Does UV use up FC like Ozone will?

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watson524

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Nov 13, 2014
182
Lake Ariel, PA
Tho we've drained, ahh-somed, never let FC get to 0 and have switched from dichlor to bleach (8.25%), I still think we're using more FC than we should be. One thing I've found is that the FC doesn't go up as high as it should when we doze based on pool math so we've taken to dosing after a soak (or a non soak day) but then testing to know actual level after a pump cycle.

We haven't been in since the 31st of July but the following are recent test results:
8/1/15 noon:
FC 4.0
CC 0.2

8/2/15 1pm:
FC 1.2
CC 0.2

So over 50% in 25 hours

Thought we'd be going in last nite so I didn't add anything. Didn't go in so at 9:30 last nite (8/2 - 7.5 hours after last test), the FC was 0.8 (a loss of 0.4)

Add 5oz of 8.25% which should have raised 500 gallons 6.6ppm on the FC. We've also decided we have more than the published 500 gallons in there and will confirm for sure when we next fill since I got a gauge for the hose. Anyway, put in the 5oz at 10pm last nite, ran a pump cycle and retested, and FC was 6.2 (vs the expected 7.4) Fine, weaker bleach, more water... we can live with that.

10am this morning 8/3/15, I tested and FC is 3.8 and CC is 0.2

So what the heck? In 12 hours with no bather waste to clean up, we lost 39%

Is it possible that my ClearRay (UV) is causing a higher than normal FC loss like an ozone system would?
 
Yes, it does. UV is what sunlight chlorine consumption is all about in a pool. You spa is covered, but you are adding UV consumption. How much I would have to leave to Chem Geek.
 
What breaks down free chlorine? Organics and sunlight. Specifically, UV light in sunlight. What is your CYA level at in the tub? Usually 20-30 is recommended in hot tubs. At that relatively low CYA in a low volume tub (compared to a pool), your UV certainly is increasing your chlorine demand. By how much? That's the question. Unplug it and repeat the 12 hour test.
 
Ahh.... so I think for a test, I'll see if I can shut the ClearRay thing off somehow (it's on with the pumps) and test and see what's what. I've been driving myself nuts trying to figure out what's up with this since the water is nice and clear and all and this just dawned on me this morning

- - - Updated - - -

JVTrain said:
What breaks down free chlorine? Organics and sunlight. Specifically, UV light in sunlight. What is your CYA level at in the tub? Usually 20-30 is recommended in hot tubs. At that relatively low CYA in a low volume tub (compared to a pool), your UV certainly is increasing your chlorine demand. By how much? That's the question. Unplug it and repeat the 12 hour test.
Click to expand...

CYA is about 30 (due to use dichlor one day next week to make up for the amount lost since switching to bleach)
 
Please do disconnect your ClearRay UV and retest your chlorine demand with no bather load and let us know. There are some who do not believe that UV systems break down chlorine, but when they are oversized beyond the minimum needed for supplemental disinfection they most certainly should show up increasing chlorine demand.
 
Ok. I'll poke around under there tonite and see how I take it out of the mix. I'll be in it tonite so I won't be able to start the ONCLT until tomorrow nite once its cleaned up all the bather waste. Also, I run the circ pump 12 - 24 hours / day (depending on how I feel about it) and the clearray runs then so that's eating up my bleach during the day too. This is definitely one of those "DOH!!" moments! I'll report back probably thursday morning once I do the test overnite on Wednesday
 
Well I found the wire from the clear ray box to the board box (going through a transformer) so I unplugged it from the board box but on the control panel, the clear ray light indicator still comes on which makes sense because there's just a switch there. But after we use it tonite and let it clean up the bather load, I'll test it tomorrow, set the FC and then hit the breaker for the ONCLT so we can be 100% sure it's off.
 
I think chem geek said in another post that the UV would be helpful to have running when you have people in the tub using it, and shortly after, as it could tackle bather waste in real time fairly well and reduce the need for some of that FC afterwards. But have it turned off otherwise, when you do you chlorine additions and daily maintenance filtering. Not sure how easy it is to automate or manually adjust the on/off though.
 

