Was the old bulb lighting up before you took it out? I assume you took it out because it lost strength and not because it "blew" out.

I find it hard to believe that a multimeter wouldn't work. I think you were given some bad information. The bulb would take whatever its rated and the tester should show what the transformer output is to the bulb. If you look at the label on the transformer it should show the output. I would certainly question this from another source such as an electrician at www.allexperts.com

Here is a link that might be useful: My pond the way it was before the freeze.

Bob, The first thing is the three bulbs you have are probably just fine. The sorched marks are the filiments heating up and this normal. The bulbs could have lost some of the mercury vapor in them and they will not light but I doubt that all three would have. As far as testing with a multimeter I have tested good and bad ballast. Sometimes the reading can be very different but other times there almost no diffence. I always plug a new bulb in to find out if it is bad and have never damaged a bulb yet. I would not leave a bulb that doesn't light pluged in for a long time as this could damage it. I have probably tested over 100 UV lights this way. Most of the time it is the bulb but with Aqua the ballast burn out quite a bit. You didn't say what size you have. I stopped selling Aqua's just because the ballast burned out so quickly. Especially on the 80 watt unit. They have gone to electronic ballast now that should last longer.
Mike

Thanks for the helpful comments, folks.

curb1, the bulb was not lighting at all when I unplugged the unit.

bugsb, as for testing the output of the transformer, I'm not sure how I'd go about that. The plug that attaches to the tube has four holes and I'm not sure what I'd look for, having never tested a transformer. But I'll check the website you suggested.

And Mike, I'll give all three tubes a try with the new transformer...thanks for the tip. My unit is a 40W Aqua and I confess that my old tube might have been burned out for a while before I noticed it and unplugged the unit. I'll pay more attention after this, now that I know that I shouldn't run the unit with a bad tube.

Many thanks again for responding.

Bob

Bob I think I may have not been clear. The old bulb did nothing to the transformer or ballast. One of three things causes the ballast to go bad. One is old age. Not the case here. Water leaking into the connect at the bulb. And the last is that the ballast in enclosed and it heats up which is normal. This heat being enclosed can shorten the life of the ballast which is what usually happens.
Mike

Thanks for the clarification, Mike. Well, it's not age and it's not water contacting the bulb connection so it must be heat. My pool and pond equipment are both in an enclosed utility room that faces south. During the summer, we regularly get temperatures over 100 and that room does get warm so maybe that's the problem. Heck, with global warming, I may be buying a new transformer every year!

Bob

Bob like I said Aqua has gone to electronic ballast so they don't get as hot as the old ones. This may solve some of the problems with the units.
Mike

To update my UV troubles, I installed a new transformer along with one of the new tubes which I had assumed had been burned out by the old transformer. As Mike predicted, the tube wasn't burned out after all...it worked!

I'm ready for another summer of crystal-clear water. Thanks again for the help!

Bob