UV sterilizer, BS or what?

[chem geek]

  I think what I'm seeing is that bromine (which is destroyed by direct sunlight) is reacting the same way with this UV wavelength from the unit. I'm going to experiment with my show tub and switch it to chlorine and see how it goes. I have some pool contracts that have big Delta UV's with no issues, they're all on Chl though. I think an incompatibility with bromine needs to be documented. My manufacturer up the original 9w unit to 18w and there's probably a threshold for the UV breakdown that's in-between there somewhere as I don't recall any customers having issues with the older style unit. Thanks for your input guys.

As I wrote in my post, both chlorine and bromine are broken down by UV.  With no Cyanuric Acid (CYA) in the water, chlorine breaks down faster than bromine at equivalent levels, but with CYA in the water as would occur if using Dichlor then the chlorine breaks down more slowly than the bromine so that may be what you are seeing or have experienced.

There is no "threshold" for the breakdown.  It's linear with output power from the UV.  If you didn't notice a problem with the older unit at half the power it wasn't because it wasn't breaking down chlorine or bromine, but rather that it was breaking it down half as quickly so it didn't seem that unusual.  Also, if the CYA level was different before than now, then for chlorine at least that will make a difference.

[summerside9000]

I recently went from Bromine to Chlorine and changed the UV light in the spa as it had expired a couple of months ago. It is a jacuzzi model/clear ray.
What I found was the Bromine demanded more interaction and the the Chlorine seems to be looking after itself at the moment with minimal chemical adjustment to keep the chlorine in the zone. Is it the light?? I thought so but chem geek is the opposite.
I am glad for the break from Bromine but will probably go back to bromine when the chemicals run out as I think the comfort is there somewhere.
UV light is like the car salesman schtick but I swear the thing reduced the chemical demand on the water.
All in all it is just great to have a tub with the awesome features going into a Canadian Winter. I'm all set to go. bromine-chlorine-whatever. its all good.

[chem geek]

When you changed to bromine, what kind of chlorine did you use?  If it's Dichlor, then that adds Cyanuric Acid (CYA) to the water where for every 10 ppm Free Chlorine (FC) it also adds 9 ppm CYA.  As I wrote, with CYA in the water, then the chlorine breakdown from UV becomes lower than bromine.  If you were using Dichlor, then your experience is consistent with what I wrote.

It's only if you were using chlorine with no CYA in the water where the chlorine would break down faster than bromine.  So if you were to use bleach or chlorinating liquid without adding any CYA (or Dichlor) first or if you were to use a saltwater chlorine generator and not add any CYA or use any Dichlor initially, then that would have the chlorine not moderated in its strength nor protected from the UV.

[summerside9000]

Nope its the Dichlor stuff. Back to adding a half a cap every three days or so.

[kitchener]

As for UV, it will use up chlorine because UV breaks down chlorine but the way it breaks apart bather waste is such that it won't necessarily reduce chlorine demand.  So a powerful UV system will usually increase, not decrease, chlorine demand (same would be true for bromine demand, but not MPS).  The main purpose for a UV system in a residential spa would be to reduce chloramines, but proper dosing of chlorine after a soak usually oxidizes the bather waste and resulting chloramines before the next soak (usually the next day).

"Not MPS", that's interesting.    Would a spa equipped with N2 and a UV system benefit from a more powerful UV system if using MPS?

[chem geek]

If you are properly maintaining a disinfectant level in the spa then there is not a need for either UV or ozone, but with the Nature² system with its silver ions and using MPS as the oxidizer, the main purpose of the UV would be to break up some chemicals that MPS doesn't normally oxidize.  MPS is slow to oxidize ammonia, for example.  Normally one needs to add chlorine every now and then to keep the water clear in a Nature²/MPS system and usually people have ozone in which case you don't normally need the occasional chlorine.  I'm not sure if the UV would be as effective as ozone for this purpose.