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I made the comment with regard to having ozone turned on right after one's soak to help oxidize bather waste. With UV it's more of a mixed bag since it's not designed to oxidize bather waste though with an oversized UV which this appears to be it will tend to break up molecules into smaller pieces. That doesn't always help and can sometimes produce more disinfection by-products (there are several papers by Ernest Blatchley on UV's effects on bather load chemicals). Nevertheless, what you wrote is reasonable in that you can run UV while in the tub since unlike ozone it won't outgas.
 
chem geek said:
With UV it's more of a mixed bag since it's not designed to oxidize bather waste though with an oversized UV which this appears to be it will tend to break up molecules into smaller pieces.
Click to expand...

What is it designed to do then? Other than eat up my bleach :) I thought it would disrupt the DNA of the nasties but I guess that doesn't sort out anything with bather waste while in there but stop them from multiplying?
 
Chlorine kills almost everything that would be common in pools were they not disinfected, including most bacteria, viruses, and some protozoan oocysts (see the "Chlorine" column in the table in this post). There are only a few pathogens that chlorine doesn't handle, namely Cryptosporidium parvum (and more rare items like iron bacteria), but unless you invite someone over who has this with diarrhea, it won't get into your pool. Unlike bacteria or viruses that are added from regular fecal matter or mucous or from blown-in soil or algae spores, Crypto is not in the general environment. Chlorine is the primary disinfectant. UV or ozone are used in some commercial/public pools for supplemental disinfection mostly to handle Crypto though ozone is also used to handle the bather waste from higher bather-load and UV is sometimes used indoors to manage chloramines.
 
There is a questionnaire which people must fill out before entering the tub and amongst the questions like "what countries have you traveled to in the last 6 months?" are things like "have you had even a sniffle in the last 5 days"..... no but seriously, I'm not quite that anal (quite...) So net net, I'm thinking that I won't bother with the clearray system since it just eats my bleach and costs me annual money (albeit just over a dollar a week) in bulb replacement.
 
Well, what's it called when you get an unexpected outcome? With the clear ray going, I had a 39% FC loss. Last nite, I tested at 8pm and had 2.0ppm, today at 8am, 1.2ppm for a 40% FC loss without clearray running.

I confirmed that I shut it off the right way on the other forum where a Jacuzzi dealer confirmed that the plug I took out of the spa pack shuts off the unit even tho on the panel, the indicator light is still on. That's just telling us it's sending power to the unit but it's definitely unplugged.

So...... huh. Maybe our spa just uses the 40% number as its norm. We've ahh-somed, keep it covered, have crystal clear water never getting to 0 FC and this fill, since the ahh-some, it's only been the two of us in it.
 
That's awfully high for chlorine demand with no ozonator or anything else consuming chlorine. Usually it's < 25% in those situations and its only that high because of the hotter spa temperatures. Of course, you need to measure such demand well away of using the spa since bather-load does consume chlorine, but most of that bather-load is usually dealt with in the 12-24 hours after a soak. If you haven't recently done a water change, then you should test your chlorine demand again soon after you next replace the water. If you have a period of time where you aren't using the spa for several days, then you can see the chlorine demand over that time to see if it is dropping so that maybe you just have a buildup of slower-to-oxidize organics in your spa (that's why testing soon after a fresh refill is helpful).

So the good news is that it seems that your UV system is sized lower for only disinfection and not (much) oxidation, but it also means that it doesn't save on the amount of chorine needed to oxidize bather waste.
 
We haven't been in since Sunday nite. So I tested overnite Monday with the UV and overnite Tuesday without UV. Changed the water 5/29 so not ready to do that again. I didn't add anything today and am going to test it tomorrow morning to see what happened to the 1.2 in 24 hours today. I know I shouldn't let it be that low but if it falls around .7 (so with the drops, 0.6 - 0.8) then I guess I just assume the tub loses 40% overnite.... who knows. Still makes me feel like I won't bother spending the money on a new UV bulb when this one is up.
 
You should probably also see what happens at higher FC just to be complete. So does it go from 4 ppm down to 2.4 ppm FC over 24 hours, for example. Strange that it's that high in your spa. The chlorine is oxidizing something or is outgassing more than usual. I wonder if your CYA level isn't as high as you think it is. If that's low, then the active chlorine level will be higher and you'll lose chlorine at a faster rate. The CYA should be around 30-40 ppm.
 

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