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Name: Suart
Status: Student
Grade: 9-12
Location: MA
Country: United States
Date: July 2006


Question:
Is it true that it is better to leave a fluorescent light on rather than turning it on or off over a short period of time? If this is true, does the rule hold true for all kinds of fluorescent lights? What is the threshold point in terms of time for leaving fluorescent lights on or turning them off? Thanks for answering these questions.



Replies:
Hi Stuart,

The practical answer is, no, it makes no sense to leave a fluorescent light (of ANY type) on instead of turning it off.

This is an story that seems to have started over 50 years ago when someone noticed that the starting current for the first second or less, of a fluorescent lamp, was higher than its current draw after that time.

Starting current of a typical fluorescent lamp is in the neighborhood of 50% more than its normal operating current. But as I stated above, this only lasts for around a second or so. Even much less with many types of fluorescent lamps.

Let us take a standard 40 Watt fluorescent tube as an example. When you turn it on, it draws perhaps 60 Watts for the first second, then 40 Watts from then on. So, for the first second, it is using 20 Watts more than normal.

Now, let us say you are walking out of the room for just 10 minutes, and you decide to leave the lamp on, because someone claimed it would use less energy than if you turned it off, then on again 10 minutes later. That lamp will be wasting 40 Watts of electrical energy for 10 minutes, before you return to the room.

But if you switched it off, it would consume no power at all for that 10 minutes, then when switched back on, it would consume an extra 20 Watts or so for a mere second or less. So in this example, leaving it on would use "waste" 40 Watts for 10 minutes. Switching it off would "waste" 20 Watts for a second or less. Clearly, that old story about leaving fluorescent lamps running to save power is utter nonsense!

Regards,

Bob Wilson


Stuart-

I really do not know; it depends on details. I bet different lights differ in that regard. I suspect the 8-foot-long instant-on fluorescents having only one pin at each end are better for multiple restarts, than the usual type with two pins on each end. But I have not really investigated whether they, too have some little curly-cue pigtail of filament in each end, which might erode. The filaments enable starting and make the voltage drop from metal-to-gas smaller even after warm-up, and they are the things which age worst usually. If you investigate those innards on the Internet and by breaking old bulbs (careful of broken glass, wear gloves and dust mask, wash hands, phosphors can be toxic and there is a bit of mercury too.) then you will actually be learning something and teaching others.

My idea of a fluorescent tube would have no filaments, instead have sturdy metal hollow-cathode cups on each end with magnets to enhance them, and a little radioactive gas inside or a little Tesla coil on the outside to help it start. I bet this bulb would not wear out until metal atoms sputtered off the cathode finally buried all the mercury vapor the tube came charged with. Hard to stop that process. Maybe the "metal cup" would only be outside the glass, and the AC voltage would be 100KHz to go through the glass by capacitance. Sputtering happens very gradually during hours of use. So this kind of tube, if you could find and afford it, would definitely be worth turning off as often as possible, both to save power and extend life. But like I said, they do not make it yet.

I lean towards just turning the thing off, if the switch is convenient. The power savings you know exists, the replacement penalty you do not. The power penalty of start-up is almost non-existent, only equivalent to 1-10 seconds of on-time. and often the light's intensity and color are not quite up to par for the first minute or so of warm-up, but usually that does not stop you from using the room.

Jim Swenson



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