how conductive is rain water?

[Machine Elf]

I remember demonstrations from high school science classes showing that pure water, devoid of any dissolved ionic material (e.g. salt), is very non-conductive.

So what's the deal with rain water? If I have the fuse box on a motorcycle exposed to rain, would one expect the rain to cause short-circuits?

[randomface]

Yes. It takes very little amounts of dissolved solids to make water conductive. The CO2 that is in the air also will produce a mild acidic solution.

[johnpost]

rain if it is not pure water might be conductive, depends on what is in it and how much.

an object may have dry salts on it, these might be dissolved by pure water and provide a conductive path for electricity. so pure water could cause a short.

[gazpacho]

It doesn't matter how pure the rain water is. Your fuse box is dirty and the water will be contaminated once it hits the fusebox.

[VOW]

Duct tape!


~VOW

[boytyperanma]

Originally Posted by gazpacho

It doesn't matter how pure the rain water is. Your fuse box is dirty and the water will be contaminated once it hits the fusebox.

Pretty much this.

Rain water is not completely pure to begin with so is conductive. Even pure water is conductive enough to short between the fuse terminals. Even it it wasn't pure water is corrosive and would absorb enough off the fuse terminals to make it so almost immediately.

[panamajack]

Originally Posted by boytyperanma

Even pure water is conductive enough to short between the fuse terminals.

Considering that in the context of the OP, "pure water" means deionized, this isn't at all true - the conductivity is somewhere around 0.1 µS/cm or better. That's negligible for a wet fuse box.

[Exapno Mapcase]

Lightning seems to conduct pretty well in rain.

[Machine Elf]

Originally Posted by Exapno Mapcase

Lightning seems to conduct pretty well in rain.

Lightning ionizes the atmosphere itself; it's not reliant on rainwater to form a conductive path to the earth.

Thanks to everyone for clearing up the details. The science experiments were of course done with deionized water poured from a sealed vessel into a clean beaker, whereas in my OP we're talking about rain droplets that have been exposed to atmosphere for quite some time with a favorable surface-area-to-volume ratio (and so might be slightly acidic due to ambient CO2) and then likely also contain dissolved solids from whatever crud happens to reside in the fuse box. Makes sense.

[IAmNotSpartacus]

Originally Posted by Exapno Mapcase

Lightning seems to conduct pretty well in rain.

Lightning conducts pretty well in the absence of rain as well. What's your point?

[mac_bolan00]

hold it a second... water, whether pure or holding chemicals in solution, is conductive. you just have a change in resistivity (i prefer to think of it that way.) more dissolved ions lowers resistivity.

[california jobcase]

Motorcycle circuits are 12 V these days, aren't they? The amount of current that could flow through rainwater at 12 V should be negligible. It could be acid rain, however. You should dry out the fusebox to prevent corrosion. Back in the days of points and distributor caps (15,000 V and up running through mechanical contacts), water in the ignition system could be conductive enough to cause some real problems. Remember, amps = volts divided by ohms (resistance).

[Exapno Mapcase]

Originally Posted by IAmNotSpartacus

Lightning conducts pretty well in the absence of rain as well. What's your point?

Well, I could ask you where in the world you could find air without water vapor. But mostly I'd ask you if you've never heard of humor. If you don't know what it is, post #8 has an example.

[Fubaya]

I don't know about motorcycle fuseboxes, but you can put a snorkel on the air intake of a Jeep and drive it underwater (youtube link). The fuseboxes aren't sealed, they actually have holes in the side. The main concern is that they tend to float, making it hard to drive out.

If you've ever seen those tests where someone drops a car in a lake and tries to get out of it, the electric windows usually work for a couple of minutes under water as well.

[Simple Mind]

Originally Posted by california jobcase

Motorcycle circuits are 12 V these days, aren't they? The amount of current that could flow through rainwater at 12 V should be negligible. It could be acid rain, however. You should dry out the fusebox to prevent corrosion. Back in the days of points and distributor caps (15,000 V and up running through mechanical contacts), water in the ignition system could be conductive enough to cause some real problems. Remember, amps = volts divided by ohms (resistance).

