# fluorescents don't light--high humidity? **URL:** \ **Category:** Factual Questions\ **Created:** [August 9, 2003, 1:11pm UTC](https://boards.straightdope.com/t/fluorescents-dont-light-high-humidity/194014 "2003-08-09T13:11:14Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [August 9, 2003, 1:11pm UTC](https://boards.straightdope.com/t/fluorescents-dont-light-high-humidity/194014/1 "2003-08-09T13:11:14Z")
I have two fluroescent plant stands that are on timers. During this last week, neither has lit. They way I get them lit–believe it or not–is to touch the bulb or even just let my hands float near it (after the current has been on a while, to be sure). That both of them stopped at the same time seems too much of a coincidence. The only thing I can think of is that every morning this week the humidity has been in the high 90% range. Could this have affected the choke in some way? Any thoughts or suggestions? ---
**Author:** ![Mr.Blue\_Sky](https://avatars.discourse-cdn.com/v4/letter/m/d26b3c/32.png) [@Mr.Blue\_Sky](https://boards.straightdope.com/u/Mr.Blue_Sky)\ **Post date:** [August 9, 2003, 1:52pm UTC](https://boards.straightdope.com/t/fluorescents-dont-light-high-humidity/194014/2 "2003-08-09T13:52:55Z")
Bad starters or ballasts, maybe. ---
**Author:** ![Podkayne](https://avatars.discourse-cdn.com/v4/letter/p/edb3f5/32.png) [@Podkayne](https://boards.straightdope.com/u/Podkayne)\ **Post date:** [August 9, 2003, 4:31pm UTC](https://boards.straightdope.com/t/fluorescents-dont-light-high-humidity/194014/3 "2003-08-09T16:31:59Z")
We have the same problem with the florescents in our bathroom. After someone has taken a shower, or in the summer when it’s quite humid, you have to gently stroke or touch the tube to get them to start. I’ve experimented–you can leave them on for a long time, but they won’t light up until you touch them–and they always light right up with the first stroke. ---
**Author:** ![AskNott](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/asknott/32/5790_2.png) [@AskNott](https://boards.straightdope.com/u/AskNott)\ **Post date:** [August 9, 2003, 5:06pm UTC](https://boards.straightdope.com/t/fluorescents-dont-light-high-humidity/194014/4 "2003-08-09T17:06:01Z")
Are your bulbs dark at the ends? If so, they are burnt out. Even if they still light, a bulb in that condition puts out only half the light it should. For your plants, this means slow growth. In **Podkayne’s** bathroom, it could mean shaving cuts. By the way, **Hari Seldon** , the light distribution from a flourescent tube is not at all even. It’s strongest at the middle, and falls off in a bell curve except for a bulge at each end. It also falls off dramatically as you get further from the tube itself. Your best plant growth will come from getting the plants as close to the tube as possible. The tube is relatively cool, but direct contact with the tube will cause leaves to go brown. Keep the tubes clean; dust can cut light intensity by up to 30%. ---
**Author:** ![sailor](https://avatars.discourse-cdn.com/v4/letter/s/a587f6/32.png) [@sailor](https://boards.straightdope.com/u/sailor)\ **Post date:** [August 9, 2003, 5:19pm UTC](https://boards.straightdope.com/t/fluorescents-dont-light-high-humidity/194014/5 "2003-08-09T17:19:43Z")
This is a common problem with some fixtures which require the metal reflector to be grounded. If it is not grounded the tubes have trouble starting but if you touch them they will start. I believe the distance between the grounded shield and the tube has to be less then 1/2" . ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [August 9, 2003, 6:54pm UTC](https://boards.straightdope.com/t/fluorescents-dont-light-high-humidity/194014/6 "2003-08-09T18:54:01Z")
The tubes are not dark at the end. The plug is a grounded plug and it was rather hard to find a three prong timer, this one is, although I have never tested to see if it works. Somewhere I have an ohmmeter, although it probably needs a battery. I do place the tubes as close to the plants as possible. Actually, almost all the plants are outside for the summer, but I am concerned for the fall. Any source of light falls off as the square of the distance; there is nothing special about fluorescents on this score. The thing was that both fixtures started doing it at the same time and this coincided with the extraordinary high humidity we have been having. Is that _post hoc_ reasoning or a real effect? From Podkayne’s experience it might be a real effect. Fortunately, I don’t shave under fluorescents–or at all. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [August 9, 2003, 7:18pm UTC](https://boards.straightdope.com/t/fluorescents-dont-light-high-humidity/194014/7 "2003-08-09T19:18:28Z")
