Shocking doesn't eliminate CC

[twilson]

I have a 355 gal hot tub, and as a result of CC creeping up to 0.5 - 0.6, I've decided to shock the spa. My CYA is 35, and consulting the Cl/CYA chart, I see I need an FC level of 14 ppm. To be safe, I raise FC to 16, and test again later in the day. FC has dropped to 12 or so. I raise it back up to 16. This continues several times, over three days, but my CC is always exactly where it was when I started. I double-check the CYA level to be sure I have it right. The same thing happened to me last draining cycle when I tried to shock the spa mid-cycle. What should I be doing differently?

BTW, I'm testing FC/CC using Lelie's FAS-DPD kit (#81329). I normally use the 25ml sample (with 1 drop = 0.2 ppm), but occasionally I use the 10ml sample (with 1 drop = 0.5 ppm), especially when I know FC will be high. In connection with this kit, I do have a few questions:

1. The test calls for 2 little spoons of DPD powder (R-0870), regardless of whether I'm using the 25ml or 10ml sample. Does that mean that the test is insensitive to the amount of powder I add, as long as it's above some minimum threshold? The reading won't be off if my spoons of powder are a little bigger or smaller than usual?

2. The CC test requires me to add 5 drops of DPD Reagent #3 (R-0003) after finishing the FC test, again regardless of the sample size (and which again seems odd). What if I mistakenly add 6 drops instead of 5? Also, what if I accidentally add an extra drop of the titrating reagent (R-0871) when testing FC? Will that invalidate the subsequent CC test? I guess it's not possible to test CC without first testing FC.

3. Finally, I've noticed, when testing CC, that after adding enough drops of the titrating reagent to make the sample clear again and recording the CC level, the sample will become pink again after sitting a minute or two. Is that normal?

Thanks!

 

[JasonLion]

Unless you leave the spa exposed to sunlight most of the time, it isn't always possible to get rid of all of the CC with chlorine alone. Often using MPS will help get rid of the CC, but that can get confusing because MPS will read as CC on the test (so you don't know if you really got rid of the CC until the MPS dissipates).

1) Correct. The test can be wrong if you use too little powder, but extra powder within reason is fine and has no effect on the results.

2) Extra R-0003 has no effect. Again, using too little could be a problem, but a little extra is fine. Yes, you must do the FC test correctly for the CC number to be correct. Extra drops during the FC test after it turns clear invalidate the CC result.

3) Yes the sample will turn pink again if you leave it sitting for a while. That is normal.

 

[Isaac-1]

Taylor makes a special test K-1518 if your using MPS based non chlorine bleach, they also offer it as an upgrade addition to the standard FAS-DPD kit, it comes in two sizes the larger is 2 oz K-2042, the upgrade for people with existing kits it include the reagent, eyedropper for it, instructions, etc. (total with shipping direct from Taylor is $31.35 as of last week ($23.85 + $7.50 shipping) The smaller size is .75 oz and is only about $4 less ( I don't remember the number).

Ike

 

[PaulR]

My pool habitually has 0 CC but the spa typically has some (<= 0.5) CC, using a 10ml sample. If I used the 25ml sample I might be inclined to let 0.6 CC ride, not worry until it was up toward 0.8. But if your tub normally runs 0 to 0.2 CC, then I might feel differently.
--paulr

 

[chem geek]

I wouldn't worry about the CC, especially if you don't notice any bad chlorine-like smell. It's most likely some sort of organic chloramine, possibly some combination of some chemical that might have been on your skin (do you use any lotions?) with chlorine that is slow to oxidize. Though in a pool the most commonly measured CC is some form of chlorourea since urea is the largest component of sweat and urine, that is less likely in a spa since the higher temperatures tend to break down urea much more quickly (in a cooler pool, it can days to get oxidized by chlorine) and most find that any CC is nearly gone by the next time they go in for a soak.

How often are you using the spa? Is it hot at 104ºF? If you are using it more frequently, say twice a day, or if many people are using it for higher bather load, or if it's kept cooler, then that can result in a higher sustained CC. Again, not a problem if it's not irritating. By your keeping the active chlorine level low the most offensive CC which is nitrogen trichloride is kept to a minimum (and is detected by your nose long before it would be picked up by a test kit since 0.02 ppm of nitrogen trichloride is irritating).

As for the pink returning in the CC test, that happens with certain CC where the chlorine is fairly tightly bound to the organic so that it takes longer for the potassium iodide to react with the chlorine to form iodine that then results in the pink color in the test. This is further indication that the CC you have is harder to oxidize by chlorine.

Richard

 

[savdoc2]

I have been having a similar experience.

I have approximately a CYA of 30 - haven't tested since it is a couple days shy of a full week since adding the last bit. I have been keeping my FC above shock level 18-22 range. Whenever we take the cover off there is a strong chlorine smell. The hot tub gets no direct sunlight and very little indirect - only when I am testing. I am currently testing 3-4 times a day. CC is always in the .5-1 range. I am losing a small amount of FC 1-2 per day when the tub has not been used. This leads me to believe that I am working on killing something. The pump cycles off and on and the temps range 97-101 degrees. I did try adding some MPS about 4oz the other day - though I can't find a good recommendation on how much to add. Not sure what to do next.

