Is CYA testing relevant to small spas?

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Lee In VT

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Jan 12, 2014
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I'm switching over from Pristine Blue (copper) and after-use shock with Dichlor to the TFP system with BBB for my 250 gal hot tub. The tub is currently quite clear and clean, getting almost daily use.

I've been reviewing 'Pool School' recommendations. Playing with the 'pool calculator' by entering arbitrary numbers, I noticed that the recommended amount of bleach to raise my FC from 2 to 6 (for example) was no different if I entered a CYA of 0 vs a CYA of 60. TFP stresses CYA testing because a higher CYA would require more chlorine for adequate FC level, if I understand correctly. Does the pool calculator adjust its chlorine advice for different CYA's or are they independent variables?

Seems like the CYA level helps you choose an appropriate goal FC range. But the difference in how much bleach to add at different FC goal levels for a 250 gallon volume is minimal, tenths of an ounce. I'm tempted to not bother testing my CYA level (bought as a separate test kit it's about $17) and just pick a reasonable FC range that assumes some CYA present (I've been shocking with Dichlor, which you say adds CYA, the spa gets no direct sunlight).

Any thoughts?
Lee
 
It is relevant so you know how much FC you need to maintain. If you have been using dichlor for awhile, then your CYA may to much too high. The CYA does not affect the amount of bleach required for a given rise in FC.

PoolMath will adjust the suggested FC levels based on the CYA you input, but the dosing is purely based on your Now and Target input FC levels and is independent of the CYA.

If you started with a clean fill and then follow the dichlor then bleach method, then you might be able to get away without testing the CYA because you would have calculated your CYA based on how much dichlor was used.
 
Thanks Jason, makes sense. The goal FC is a 'range' (e.g. 2-4, 3-5) with a fair degree of overlap between the recommended range for high vs low CYA, right? Seems like in a small tub like mine, the effect due to my actual CYA level (a higher or lower goal FC range) would probably translate to a pretty tiny adjustment in the actual amount of bleach to add. The difference could be huge in larger pools of course. So being cheap and lazy (I just ran out of the free chemicals they gave me) I thought perhaps just to pick a midrange FC goal and see how it goes with TFP guidance.

I first filled the tub 2 months ago with our clean well water, pH and TA have been stable. I've used a total of 8 oz of Sodium Dichloro...('Pristine Extra') and 10 oz of Potasium Peroxymonosulfate (Pristine Power) as after use-shocks, and used their copper solution (Pristine Blue) at recommended rates. Is there any way, based on that, to guesstimate my CYA?
 
I would not say there is a lag overlap ... have you see the FC/CYA Chart?

There is not really a "range" as much as there is a minimum above which you need to keep the FC. If your CYA is 20-30ppm, then you need to keep the FC above 2ppm. If your CYA is 100ppm, then you need to keep the FC above 7ppm.

Use the Effects of Adding Chemicals at the bottom of PoolMath. For 250 gallons, 8 oz of dichlor would add 121ppm of CYA ... see, likely WAY too high.

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I would dump it and start over ... both to get the CYA under control and to get all that copper out of there to prevent staining or green hair.
 
Right, I see your point. I hadn't noticed the 'Effects of Adding Chemicals' in PoolMath. pretty nice. Guess I'll dump and do over as you suggest, next time we're above 32 degrees for a day. Not many things in life allow such an easy do over. Of course I still won't know the CYA level in my water, which is where I started. ;-) Maybe the pool store does that test.
 
When you refill, CYA will be zero. Use PoolMath to calculate how much dichlor will raise it to 30ppm. Then only use that much (spread out over time so the FC is not way too high). After that, use liquid chlorine for all your FC applications assuming you CYA is right around 30ppm.

IF things go south for some reason, start over ... you are right it is easy in this case.

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BTW, CYA happens to be the test that the pool stores are notoriously the worst at getting correct ... or even consistent.
 
Spas usually don't get algae because they are tightly covered with no light reaching the water and they usually run much hotter (100+ºF) especially when frequently used. Nevertheless, if one uses Dichlor-only and lets the CYA get high, then the water can turn dull/cloudy and need a water change more quickly than the Dichlor-then-bleach method that attempts to keep the CYA at a more constant level. Also, with a higher CYA level (especially above 100 ppm), the risk for getting hot tub itch/rash/lung increases since bacterial biofilms may form.

