How does water hurt a flashlight?

how does water that enters into a flashlight hurt it so that it goes off? does this happen only in ones with drivers and such or does it also turn off ones that are direct drive?

More than likely, water will do more harm to the battery than anything else in a flashlight. The batteries could be shorted, and due to electrical current, will probably corrode as well, if exposed for enough time. Corrosion is probably the biggest problem for any flashlight, and it's short circuiting that is really the problem for lights with IC's and boards. A short circuit could ruin the electronics, but if there are no batteries in the light (and capacitors are discharged) and it is flooded with water, chances are it will not sustain damage -- assuming it doesn't get corroded. An example of this is the Inova line of flashlights -- CPFers open them using water and freezing, and aside from physical damage, the lights come out relatively unharmed, except for broken seals.

so what would be the best way to waterproof a light? would o-rings be good? and which ones? how do say the ones on the cheap ebay clone differ from the ones on the arc?

O-Rings and grease the threads is one way to add "water-proofness" but more like "water resistance" to said flashlight. Other than that I don't know. But exactly what Carrot said above regarding your question.

Water doesnt actually conduct as much electricity as people think. Ever seen a car crash into a river or lake? Their headlights can stay on for hours under water.

I recall seeing a picture of one of Juan_Cho's MAG85's light up under water...but then again, there are no electronics in one of those things.

It may be just as likely that a wet light won't turn off. Some manufacturers (like Pak-Lite) state that fresh water won't adversely affect their circuit boards -- but you might not be able to get the light to turn all the way off while it's still wet. They recommend using a hair dryer to get the water out. I got one wet inside once and it wouldn't turn off -- I blew the water out with compressed air and everything went back to normal.

Fresh water probably doesn't do too much in limited exposure and with a thorough drying out ASAP.

Now, salt water is another story. Even a very limited exposure to salt water can be bad Mojo, even for lights w/o circuitry. The corrosion can be amazing and fast.

Here's a tip from my (long ago and partially mis-spent) youth, when I was into sports and nature photography -- remove the bezel (or use a filter/tip off cover), take a heavy duty plastic zipper bag, put the light in it, put the bezel or filter back on the light so that it cuts a hole out of the bag, close up the bag and you're good to go in most wet conditions. I used to do that by using a screw on filter over the camera lens. It cuts out a perfect hole and leaves what's in the bag easy to use. It is not waterproof, but it is certainly weather resistant and very cheap.

Be sure to try a few different size bags since getting one that's too tight makes it hard to use the switch and to be able to focus the beam. Trust me, the first one you choose is probably going to be too small, so play with a few different sizes of bags to see what works best. Sometimes one that seems way too big actually is the best choice -- remember, the bezel or filter is going to firmly hold the bag to the business end of the light so extra room won't be as big a deal as you might think it would be and works to your advantage by making it easier to work the light. I used to use this trick with a Nikon camera with no problems and could use the camera easily in a downpour, and it works well with most lights, too.

I have used the zip-lok plastic bags to take pictures outside in the snow and rain with no problems, I always put the bag between a filter and the lens, or between a lens shade and the filter, ( this keeps rain and snow off the filter )
I would not go underwater with this set up.

For a flashlight I would just put the light in the bag and close it up. shine the light through the bag, you will lose some brightness, but the light will be protected.

now - let me put this into perspective, I generally use a SLR camera - value - over $300. flashlight - generally under $20

the flashlight can be taken apart fairly easily (I use Maglites ) and dried off. The only area that would be tricky is the switch.

I have a light setting in the sink right now....underwater and lit....Cleaning mud off of it.

I also tried the trick of changing a AAA battery in a light underwater and it worked fine.

If the light is potted properly, you shouldn't have problems in the water with your light.

Like others have said..... pure water is safe but the impurities in and around the water that enters can conduct and unless the light is designed to fend off water rust/corrrosion is a big issue. You could dunk a light in pure water and the light itself may have dust/debris in/on it that could be washed into the circuit. Any parts that are subject to oxidation (rust) would be at risk, sometimes the rust can either insulate the circuit or connect it causing the light to be unable to work or slowly draining the batteries. I have had a few batteries rust in lights perhaps due to condensation caused by humidity and temperature pressure changes drawing moisture into the light and cooking inside of it accelerating rust. A little water inside a light that turns to steam can also help to coat the reflector and debris or dust can stick on it like dirt on the inside of a car windshield and trying to remove the dirt can scratch or even destroy the reflector. A super high output bulb that is runnin and very hot may possibly even break when subjected to cold water suddenly.

lynx arc says it best....

If you were electricity would you choose a nice, highly conductive path of aluminium or steel, or would you choose the hard,resistive path of water?

The main problems of water and moisture comes from corrosion, the PCB breaking down and the lamp exploding.

I think someone on these forums exploded an M3 in their bath

If I were lazy like direct current, and always following the path of least resistance, (and I am lazy) I'd cut straight around the battery through the water bypassing the wires and switches and such. if there was very little resistance in your lamp I'd consider going in there I suppose, but it's way easier to just short the battery, especially when your switch is in the off position. Water is BAD for flashlights and electronics period.

:touche:

There is a thread on this board somewhere I think about two years ago where someone ran and E2e completely flooded with water. It ran fine and for some time with no ill affects.

To be completely accurate, water doesn't conduct electricity at all, the current is carried only by impurities in the water.

The fewer the impurities, the less the conductivity, thus tap water is never going to be a great conducter. Better than distilled, but nowhere near as good as say, the brown water found in a bucket full of rusty nails.

Still, even though tap water isn't a great conductor, it still has enough impurities to conduct so I don't recommend testing this by making toast in the tub.

Salt water is very bad for flashlights, batteries can short, the plating on the reflector can go away, electronics can die and everything that can corrode will do it much faster when there is saltwater and electric current flowing.

I've had all of the above happen with dive-lights that had faulty O-rings or other problems.
Often lights will work for a while, but the salt and humidity will corrode contacts, reflectors and everything else.

So clean fresh water is usually no real problem, but the more inpurities the water contains the worse it gets.