 

[Isaac-1]

I know nothing about hot tubs, but I suspect you have a war going on between the chlorine and something else growing in the hot tub, without UV light from the sun the chlorine is having to do all the fighting on its own. Since you already have it I would try adding a good bit more MPS ( I don't know the hot tub dosing), maybe in 2 or 3 doses each a half hour apart, MPS burns off fast. This may give the chlorine the exta help it needs to win the battle.

Ike

 

[chem geek]

A 1-2 ppm FC drop from shock level when the water is at hot spa temperatures is not indicative of fighting anything in the tub. Hot temps outgas chlorine faster (which is why you smell it more from a hot tub, especially at shock levels) and it also has chlorine react with tub surfaces and covers [EDIT] and slowly oxidizes CYA itself [END-EDIT]. So just because you were reading a 0.5 to 1.0 ppm CC level in your spa, I wouldn't be concerned about it unless you detect an odor when the FC level is lower (< 4 ppm) and you aren't using the spa for at least a day. You can try keeping the cover off for at least an hour when you are shocking to see if that helps by outgassing some byproducts of the shocking, but again a drop of 1-2 ppm FC at hot spa temps is not at all unusual and at shock level that is actually low.

[EDIT] In fact, at hot spa temps in a fresh tub, one usually has a 25% drop in FC over 24 hours, so from 4 ppm to 3 ppm, for example. At 20 ppm, that would be a drop of 5 ppm. We know that hot tubs with around 4 ppm FC and 30 ppm CYA seem to lose around 5 ppm CYA per month, probably from oxidation by chlorine, and that implies a daily chlorine demand of around 0.4 ppm FC so accounts for nearly half of the chlorine demand normally seen. At 20 ppm FC, this may translate to a daily chlorine loss of around 2 ppm FC per day just from chlorine oxidation of CYA. So your 24-hour drop at shock levels is explained, assuming your spa is kept at hot temperatures. In pools, the loss is lower due to the lower temperatures -- at 88ºF the monthly CYA loss is usually lower at perhaps 1.7 ppm and the daily FC loss due to oxidation of CYA is perhaps around 0.14 ppm FC per day

I would look to see if there is anything foreign getting into the spa such as lotions or other chemicals. Normal sweat and urine don't usually result in CC at spa temps unless the bather load is exceptionally high (more than 2 person-hours of soaking per day in 350 gallons).

 

[savdoc2]

Thank you! I understand better now. I keep trying to apply "pool" principles to the hot tub. It is entirely possible that there are some lotions or other things getting into the tub. Thanks for the explanation, I will quit worrying so much.

Is there a good recommendation for how much MPS to use?

 

[chem geek]

The rough rule-of-thumb for the amount of oxidizer needed to get rid of bather waste, assuming that there is no ozonator being used, is that every person-hour of soaking in a hot (104ºF) tub requires around 5 fluid ounces of 6% bleach or 3-1/2 teaspoons of Dichlor or 7 teaspoons of non-chlorine shock (43% MPS). Just remember that MPS will show up as CC in the chlorine test unless you use the Taylor K-2042 MPS interference remover.

 

[savdoc2]

How long will the MPS show up as CC?

 

[chem geek]

Until the MPS gets used up oxidizing something. Usually in a hot spa it won't last more than a couple of days and if you add it after a soak it might get used up in an hour -- it all depends on what there is to oxidize in the spa water.

 

[twilson]

Thanks for the information in this thread. I'm motivated to try MPS, and have looked into the Taylor K-2042 MPS kit, as mentioned in this thread. Taylor also sells the reagent, R-0867, separately, which seems all you would need if you already have a FAS-DPD kit from Taylor (or Leslie's), especially since the instructions are on-line.

One interesting thing about the instructions: in the FC/CC test, besides the extra 1 ml of R-0867 used before the FC test, they also call for 10 drops of R-0003 and a wait time of 1 min before the CC test. My FAS-DPD kit calls for 5 drops of R-0003 and no waiting. JasonLion pointed out that the extra R-0003 shouldn't matter, but other discussion on this thread suggested how and why the CC sample gets pink(er) after waiting, which may indicate a real need to be more specific about how long one waits during the test. Also, the instructions assume a 25 ml sample, and I wonder whether it is also safe to use them for a 10 ml sample (including using maybe less than 1 ml of R-0867). The instructions themselves didn't say so.

Finally, are there any guidelines for how much FC should be added, and on what schedule, to a tub that is not being used? The recommendations that chemgeek listed (5oz 6% bleach, or 3.5 tsp Dichlor, or 7 tsp 43% MPS, per person/hr of soaking) were under bathing conditions, but it would also be nice to have baseline recommendations. Thanks again!

 

[chem geek]

When the tub is not used, the chlorine demand is mostly a function of the water temperature. If it's kept hot, then shortly after a fresh fill the chlorine demand is usually around 25% of the FC level, so if you start with 4 ppm FC you will use up 1 ppm FC over a day. As you use the tub, there is some buildup of slow-to-oxidize organics and the chlorine demand may rise to around 50% of the FC level, perhaps after 3-6 months depending on bather load.

This all assumes that there is no ozonator. An ozonator increases chlorine demand when the tub is not used since the ozone can oxidize the chlorine to chlorate and possibly force more outgassing of it as well. When you use have bather load, then an ozonator helps to reduce chlorine demand since the ozone oxidizes some of the components of sweat and urine so that chlorine doesn't have to.