With Dichlor-then-bleach Method, Check Your Cya After 2-3 Months

[chem geek]

Now that there are quite a few of you using the Dichlor-then-bleach method, I would like those of you who have been using this method for at least 2-3 months since the last water change to test the Cyanuric Acid (CYA) level and compare this reading with what it was (or should have been) after initially using Dichlor. The reason is that chlorine can slowly oxidize CYA and this should occur more quickly in a hot spa than in a pool. So it may be necessary to use some Dichlor after some period of time, perhaps after 3 months, if one plans to go longer without changing the water.

So I'd like to get some real-world feedback in order to make a recommendation (if needed).

Thanks,

Richard

[oldparr]

Just did that yesterday, after 3 months. I was suspecting that my CYA was low. Tested it and the reading was below 30ppm (the lowest that my test kit can go. The reading after 10 days of the fill was 45 PPM. I think it is good to put some dichlor after 2 months.

OldParr

[oldparr]

I forgot to tell the temp: most of the time 90ºF

[mcw53]

I started out using the Nature2 low chlorine recipe, but shifted away from MPS towards chlorine. It took 1 month for me to add 26 teaspoons dichlor to my 450 gallon spa before I tested my CYA and switched to dichlor-then-bleach. By my calculations, that is 40 ppm FC and 36 ppm CYA. The Taylor K-2006 CYA test measured 30 ppm. Retested today after 2 additional months. The dot was still visible at the 30 ppm mark (the lowest measure with the Taylor kit). I filled the tube to the brim and the dot was still faintly visible. If I had to guess, I would say CYA is now somewhere between 20 and 25 ppm.

Over the last 3 months, we have averaged about 50 min/day soaking in 101° water. The last 2 months we have been using a mix of bleach and MPS at a rate of ~5.4 ppm FC equivalent per hour soak time. I also have an ozonator (Hot Spring FreshWater® III Corona Discharge).

[chem geek]

OldParr and Mike,

Though both of your readings were low, the sample mixed with reagent wasn't clear, was it? That is, it was lightly cloudy, but not enough to obscure the black dot at the lowest reading available on your test kit, correct? If that's the case, then I'd figure it's somewhere between 10-20 ppm depending on your kit so would mean that after 2 months about 4 days of Dichlor use would be useful to add around 14-15 ppm CYA if your usage was 4 ppm FC per day, perhaps a little more (a week of Dichlor would probably be fine in most cases).

Thanks for the info. If anyone else has data, please post.

Richard

[mcw53]

Though both of your readings were low, the sample mixed with reagent wasn't clear, was it? That is, it was lightly cloudy, but not enough to obscure the black dot at the lowest reading available on your test kit, correct?

That is correct. The dot was obscured, but not totally. I estimate my CYA is about 20 ppm. In my case, that would be a 5 ppm / month degradation.

We need more data points.

[Maxemily]

Refilled the 325 gallon tub at the end of August and had 30ppm CYA in short order as we use the tub everyday. Water temp in September averaged 95F, in October 97F. I add 3oz Clorox per day.

The CYA test is now clear.

Glad you brought this up.

[bart6453]

Refilled the 325 gallon tub at the end of August and had 30ppm CYA in short order as we use the tub everyday. Water temp in September averaged 95F, in October 97F. I add 3oz Clorox per day.

The CYA test is now clear.

Glad you brought this up.

Funny...I noticed my CYA dropping too....but I chalked it up to splashing water out. This last fill we have not lost much water and after 2 months we lost about 10ppm CYA.

I switched to di-chlor for 12ppm FC to get my cya up to 30ppm.....hmm....

[quantumchromodynamics]

Richard, I'm guessing that the chlorine is oxidizing only the nitrogen to N₂ gas, and the rest will be converted into carbon dioxide, chloride and water?

What effect do you think is caused by ozone, MPS, hydrogen peroxide or other oxidizers?

Also, what effect do you think the pH will have on the degradation rate?

C₃H₃N₃O₃

C₃N₃(OH)₃

[chem geek]

I wrote about the degradation of CYA by various methods in this thread. You are correct about the end products. I don't know how rapidly CYA would break down with the other oxidizers. Dupont told me that MPS had little effect on CYA, but that doesn't mean it's just slow; my hunch is that without chlorine attached to the ring, MPS isn't fast enough by itself to oxidize CYA in the timeframe a spa would be used (perhaps 6 months with the Dichlor-then-bleach method). Ozone is a stronger oxidizer, but thermodynamics tells you if something is possible, not how quickly it will occur, and many of these oxidizers are selective and specific in terms of what they oxidize quickly.

As for pH, there are multiple sources that indicate that the reaction is faster at high pH. Intentional breakdown, as with the purification of aqueous waste streams, has a patent here where the breakdown is most rapid at a pH of 9-10 and with optimal molar ratios of sodium hypochlorite to CYA of 6:1 to 8:1 where the reaction time for fairly complete breakdown is in hours (these molar ratios correspond to a ppm ratio of chlorine to CYA of 3.3 to 4.4; dominant species are HClCY^- and OCl^-). According to another source (Wojtowicz), the temperature dependence of the reaction at much lower ratios and more normal pool pH may be stronger than average with a rough doubling in rate for every 10F (as opposed to every 10C).

