How can one safely tell if a UV lamp is working? This is a UV tube in a pond filter system which lives in a lightproof (well, UV proof I suppose) tube with the water going to the filter flowing past it.
It has standard fluorescent gear, a choke and a starter.
Often they have a little fluorescing plastic indicator in the case, to show it is picking up UV. You should change the tube annually anyway (assuming 24x7 operation), as UV output drops, and in continuous operation the output will become too low before the tube fails in the conventional ways (i.e. not starting). If you are doing this and the unit is drawing roughly the right current, I would assume it's working unless you have some other reason to suspect it (e.g. pond going unexpectedly mankey). When operating, it may generate some ozone which you might be able to smell from the casing, but they don't all do this.
I hope the unit has interlocks to prevent you looking at the tube operating, which is dangerous. However, if you are tempted to try defeating the interlocks, put a sheet of glass (not plastic) between you and the lamp, stand well back, and only operate the lamp for a few seconds. There is normally a dim visible glow from the clear tubes.
Is it interlocked not to come on if the box is open? Defeat the interlock, probably just a switch on the cover then in a dark area switch on and see if a sheet of ordinary white photo copier type paper fluoresces. You may also see a dim blue glow. Obviously don't stare at the tube or spend any longer than required exposed but a second or three isn't going to hurt you.
Thanks, we have two spare tubes (it's ex. my mother-in-law) and I doubt very much if the tube has been replaced within living memory so I'll change it anyway.
I would have thought most plastics would block UV more than glass. Most UVB and almost all UVC are blocked by virtually any material apart from quartz.
You can't (without vast expense) -- output falls with time, so bactericidal UV lamps are time-rated. I'm not sure about algae though?
As to a simple "Is it still working electrically" then they nearly all have visible glow too. If you can't defeat the interlocks, then stick a piece of phosphorescent plastic in there and see if it's charged up by it. It's nearly Hallowen, I'm sure you can find some glowing vampire teeth.
That's a tough one.
I have something similar used as a steriliser for a reef tank.
The problem is that they don't suddenly stop working, but rather the output degrades.
Generally, therefore, one replaces them according to time - mine is changed every 6 months.
For a pond system where the UV is usually used to control algae, one could look on the basis of whether the algae is being controlled. However, as you probably know, algal growth depends on sunlight, temperature, nutrient availability and your inside leg measurement, so it is not always easy to make an assessment of whether the UV treatment is doing its job.
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