Spas are different than residential pools in that the bather load is extremely high so the swing in FC is very high. One usually starts their soak with 1-2 ppm FC to minimize chlorine/chloramine smell, but then one adds sufficient chlorine after their soak to oxidize bather waste and still have 1-2 ppm FC for the start of their next soak. The amount to add is proportional to bather load and depends on whether there is supplemental oxidation such as an ozonator.
 
The pool store clerk claims they have a computer that reads their dip strip, including CYA, which she call CIA. She also strongly warned me to avoid using household bleach, including Clorox, which she claims contain animal products.(!) I'm looking forward to the restart.

Chem geek, I see what you mean about the big swings in bather load, relative to my small water volume. Seems to me what you recommend is sort of the equivalent of 'shock after every use' approach, which may well be the best way to go with such a small tub as ours. If I understand your advice, (and I may not) following each use I should test and add enough bleach (I've got 8.5% regular Clorox) to bring the FC level up to the target FC (based upon my CYA). Perhaps add a little more if heavy bather load. What confuses me is how one controls the prebathing FC level. Do you just not worry about it (if too low) knowing you'll be adding more bleach after bathing?

Hope I'm not overthinking this, it's the kind of person I am.
 
No, you do not test after your soak and bring up the FC to the level recommended for the CYA level. If you start your soak with a very pleasant 1-2 ppm FC, then you will likely have close to 0 ppm FC after your soak and will likely have CC instead. That's OK since your soak isn't very long and the bacteria are still killed before they can reproduce and they don't double faster than every 15 minutes at best anyway. The amount of chlorine you add after your soak is a function of your bather load and has absolutely nothing to do with the FC/CYA chart. It is also independent of the size of the spa since it's not related to the chlorine concentration (i.e. FC) but to the absolute amount of chlorine needed to oxidize bather waste. The elevated FC after your soak also kills off anything that might still be alive. For spas, a CYA level of 30-40 ppm is a decent level since you don't want it too high, but you need some to moderate chlorine's strength.

If you have people over using the spa and you are worried about person-to-person transmission of disease, then you can start with a higher FC level such that it lasts through your soak, but you will likely smell the chlorine/chloramines in that case -- it will be more like a commercial/public spa with higher chloramine smell except that chlorine's strength will be moderated in your spa from the CYA.

If you have no ozonator, then the rough rule-of-thumb is that every person-hour of soaking in a hot (104ºF) tub needs 5 fluid ounces of 6% bleach (3-1/2 fluid ounces of 8.25% bleach) or 3-1/2 teaspoons of Dichlor or 7 teaspoons of non-chlorine shock (43% MPS) to oxidize the bather waste and end up 24 hours later with roughly 1-2 ppm FC for your next soak. With an ozonator, you may need only half as much since the ozone will oxidize some of the bather waste. The real rule, however, is to add whatever amount you need to such that you measure a small residual of chlorine (1-2 ppm FC) for the start of your next soak. If you don't soak the next day, you may need to add chlorine in between soaks. As for when to test, that would be just before your soak, not afterwards. You can also test 24 hours after your soak if you aren't going to be soaking again. You want to make sure there is chlorine in the water at all times and if you want to maintain more than 1-2 ppm FC in between soaks, that's fine, but for the start of your soak you will likely find it more pleasant if you target 1-2 ppm FC for the start of your soak.

Clorox does not contain animal products. The pool store clerk does not know what she is talking about. The main reason that the spa industry doesn't recommend bleach is that if you were to use ONLY bleach with no CYA in the water, then the chlorine would be too strong. It would outgas faster and oxidize the hot tub cover faster and would oxidize your skin, hair and swimsuit faster and corrode equipment faster. It also won't last as long and if you don't make other adjustments such as lowering the TA level and using 50 ppm Borates then the pH can get too high from carbon dioxide outgassing and this can cause calcium carbonate scaling.
 

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With the FC swinging about and averaging around 3-4 since it ends up around 1-2 before the soak, a CYA of 20 ppm isn't enough to moderate chlorine's strength so that it isn't too strong. Also, the hotter water temperature has there be more active chlorine for a given FC/CYA ratio. 30-40 ppm CYA seems to be a decent compromise moderating chlorine's strength but not by too much.
 
