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  #1  
Old 08-18-2011, 01:23 PM
Machine Elf Machine Elf is offline
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how conductive is rain water?

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?
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  #2  
Old 08-18-2011, 01:35 PM
randomface randomface is offline
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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.
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  #3  
Old 08-18-2011, 01:36 PM
johnpost johnpost is online now
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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.
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  #4  
Old 08-18-2011, 01:37 PM
gazpacho gazpacho is online now
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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.
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  #5  
Old 08-18-2011, 01:39 PM
VOW VOW is offline
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Duct tape!


~VOW
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  #6  
Old 08-18-2011, 01:59 PM
boytyperanma boytyperanma is online now
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Quote:
Originally Posted by gazpacho View Post
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.
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  #7  
Old 08-18-2011, 02:17 PM
panamajack panamajack is offline
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Quote:
Originally Posted by boytyperanma View Post
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 S/cm or better. That's negligible for a wet fuse box.
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  #8  
Old 08-18-2011, 02:24 PM
Exapno Mapcase Exapno Mapcase is offline
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Lightning seems to conduct pretty well in rain.
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  #9  
Old 08-18-2011, 02:38 PM
Machine Elf Machine Elf is offline
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Quote:
Originally Posted by Exapno Mapcase View Post
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.

Last edited by Machine Elf; 08-18-2011 at 02:40 PM..
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  #10  
Old 08-18-2011, 07:30 PM
IAmNotSpartacus IAmNotSpartacus is offline
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Quote:
Originally Posted by Exapno Mapcase View Post
Lightning seems to conduct pretty well in rain.
Lightning conducts pretty well in the absence of rain as well. What's your point?
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  #11  
Old 08-18-2011, 07:37 PM
mac_bolan00 mac_bolan00 is offline
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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.
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  #12  
Old 08-18-2011, 07:39 PM
california jobcase california jobcase is online now
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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).

Last edited by california jobcase; 08-18-2011 at 07:44 PM..
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  #13  
Old 08-18-2011, 08:16 PM
Exapno Mapcase Exapno Mapcase is offline
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Quote:
Originally Posted by IAmNotSpartacus View Post
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.
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  #14  
Old 08-18-2011, 11:31 PM
Fubaya Fubaya is offline
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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.
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  #15  
Old 08-19-2011, 07:26 AM
Simple Mind Simple Mind is offline
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Quote:
Originally Posted by california jobcase View Post
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.
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  #16  
Old 08-19-2011, 11:11 AM
Crafter_Man Crafter_Man is offline
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Quote:
Originally Posted by mac_bolan00 View Post
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.
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  #17  
Old 08-19-2011, 11:47 AM
engineer_comp_geek engineer_comp_geek is offline
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Quote:
Originally Posted by Crafter_Man View Post
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.

Quote:
Originally Posted by Simple Mind View Post
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.
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  #18  
Old 08-19-2011, 12:19 PM
ZenBeam ZenBeam is offline
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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.
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  #19  
Old 08-19-2011, 04:10 PM
chorpler chorpler is online now
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How does the conductivity of pure water compare to the conductivity of air?
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  #20  
Old 08-19-2011, 04:37 PM
lazybratsche lazybratsche is online now
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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.
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  #21  
Old 08-19-2011, 05:37 PM
WarmNPrickly WarmNPrickly is offline
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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.
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  #22  
Old 08-19-2011, 07:36 PM
panamajack panamajack is offline
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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.

Last edited by panamajack; 08-19-2011 at 07:37 PM.. Reason: extraneous *
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