say, 104F) to Celsius (say, 40C).

Richard

Ah, OK, thanks - it works as advertised. I fill my gas tank with gallons, bake bread with cups, teaspoons and tablespoons and complain about the weather in degrees Fahrenheit. But due to my background I like to work metrically when dealing with chemistry. Now I know how to do it with your spreadsheet. I continue to be impressed as I use more features of it, and haven't found any errors yet in the formulas.

Don

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Richard, I think Jason's Pool Calculator has you beat on this one... if I switch it from U.S. to Metric, it automatically converts the gallons and temp for me.

--paulr

That's a nice feature. My spreadsheet isn't intended for novice users and people aren't usually switching back and forth between U.S. and Metric -- they usually pick one and stick with it. Though I know how to attach a macro to a button, I'm not even sure how to attach one to a pull-down menu selection (in Excel) and it would take a macro to "update" a type-in field (as opposed to a calculated field that could simply look at the pull-down menu setting). I probably won't be changing this.

Hi Richard,

FYI Excel does have the ability to run macros when cell values change. Here's a web page describing the ability:

http://www.ozgrid.com/VBA/run-macros-change.htm

Don

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Don,

Thanks for the tip. Unfortunately, I cannot use this method because I have a Mac and am forced to use Excel 2004 for Mac for my spreadsheet because Microsoft removed macros completely from Excel 2008 for Mac and I want the spreadsheet to be usable by both Windows and Mac users so I keep it in Excel 2004 for Mac (2003 for Windows) format. Supposedly, macros are coming back in a future version of Excel for the Mac, but who knows when that will be.

Richard

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