Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!crdgw1!uunet!mcsun!hp4nl!dnlunx!pttrnl.nl!walvdrk_r From: walvdrk_r@pttrnl.nl Newsgroups: sci.electronics Subject: Re: Economy of turning off fluorescent lights Message-ID: <1991May19.164508.65064@pttrnl.nl> Date: 19 May 91 15:45:08 GMT References: Distribution: sci Lines: 46 In article , heskett@titan.tsd.arlut.utexas.edu (Donald Heskett) writes: > The question recently came up here of the minimum duration of absence > from a room (an office in this case) that justifies turning off the > fluorescent lighting. I turn them off whenever I'm going to be gone > for more than two or three minutes. One fellow said he had heard that > you should leave them on unless you'd be gone for more than forty-five > minutes. Is there someone out there who can provide a firm answer to > the question? If you'd like to conserve power, it's worth while turning off the lights for a few minutes. But ... the tubes have a very long lifetime. Only the heated wires at both ends of the tube haven't. These wires are only heated during the start-up of the tube and that's when they suffer more than during the normal "burning" operation of the tube. However, I don't know how many start-ups a standard tube is supposed to last, but suppose it is 5000 times. Lifetime of the tube itself is something like 10000h. So if, on the average, the on-time is longer than 2h, the tube will end it's life because of the tube, rather than because of the heated wires. So it's -without any doubts- defendable to switch of the tubes for a short time as long as the average on-time stays at 2h or more. If the average on-time is shorter, the tube will end it's life because the heated wires are worn out. Your energy saving has to account for the premature replacement of the tube. Worst case: the price of a new tube. Suppose the price is $5.00 and the kWh price is $0.10 a new tube equivalents 50*(1000/32) hours of on-time (for a 32W tube). So, if you save more than 1563h on-time during the life time of a tube, it's certainly more economical to switch it off. In normal circumstances you've only to make up for the price difference, so it's already more economical to switch the light off with less than the (worst case) 1563h of saved on-time. So I guess, you'll have to split the lighting of your office in 2 parts, connect kWh meters to both halves and set up an administration of when your tubes have to be replaced. Switch one half with one strategy and the other half with the alternative and after a number of years ... ;-) -- Kees van der Wal e-mail: JC_vdWal@pttrnl.nl ---------------------------------------------------------------------------- PTT Research Neher Laboratories P.O. Box 421 2260 AK Leidschendam The Netherlands Phone: +31 70 3326295