I am using the BBB method and my last fill was about 3 months ago. I used dichlor initially to achieve ~30ppm CYA. I tested recently and found the CYA is less than 10ppm now. I remember seeing that CYA needs to be periodically added to maintain its level but forgot about it.

My wife has been experiencing skin irritation when using our hot tub in the past few weeks. I understand that high CYA levels require higher FC levels to maintain sanitation. My question is the reverse, that is do low CYA levels result in effectively higher FC levels. For example, if I add bleach to 4.5ppm at <10ppm CYA, is that effectively a much higher FC level that could be causing skin irritation?

Thank you

Yes, it is a higher effective chlorine level at a lower CYA. Whether that alone would cause the irritation, I don't know. pH issues or low chlorine would be things I'd think more likely to cause an irritation.

I watch PH pretty closely. It stays at 7.5 or 7.6. I believe TA is around 50, but I don't usually test it as long as the PH stays stable. The CC was 1.0 so I shocked with MPS about a week ago, but my wife had the same irritation reaction the next time she went in a few days later.

She used hot tubs when we went on vacation recently without any problem, but our tub before and after is causing her trouble. She is the only one in the family having the reaction.

What is MPS???

Potassium monopersulfate, non-chlorine shock. http://en.wikipedia.org/wiki/Potassium_monopersulfate

MPS can be irritating to some, though it sounds like she was experiencing irritation before you added the MPS. Also, as you point out, the MPS usually gets used up within a couple of days, depending on bather load.

The CYA level usually drops by around 5 ppm per month in a hot spa so you probably need to raise it. You could also target 40 ppm instead of 30 ppm to give you a little more leeway. You can also start your soaks with 1-2 ppm FC and add more chlorine afterwards. That should minimize chlorine exposure during the soak itself.

Hey chemgeek...why is this?

Would a OCLT (http://www.troublefreepool.com/pool-school/overnight_fc_test)be any help here...in case the CC measurement is indicative of persistent organics?

See the "CYA Degradation by Oxidation from Chlorine" section of this post.

For a spa, a 24-hour loss test with no bather-load is usually better and should show around 25% FC loss the water is relatively new while it could be closer to 50% FC loss if it's close to needing a water change.

If the CYA level is low, then the higher FC/CYA ratio will produce more nitrogen trichloride and that can be very irritating, though it is volatile and generally smelly and irritating to breathe (not so much for the skin, as far as I know).

Thanks for the link...I will get started on it!

I am glad you mentioned this. I have been experimenting with the 1 ppm loss overnight and that has not been working well for me. Is the main reason for this the high water temp, or something else? Is the 50% FC loss in 24 hours a good general criteria to get new water? How do you differentiate from say poorly maintained water getting organisms verses needing to replace due to build up?

Does that dissipate if one leaves their cover off for awhile before soaking (at least vapor phase above the water), or does it keep being produced in high quantities while soaking (i.e. is it a build up due to having the cover on)?

The main reason for the higher initial 25% per day loss is the high water temperature. It makes all chemical reactions faster including chlorine oxidation of spa components (including the cover), faster outgassing, etc. If you think that's high, try adding chlorine without any CYA in the water and you'll see much faster loss rates -- probably where the myth that bleach "evaporates" and doesn't last comes from. The higher 50% per day that develops over time is from slow-to-oxidize organics that build up. The spa is still sanitary so long as there is measurable FC, but one usually changes the water around that 50% daily loss timeframe so as to not be too inefficient in chlorine usage. Also, around that time the salt level will have built up. Roughly speaking, using the Dichlor-then-bleach method, one gets around double the normal water replacement interval time, so in days this is around (2/9) x (Spa Volume in Gallons) / (Person-Hours of Soaking Per Day).

Nitrogen trichloride will dissipate faster if you leave the cover off, but normally the chloramines get oxidized properly when you add chlorine after a soak. Ideally one would leave the cover off for an hour or so after they are done soaking, but most people don't do that so taking the cover off 5-10 minutes before getting in for the next soak helps minimize smell (not only of any chloramines, but of chlorine built up under the cover. Normally one soaks with lower chlorine levels, around 1-2 ppm FC, so though monochloramine may be produced, the other chloramines usually occur after the soak with the much higher added amount of chlorine to handle the bather waste. If the FC/CYA ratio is too high, especially if FC is higher than 2 ppm and the CYA is lower than 20 ppm, then this oxidation can occur during the soak which can be more smelly and possibly irritating.