So far, it looks like perhaps adding Dichlor about two or maybe three days per month for a typical spa (assuming 4 ppm FC per day) might be all that is needed to maintain reasonably consistent CYA levels after the initial one week or so Dichlor dosing. After we get as many data points as possible, then we can make a decent recommendation.

[oldparr]

OldParr and Mike,

Though both of your readings were low, the sample mixed with reagent wasn't clear, was it? That is, it was lightly cloudy, but not enough to obscure the black dot at the lowest reading available on your test kit, correct?

Richard

That's correct. I edited my post. I said below 20ppm, but that is incorrect. My test kit lowest reading is 30ppm and the dot was barely visible at this point. So I presume my CYA was between 20 and 25ppm. Now I will use dichlor for 3 days and test again.

[aschwartz]

OldParr and Mike,

Though both of your readings were low, the sample mixed with reagent wasn't clear, was it? That is, it was lightly cloudy, but not enough to obscure the black dot at the lowest reading available on your test kit, correct?

Richard

That's correct. I edited my post. I said below 20ppm, but that is incorrect. My test kit lowest reading is 30ppm and the dot was barely visible at this point. So I presume my CYA was between 20 and 25ppm. Now I will use dichlor for 3 days and test again.

I am new to this forum but I am at total loss. There is no reason to have CYA in a spa. CYA was introduced because it supposedly shielded chlorine from UV degradation. This claim has now been challenged and no solid research, publications, or patents can be found supporting this UV schielding effect. Multiple state health departments have banned or severly restricted CYA usage in outdoor commercial pools at this point.

One fact about CYA that is not in debate is it act as an oxidation retardent. Why would we want to retard the oxidation/sanitization process of chlorine. Every instruction manual tells the user to add more sanitizer when adding CYA!!! I has checked spas where the user was have clarity and chlorine reading problems and most of them had high levels CYA. Even at 20 to 30 ppm of CYA, chlorine retardation is taking place.

[Nitro]

One fact about CYA that is not in debate is it act as an oxidation retardent. Why would we want to retard the oxidation/sanitization process of chlorine. Every instruction manual tells the user to add more sanitizer when adding CYA!!! I has checked spas where the user was have clarity and chlorine reading problems and most of them had high levels CYA. Even at 20 to 30 ppm of CYA, chlorine retardation is taking place.

Because without CYA, the Chlorine would be too strong at normal levels (3-6 ppm FC). You would need to keep FC <1 ppm, which would be to difficult. Read this post for more info.

Chem geek has already given you detailed explination, HERE.

[mcw53]

I started out using the Nature2 low chlorine recipe, but shifted away from MPS towards chlorine. It took 1 month for me to add 26 teaspoons dichlor to my 450 gallon spa before I tested my CYA and switched to dichlor-then-bleach. By my calculations, that is 40 ppm FC and 36 ppm CYA. The Taylor K-2006 CYA test measured 30 ppm. Retested today after 2 additional months. The dot was still visible at the 30 ppm mark (the lowest measure with the Taylor kit). I filled the tube to the brim and the dot was still faintly visible. If I had to guess, I would say CYA is now somewhere between 20 and 25 ppm.

Over the last 3 months, we have averaged about 50 min/day soaking in 101° water. The last 2 months we have been using a mix of bleach and MPS at a rate of ~5.4 ppm FC equivalent per hour soak time. I also have an ozonator (Hot Spring FreshWater® III Corona Discharge).

Though both of your readings were low, the sample mixed with reagent wasn't clear, was it? That is, it was lightly cloudy, but not enough to obscure the black dot at the lowest reading available on your test kit, correct?

That is correct. The dot was obscured, but not totally. I estimate my CYA is about 20 ppm. In my case, that would be a 5 ppm / month degradation.

We need more data points.

I have added 15.4 ppm CYA over the last 3 days. Measured CYA this morning at 37 ppm +/-. So, my CYA had dropped from 30 to 22 ppm over the last two months. Bottom line, I have observed ~4 ppm / month CYA degradation.

[bart6453]

I started out using the Nature2 low chlorine recipe, but shifted away from MPS towards chlorine. It took 1 month for me to add 26 teaspoons dichlor to my 450 gallon spa before I tested my CYA and switched to dichlor-then-bleach. By my calculations, that is 40 ppm FC and 36 ppm CYA. The Taylor K-2006 CYA test measured 30 ppm. Retested today after 2 additional months. The dot was still visible at the 30 ppm mark (the lowest measure with the Taylor kit). I filled the tube to the brim and the dot was still faintly visible. If I had to guess, I would say CYA is now somewhere between 20 and 25 ppm.

Over the last 3 months, we have averaged about 50 min/day soaking in 101° water. The last 2 months we have been using a mix of bleach and MPS at a rate of ~5.4 ppm FC equivalent per hour soak time. I also have an ozonator (Hot Spring FreshWater® III Corona Discharge).

