This sounds like a good topic to get into about UV radiation and the pros and cons about it.

This sounds like a good topic to get into about UV radiation and the pros and cons about it. Through the conversation is very good, I think I’ll add a few wood chips to the fire.

There are several good points being brought up here by everyone and if people think that making a UV Light is easier and cheaper than just going out and buying a professionally made one, well think again. On the surface UV Lights seem like a very simple device to make and anyone could make one for half the price than one you can buy. Manufactures use VHO and UHO Lamps for most UV Lights, that means special electronic ballast and not the cheep ones you buy at a hardware store that are specifically made for Low Output Lamps and not for high output lamps. Yes, you can buy cheaper ballast at a hardware store, but wiring them up correctly, well that’s if they will work at all, is another story from what I’ve heard through the grapevine.

Plus, and this is a big plus, that already has been brought up here; if you do the job yourself and get shocked or accidental death should be the outcome, you will have no recourse of tort in a court of law for such. Buying a manufactures UV Light means they and not you take all responsibility that their devise is safe to use around water.

Syd is right in saying that a three inch diameter UV unit will give you a better kill rate than a two inch diameter unit would by over 50% better. Bigger is not better when using and improper UV Lamp to size of ID (inner diameter) of a unit. The bigger the diameter of the unit like 6 inches would need a VHO T5 Lamp with over a 55Watt rating.  However, you cannot compare UV Units/Sterilizers by the watt ratings alone. The watt rating is just a starting point in properly sizing a sterilizer to water output.

As Syd has said: The full amount of UV energy required to kill a microorganism must hit the organism after the energy leaves the lamp, after it leaves its quartz sleeve, after the lamp has aged and after it has passed by any turbidity and color that blocks it.

Then you get into UV Lamps, like low-pressure mercury type lamps that 40% UV energy efficiency rate between input watts and UV output watts. UV-A lamps produce some ozone and UV-C lamps do not. As Dave has stated, Medium- and high-pressure mercury-type lamps are best suited for treatment involving chemical byproducts associated with industrial wastewater but not germicidal action. Their output range falls way outside of the germicidal range. You must also remember that it is the intensity of light that is doing the killing not just watts and how much of that energy is reaching the target we wish to kill or eradicate.

So placing a UV Lamp inside a filter (9 Watts is a joke) is a guessing game to say the least. You must know how much light energy to use, and how much is reaching the target. All factors like turbidity, even some dissolved salts, like Sodium Thiosulfate affect the efficiency along with lamp temperatures. Temps of 110F give maximum UV output; that’s why quarts sleeves are used to keep temperatures of lamps hot so cooler pond waters don’t reduce lamp output and their effective kill rate.

Years ago Tetra made a filter that had a UV Lamp on the inside of the filter cover/top with a reflective SS behind it. It failed because the lamp was not protected and could be busted easily and its effectiveness was too untrustworthy and too many variables came into play that RD did not show. Good ideal or bad it failed!

As far a looking into a UV Lamp when on, take it from someone that’s been there done that. In the Navy several of us were exposed to UV Lamp radiation by accident for over an hour. Because of the distance we were at (about 1-2 feet), we only suffer from what is called “sand in the eyes” for about a day and then all went away. So looking at a UV Light from 2-3 feet away or more will probable do very little harm to ones eyes for even a few minutes. It’s a caution the manufactures give to protect themselves not you in a lawsuit. Your eyes are being exposed to more UV light when you walk outside in direct sunlight than from a UV lamp at 2-3 feet away.

Also, UV lamps lose about 40% of their output in about 6 months so a controlled distance inside a cylinder is still the best way to go and refracting and reflecting UV light also plays a part in the effectiveness of the UV units that we buy along with dwell time that is scientifically regulated by the units ID, how long is the unit, and if it’s tangential and allows the water to circulate around the UV lamp for a longer dwell time. In a filter dwell time can never be calculate to an exact UV exposure for waterborne algae that requires a 22,000µWs/cm2. Only as Tetra found out that this dwell time can be regulated with a cylinder and not inside a filter.