> [@](#): > > \*Originally posted by sailor \* > \*\*This is a common problem with some fixtures which require the metal reflector to be grounded. If it is not grounded the tubes have trouble starting but if you touch them they will start. I believe the distance between the grounded shield and the tube has to be less then 1/2" . \*\* Eh? No, I don’t think so. I’ve never seen a fluorescent fixture with grounded plug, and even if there were, it would have nothing to do with the tubes igniting or not. The starter circuit is just a switch to turn on the filaments to vaporize the mercury. The problem in the OP sounds more like a ballast problem. Touching an energized fluorescent tube creates a shunt for current to flow, apparently just enough to start the ioniziation process in this case. You can see this effect clearly if you energize one side of a tube with a high voltage of a few kV. Touching the tube it will glow bright from the energized end up to where you touch it. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [August 9, 2003, 7:22pm UTC](https://boards.straightdope.com/t/fluorescents-dont-light-high-humidity/194014/8 "2003-08-09T19:22:28Z")
Iwant to clarify that I am not suggesting there are no fluorescent fixtures with grounded plugs. There certainly are, I’ve just never seen one, that I can recall. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [August 10, 2003, 3:51am UTC](https://boards.straightdope.com/t/fluorescents-dont-light-high-humidity/194014/9 "2003-08-10T03:51:28Z")
> [@](#): > > \*Originally posted by Q.E.D. \* > **Eh? No, I don’t think so. I’ve never seen a fluorescent fixture with grounded plug, and even if there were, it would have nothing to do with the tubes igniting or not.** Sorry **Q.E.D.** , but you’re incorrect. Some fluorescent fixtures _do_ require the reflector to be connected to earth ground in order to properly start: [http://www.repairfaq.org/REPAIR/F\_flamp.html#wgf](http://www.repairfaq.org/REPAIR/F_flamp.html#wgf) > [@](#): > > Many fluorescent fixtures will not start reliably unless they are connected to a solid earth (safety) ground. This is most likely the case with rapid or trigger start magnetic ballasts. These will usually state on the label: “Mount tube within 1/2 inch of grounded metal reflector”. If this is not done or if the entire fixture is not grounded, starting will be erratic - possibly taking a long or random amount of time to start or waiting until you brush your hand along the tube. > > The reason is straightforward: > > The metal reflector or your hand provides a capacitive path to ground through the wall of the fluorescent tube. This helps to ionize the gases inside the tube and initiate conduction in the tube. However, once current is flowing from end-to-end, the impedance in the ballast circuit is much lower than this capacitive path. Thus, the added capacitance is irrelevant once the tube has started. > > The reason that this is required is probably partly one of cost: it is cheaper to manufacture a ballast with slightly lower starting voltage but require the fixture to be grounded - as it should be for safety anyhow. **Sailor** and I are not saying this _is_ the problem; we’re saying that this _could_ be the problem… ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [August 10, 2003, 6:20am UTC](https://boards.straightdope.com/t/fluorescents-dont-light-high-humidity/194014/10 "2003-08-10T06:20:44Z")
Well, whaddaya know? Learn something new everday. Thanks, **C\_M**. ---
**Author:** ![sailor](https://avatars.discourse-cdn.com/v4/letter/s/a587f6/32.png) [@sailor](https://boards.straightdope.com/u/sailor)\ **Post date:** [August 10, 2003, 9:40am UTC](https://boards.straightdope.com/t/fluorescents-dont-light-high-humidity/194014/11 "2003-08-10T09:40:23Z")
Basically there are three types of fluorescent lamps: pre-heat, rapid start and instant start. Pre-heat have starters. Rapid start requires a grounded plate near the tube as an aid to reliable starting. ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [August 10, 2003, 1:58pm UTC](https://boards.straightdope.com/t/fluorescents-dont-light-high-humidity/194014/12 "2003-08-10T13:58:54Z")
This fixture has no starter I can see. I will eventually take it apart and see what’s inside, but it seems to me that I have seen them with the starter (small metal encased tube, right?) exposed for easy replacement. Does that help identify what kind it is? And it certainly has a grounding plug. There is no sign on it that suggests that the grounding is there for anything except safety, however. No signs at all, in fact. It just sits there. And the reflector is just a hair over 1/4" from the tubes. I really have to find out if it is really grounded. All I know is that the socket is relatively recent; the timer has a three pronged outlet and everything looks right.