Contemplating a UV system

dw886

Gold Supporter
Sep 19, 2016
240
ND
To be honest, I've been contemplating the same thing - our pool is covered 99.9% of the time with an autocover, and when it's opened up, you can smell chloramines. I also don't think that my water gets enough UV exposure - I think I asked the same question about a year ago. The downside to this is obviously that UV eats Chlorine not bound to CYA, so I'd assume Chlorine consumption actually goes up to stay at the same level. The other downside is that with any system like this, there's no real way to know if it's working - all feedback is going to be anecdotal.
 

dw886

Gold Supporter
Sep 19, 2016
240
ND
Can you not give it more by keeping the autocover open more often? 99.9% covered is asking for problems. Pools need to breathe.
For our scenario, it's a safety issue with small kids around. If the pool isn't being used, the cover is closed. It also helps with the efficiency of the pool heating. For the most part, I feel like the way we operate is normal for up here in the North. The kids were in a 88-degree pool yesterday when it was 65 degrees outside - it's part of how we extend our summer.
 

Ian196499

Gold Supporter
Silver Supporter
Aug 12, 2018
77
England
I’ve decided for this season to just go with opening our enclosure more.
My wife asked me a good question for which I didn’t know the answer : which type of UV actually destroys the chloramines? Is it UVA or UVB? Since our enclosure has glass sides I could maybe get away with removing the cover if enough of the right sort of UV gets through the glass ?
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
16,574
Tucson, AZ
I’ve decided for this season to just go with opening our enclosure more.
My wife asked me a good question for which I didn’t know the answer : which type of UV actually destroys the chloramines? Is it UVA or UVB? Since our enclosure has glass sides I could maybe get away with removing the cover if enough of the right sort of UV gets through the glass ?
UV Wavelength Definitions -

400nm > UVA > 315nm > UVB > 280nm > UVC > 100nm

Chlorine and Monochloramine absorbance - 259 nm (ϵ=484.61 mol−1 cm−1 ) and 331 nm (ϵ=95.7 l mol−1 cm−1)
Dichloramine Absorbance - 255 nm (ϵ=126.1 l mol−1cm−1) and 301 nm (ϵ=307.8 l mol−1 cm−1)
Nitrogen Trichloride Absorbance - 262 nm (ϵ=635.4 l mol−1 cm−1) and 344 nm (ϵ=315.0 l mol−1 cm−1)

Most UV sterilizers use low pressure mercury discharge bulbs where 80-85% of the UV is at 253.7nm and 5-10% is at 185nm. This puts most of the UV in the UVC range which is also at the wavelength of sterilization (pathogen destruction). Sunlight is mostly a UVA where one finds lower absorbance, and therefore less oxidation, of CCs. But, given how most pools are exposed to UVA throughout long periods of the daylight hours, even lower absorption of UV and oxidation of CCs has a noticeable effect.

My pool, which is open all year round and exposed to sunlight and UV for most of the daylight hours (with very intense UV in desert climate), almost never registers CCs of any kind. I think I once had one reading of CC's that was 0.5ppm and, honestly, it may have been a fluke. A covered pool will build up CCs over time.
 

Ian196499

Gold Supporter
Silver Supporter
Aug 12, 2018
77
England
UV Wavelength Definitions -

400nm > UVA > 315nm > UVB > 280nm > UVC > 100nm

Chlorine and Monochloramine absorbance - 259 nm (ϵ=484.61 mol−1 cm−1 ) and 331 nm (ϵ=95.7 l mol−1 cm−1)
Dichloramine Absorbance - 255 nm (ϵ=126.1 l mol−1cm−1) and 301 nm (ϵ=307.8 l mol−1 cm−1)
Nitrogen Trichloride Absorbance - 262 nm (ϵ=635.4 l mol−1 cm−1) and 344 nm (ϵ=315.0 l mol−1 cm−1)

Most UV sterilizers use low pressure mercury discharge bulbs where 80-85% of the UV is at 253.7nm and 5-10% is at 185nm. This puts most of the UV in the UVC range which is also at the wavelength of sterilization (pathogen destruction). Sunlight is mostly a UVA where one finds lower absorbance, and therefore less oxidation, of CCs. But, given how most pools are exposed to UVA throughout long periods of the daylight hours, even lower absorption of UV and oxidation of CCs has a noticeable effect.

My pool, which is open all year round and exposed to sunlight and UV for most of the daylight hours (with very intense UV in desert climate), almost never registers CCs of any kind. I think I once had one reading of CC's that was 0.5ppm and, honestly, it may have been a fluke. A covered pool will build up CCs over time.
Thanks for the very useful explanation. If I understood it correctly, UV systems in a pool are using UVC to destroy CC whereas the UVA element of sunlight is destroying the CC.
given that glass absorbs only 25% of UVA then if I leave the pool bubble cover off then the pool will get some benefit of sunlight even if I leave the enclosure closed. This is really useful.
Of course it doesn’t help if one has an automated solid pool cover....
 

Ian196499

Gold Supporter
Silver Supporter
Aug 12, 2018
77
England
The post from DW886 does raise an interesting point for me.
whilst the TFP method clearly works very well in the context of an open air pool, there must be many users of this forum who have an indoor pool, a pool with an automated cover or (like me) a pool covered by a telescopic enclosure.
I imagine that most users who invest in such a covering do so for the reason highlighted by DW886 - they live in a colder climate and so it extends the season / allows them to use the pool in inclement weather. In my case, the ambient air temperature practically never exceeds the water temperature and so without using a covering the heating bills would be prohibitive.
The consequence of this is that there will inevitably be a slow build up of CC. The TFP answer is to then SLAM the pool, however my experience is that whilst this clearly works to remove the CC the lack of sunlight results in the FC level remaining. Very high for days before it gets back to the correct level. It would be much better to know how to stop the build up.
My pool / chemistry knowledge is “beginner” at best and it would be very beneficialto have a more knowledgeable TFP view on the “best” approach to solve this.
Even if the answer is “let the pool breathe” then how long does it need? Etc
I’m sure some users must have solved this already!
Cheers
Ian
 

Ian196499

Gold Supporter
Silver Supporter
Aug 12, 2018
77
England
To be honest, I've been contemplating the same thing - our pool is covered 99.9% of the time with an autocover, and when it's opened up, you can smell chloramines. I also don't think that my water gets enough UV exposure - I think I asked the same question about a year ago. The downside to this is obviously that UV eats Chlorine not bound to CYA, so I'd assume Chlorine consumption actually goes up to stay at the same level. The other downside is that with any system like this, there's no real way to know if it's working - all feedback is going to be anecdotal.
Hi, Just a bit of a follow-up on your post. I was talking to a friend who has exactly the same set-up as you- automated hard pool cover which shuts out all light to the water. He has had a UV system for a few years now and never has any build up of CC. Additionally he says the chemical usage is very low. So might be worth a few hundred dollars to see if it works for you.