Though both of your readings were low, the sample mixed with reagent wasn't clear, was it? That is, it was lightly cloudy, but not enough to obscure the black dot at the lowest reading available on your test kit, correct?

That is correct. The dot was obscured, but not totally. I estimate my CYA is about 20 ppm. In my case, that would be a 5 ppm / month degradation.

We need more data points.

I have added 15.4 ppm CYA over the last 3 days. Measured CYA this morning at 37 ppm +/-. So, my CYA had dropped from 30 to 22 ppm over the last two months. Bottom line, I have observed ~4 ppm / month CYA degradation.

Hmmm....seems to be averaging pretty close to 5ppm CYA loss per month...interesting.

[oldparr]

After 6 days, I put 15ppm of CYA and retested. Now re reading is 40ppm. So it was 25ppm. As I started with 45ppm, the CYA degradation was around 6ppm/month.

[chem geek]

Thank you everyone for your responses. I'm going to say that roughly speaking it looks like the drop in CYA levels at what is normally recommended for FC and CYA at hot spa temperatures is around 5 ppm per month. So that's 1-2 days of Dichlor addition per month (depending on spa size and daily chlorine usage). Though there was only one data point, it appears that use of an ozonator does not significantly change this recommendation.

Nitro, can you add something in your Dichlor-then-bleach writeup that says to use Dichlor for one day per month after the initial month? That should be about right for most people. You could say to add 5-6 ppm FC cumulatively of Dichlor once each month.

By the way, the oxidation of 5 ppm CYA requires roughly 12-1/2 ppm FC so over one month that's a chlorine demand of around 0.4 ppm per day coming from chlorine oxidation of CYA. This is part of the daily chlorine demand that occurs even when a tub is not in use, but is kept at high temperature. The rest of the daily demand (for unused spas) probably comes from outgassing of chlorine or the oxidation of other spa materials.

[bart6453]

Thank you everyone for your responses. I'm going to say that roughly speaking it looks like the drop in CYA levels at what is normally recommended for FC and CYA at hot spa temperatures is around 5 ppm per month. So that's 1-2 days of Dichlor addition per month (depending on spa size and daily chlorine usage). Though there was only one data point, it appears that use of an ozonator does not significantly change this recommendation.

Nitro, can you add something in your Dichlor-then-bleach writeup that says to use Dichlor for one day per month after the initial month? That should be about right for most people. You could say to add 5-6 ppm FC cumulatively of Dichlor once each month.

By the way, the oxidation of 5 ppm CYA requires roughly 12-1/2 ppm FC so over one month that's a chlorine demand of around 0.4 ppm per day coming from chlorine oxidation of CYA. This is part of the daily chlorine demand that occurs even when a tub is not in use, but is kept at high temperature. The rest of the daily demand (for unused spas) probably comes from outgassing of chlorine or the oxidation of other spa materials.

Funny, I already added this to my monthly routine...6ppm FC added via dichlor....yielding about 5ppm CYA....I put the measured dichlor in a 7 day pill dispenser. I forget everything and if I didnt have it sitting there I would forget to add it or forget if I did. So I marked the tops with tape and the months name. I will add the metered amount on the 1st of every month.

Sounds stupid...but when it comes to tasks I repeat....I can never remember if I did them or not!

[PaulR]

Not to be left out of the fun: I overshot to CYA 40 on 9/16, and today (7 weeks later) I read 30. Tub temp a bit over 100 the whole time. In line with the estimated 5ppm/month loss.

Richard, presumably this rate is influenced by temperature, so pools in southern climes with temps routinely at 90+F would also see CYA loss through this same mechanism?

--paulr

[chem geek]

Yes, pools should see some loss as well, but at a slower rate. As for how much slower, it's somewhat of a guess since the exact mechanism is not known for certain. If I assume an average of 3 ppm FC with 30 ppm CYA in a 100F spa vs. a 90F pool, the concentration of HClCY^- is 1.16 times higher in the pool, the concentration of ClCY^2- is 0.65 times lower in the pool, and the concentration of HOCl (and roughly OCl^-) is 0.59 times loser in the pool. The reaction rates at 10F lower temperature may be 0.7 to 0.5 slower though this is a guess. So the net result could be anywhere from about 0.5 times slower (that is, a factor of 2) to 0.2 times slower (that is, a factor of 5). That translates to anywhere from 2.5 ppm to 1 ppm per month CYA loss.

In my own 88F pool, it seemed that I lost around 5 ppm CYA in 3 months or 1.7 ppm per month, but at such low loss rates it is very hard to distinguish from splash-out (I have a cartridge filter, so no backwashing). Pools that operate at higher CYA levels would be expected to have proportionately higher CYA loss rates assuming the same FC/CYA ratio and temperature. So pools with 70-80 ppm, such as pools in very hot sunny climates, may lose CYA at rates closer to that of a spa. SWG pools typically have a high CYA level, but a lower FC/CYA ratio so would lose CYA perhaps at half the rate of a spa if the pool were 90F in temperature. The actual CYA losses in pools seems to vary quite a lot so there is probably something else going on that can change the reaction rates or there is more or less water dilution than people realize. Perhaps the presence of some metal ions catalyzes the reaction (see this thread).