I should think that water in the fuse box should not be a problem as long as there is a good connection on the terminals, the current will flow where there is the lowest resistance but if exposed for a long time you will risk corrosion.

[Crafter_Man]

Originally Posted by mac_bolan00

hold it a second... water, whether pure or holding chemicals in solution, is conductive.

No, pure water has very high bulk resistivity. The point everyone is making is that pure water almost never exists in natural settings; it is almost always contaminated with something. And if you took laboratory-grade pure water (quadruple distilled or whatever), and poured it on something, it would likely become contaminated (and thus conductive to a lesser or greater extent) in less than a second.

[engineer_comp_geek]

Originally Posted by Crafter_Man

No, pure water has very high bulk resistivity. The point everyone is making is that pure water almost never exists in natural settings; it is almost always contaminated with something. And if you took laboratory-grade pure water (quadruple distilled or whatever), and poured it on something, it would likely become contaminated (and thus conductive to a lesser or greater extent) in less than a second.

Pure water is electrically conductive to some degree, just due to the natural occurrence of H+ and OH- ions within it. The conductivity is fairly poor though since the concentration of ions is so low. This is starting to get into a battle of semantics, but it doesn't bother me if someone says pure water is conductive, especially if they qualify it by stating that it gets much more conductive when you add impurities to it. I don't know that there is actually an argument here. The two of you seem to be saying the same thing using different words.

Originally Posted by Simple Mind

I should think that water in the fuse box should not be a problem as long as there is a good connection on the terminals, the current will flow where there is the lowest resistance but if exposed for a long time you will risk corrosion.

Current doesn't just follow the path of least resistance. Current follows ALL paths. It's just that much more current will flow over the path of least resistance.

In the case of the OP, it depends on what else is in these circuits. Even impure water doesn't conduct electricity anywhere near as well as say a hunk of copper, so while you may get some current flowing, it may or may not be enough current to make anything weird happen. A headlight for example takes a lot of current to make it light, so you won't get any noticeable glow from it even though there may be a tiny amount of current flowing through it. On the other hand, devices that require a very low amount of current might switch on and do weird things.

And I also believe that corrosion would most definitely be a problem.

[ZenBeam]

For fuses that haven't burned out, both contacts will be at the same potential. I'd expect all the fuses in a fuse box to either be at ground, or at 12 Volts, not a mix of voltages. So I'd expect pouring even salt water into the fuse box wouldn't have any effect on the motorcycle's operation, until corrosion set in.

[chorpler]

How does the conductivity of pure water compare to the conductivity of air?

[lazybratsche]

Off the top of my head I know the resistance of pure water is 18 megaOhm per cm (if my lab's water system reads lower resistance, it's time to replace filters...). Wiki and a bit of mental conversion tells me that the resistance of air is around 2x10^12 megaOhm per cm. So air is roughly 11 orders of magnitude more resistive/less conductive.

[WarmNPrickly]

I read something recently that suggested some of the high resistance in ultrapure water was due to dissolved gases. When these gases were removed, it was more like 5 MegaOhms.

I'm speaking from memory, and have no cite. We use 18.6 MegaOhm water in our lab.

[panamajack]

Resistivity is commonly given in ohm-cm, not ohm/cm. This is known as "volumetric" as you can derive the resistance based on a given volume. You would take this value and multiply by the length, since length increases resistance. Then you divide by the cross-sectional area, as more area decreases resistance. Technically measured values are probably defined on a standard volume at a specific temperature as well.
And of course it's not uncommon to incompletely specify the units when everyone knows what they signify (as in the 18.6 Megaohm figure WarmnPrickly gave).

Referring to all water as conductive to a degree isn't a problem in itself, as long as you understand it apples to every material. By this reckoning, insulating materials do not exist. I'm not trying to be facetious either; it can be useful to remember what can happen under the right circumstances.