Thanks Chem geek, that answers a lot of questions. I just refilled the hot tub but I won't be able to add the CYA (via dichlor) till tomorrow. I don't suppose there is any alternate product sold at supermarket/hardware store that could be used to add CYA is there (the pool store is an hour away)?
 
There are no grocery store items with CYA, but you can often get dichlor or powdered CYA stabilizer at most big box stores that sell pool supplies (Lowes, Wal-Mart, even many smaller Ace Hardware stores) Just make sure it is not cal-hypo which does not have CYA in it.
 
Having refilled my tub with fresh water, and using muriatic acid to lower my high TA (250ppm) I am adding Dichlor as a source of CYA, as recommended. The Dichlor instructions recommended 1 tsp per 100 gal daily, so I have added 1 1/4 tsp per day twice now with an expected FC rise to 3 which falls to zero by the next day. total of 2.5 tsp (or 0.83 tbl). Pool math says 2 oz will raise the CYA level to 30ppm, my target. Does this refers to ounces by weight or by volume (i.e.fluid ounces)? One tablespoon equals 1/2 fluid ounce I know. So am I shooting for a total of 4 tablespoons of Dichlor to reach my goal CYA of 30ppm? Or should I measure the weight of a tablespoon of Dichlor on my scale and calculate how many tablespoons will equal 2 oz by weight (I can do this easily)? Tell me I'm making this harder than it needs to be and I will believe you.
By the way, my tub does have a factory installed ozonator.
Thanks, Lee
 
If it's of any interest, one tablespoon of dichlor (SpaGuard brand, 99%) weighed 16.5 grams. At 28 grams to the ounce one tablespoon equals about 0.6 ounces (0.2/tsp). I'm shooting for a CYA of 30 by adding 2 ounces of Dichlor (by pool calculator), so I guess I'll continue using the Dichlor as my chlorine source until I've added a total of 3 1/3 tablespoons (10 teaspoons) at which time I'll switch over to liquid bleach exclusively. I could work strictly by weight but measuring out the granules directly with a teaspoon measure (e.g. by volume not weight) is much more convenient than weighing the dose each time and I think accurate enough for this. Please tell me if my calculations seem flawed.
 
So that's a density of 16.5 grams / 14.8 milliliters = 1.115 g/ml. I usually figure that 1 ounce weight is about 1 ounce volume which implies 0.959 g/ml so off by 14%. It will depend on the granularity of the Dichlor where more powdery is less dense. Your calculations seem correct.

The only thing to keep in mind is to use Dichlor around once a month to add another 5 ppm CYA to the water since that's roughly how much gets oxidized by chlorine in a hot spa over a month.
 
Thanks Chem Geek. Would you please send me a Dichlor reminder in a month? ;-)
After reading more about it on TFP I took the borax plunge (we keep 20 Mule Team on hand). I added 12 oz and enough muriatic acid to bring pH back down to 7.5. I think the pool calculator called for 15 oz for me to reach 50ppm, but I intentionally undershot a bit. Do you agree with the 50ppm goal? Would the presence of borates suggest a different goal CYA (higher or lower)? Meanwhile I overshot in my lowering of TA, from 220 it's now down to 50ppm. I figure I'll let the soup settle down for a day or so before adding back a little baking soda if needed. I believe my TA goal is 70-80.
 
50 ppm TA is fine since it should reduce the rate of pH rise (I assume your spa is acrylic and not plaster). The CYA level is also fine and not dependent on the Borates level. Having less than 50 ppm borates is OK -- it's only there to less the rate of pH rise.
 
Thanks for all the advice C.G. It's all falling together nicely. I feel like I finally have a good handle on the overall process, or at least ready graduate from first grade (not bad for an MD with a masters degree in biochem). The more I use the 'pool calculator' the more I'm impressed with how useful and clever it is. I hadn't noticed that it gives measurement both in volume and by weight, and can work in metric as well. And the handy guide at the bottom (below the pool volume calculations, that tells the effect of adding a given chemical) is really great. Answered many of the questions that I posted here, had I only noticed it. I also like that TFP (the only forum I've used) is nicely searchable, and you experts are quick to respond to the many questions who's answers have already been posted here many times before.
 

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