Diving and lightning

[Genesis]

Here's the issue.

Electricity when discharged into water dissipates depending on the conductivity of the water. Salt water has a MUCH higher conductivity than fresh.

If you discharge a point source of electricity into the water, the measured electrical field dissipates according to the conductivity of the water and the strength of the charge originally delivered.

It is the electric FIELD that is dangerous here. Why? Because the different potential between your arms, or your head and feet, provides the ability for current to flow through YOU!

If you have 1,400,000 volts at your head, and 1,300,000 volts at your feet, then the 100,000 volt difference is going to go - guess where! :boom:

Your body is more conductive than the water in virtually all cases. Therefore, it is the low-resistance path compared to the water. But even if this were not true, basic electrical theory tells us that the current carried by differing resistances all in parallel is proportional to those resistances. So if your body path is 2 ohms, and the water is 1 ohm, you take 1/3rd of the total current and the water takes the other 2/3rds.

The current flow required to disrupt your heartbeat depends greatly on how it gets in there. Its a VERY LOW value (like 5-10ma) once it manages to get inside your chest. The "across the chest" figure assumes the current comes in one arm or hand and out the other, which, along with a "in one arm and out a leg" is pretty much the worst case.

The current levels in a lightning stroke are measured with six or more figures to the left of the decimal.... not good.

In fresh water the gradient is much greater because the resistance of the water is higher. This is why a boat that has an electrical fault and is dumping 120V current into the water, in fresh water, is a death trap. The person who gets in the water from the swim platform is likely to die, even at a significant distance from the vessel, because the difference in electrical field at different points in their body is high enough to cause current to flow. The same fault in salt water might not even be detectable, although the current flow FROM THE BOAT will be much higher.

The other problem is that very small electrical currents - in the milliamp range - are enough to screw up your muscles. That's how they're signalled to contract, after all. So even if your heart isn't stopped, you could easily find yourself unable to move - or breathe.

When it comes to lightning I'm not sure it matters whether the water is fresh or salt, simply due to the current and voltage levels involved. People swimming have been killed even though they are not struck, simply due to the gradient.

If there is lightning in the area, and you can, get the hell out of the drink. Its extraordinarily unsafe for you to be diving under those circumstances.

 

[boomx5]

Are you safer the deeper you are? And, how deep will lighting strike?

 

[Aquamaniac]

No Doubt Genesis will argue with me on this (he has a habit of doing it).
But Theoretically yes, the deeper you are the more resistence you are putting between yourself and the source.
So you are safer at depth.

Dave

 

[Genesis]

the safer, but its all relative.

I could run the field strength calculations for typical sea water, but the problem is that I don't know the proper initial values and neither does anyone else (they're all guesses, since lightning is rather, uh, difficult to measure the strength of accurately )

Immersed in sea water, a potential difference of 40-50V across any two parts of your body is probably enough to kill or disable you. I use those values because they lie just outside of the "safe for wet locations" numbers that the various occupational health folks use, although they're more than a bit optimistic on that - you CAN electrocute yourself with a 12V battery under the right (or wrong, depending on your point of view) conditions. Unfortunately some part of your head (even if just your mouth near where your reg is) is typically exposed to the water, even if you're diving dry.

You are probably safer in a full dry suit with dry gloves than in a wetsuit, as there are "fewer" bad paths through your body. You're even safer in one with an integrated hood that seals around the face (as opposed to sealing around the neck.) And a "traditional" dry suit with gloves, which leaves your head and hands in contact with water, is probably better than a wetsuit, where your entire body is, but the difference there is not likely to be terribly large.

Salt water is "safer" than fresh, as I noted above, in that the field will deteriorate faster due to its greater conductivity. How much safer depends on how "fresh" is "fresh" - higher ion content makes it MORE safe, ironically.

The problem is that the "safer", "better", etc. types of designations are all very much a relative thing and when you're talking about lightning being kinda dead and very much like a boiled hotdog don't really make much of a difference on the outcome.

If you can get out of the water, do so. If you can't, then you can't. If you're caught and can go deeper, you probably want to do that, as its all about the field strength and the further away from the point of impact with the water the better. You DEFINITELY don't want to be on the surface with your tank sticking up on what is otherwise a flat, featureless body of water!

BTW, lightning DOES hit the water. All the time. I live on it and see bolts hit the bay pretty regularly, and I've been caught out in some hellacious thunderboomers in my boat before.

 

[MikeFerrara]

Anyone ever hear of a diver being killed? I have heard od two instances of divers in the water getting a jolt but no seriouse injuries. I have seen lightning apear to hit the water many times but no dead fish why?

Lightning is a static charge. Even if there is a strike at the surface of the water that may not mean that there is a significant path throught the water.

Determining an equivilant circuit to look at may also be hard. It wouldn't be as simple as two sides of a potential difference with water between for a load.

 

[Genesis]

and the water is a pretty homogeneous thing (that is, its conductivity is pretty much constant across the area under consideration)

 

[FredT]

The Lightning Protection Institute (www.lightning.com) has some free info, but the actual standards and methods of lightning protection are sold to help defray the cost of running the Institute.

The $s for the standards are well worth the cost if you have any intrest in doing protection design. I've done several very remote sites to protect sensitive electronics that are visited only every 2 to 6 years, and then only for minimmal maintenance.

Volume resistivity is a big part of it, and so is the fact that a lot of the lightning charge runs on the surface of a conductor due to it's high frequency components. Volume resistivity is why you don't lay down during a storm. Past experience is that skin is a pretty bad surface for this surface current flow, but bones seem to work well. A direct hit tends to cook you from the inside out. OTOH more people die from a side strike than from being hit directly.

FT

 

[MikeFerrara]

From the reading I'v done it seems that as far as the earth is concerned lightning is mostly a surface thing. Which sort of explains why the fish survive it so well. There is some good stuff on the net.

 

[Scuba]

I found this on Physlink on the net.

"Another phenomena that can happen once you try to expose water to a high electric potential is that it starts ionizing, i.e. you start creating ions in the water just from the strong electric field. Then you might end up with an ion trail being created, very much like the one created in the air during lightning, where the conductivity becomes very high. This trail becomes the passage for the current with almost no resistance"

posted by Anton Skorucak

 

[Genesis]

Mike....

My dive buddy has shot a fish with a speargun, discovered it was short (oops), left it on the bottom in no current and a couple of minutes later its magically gone.

Well, ok, it wasn't really "gone". It went into something else's stomach. We just couldn't see it any more

And a fish that dies at depth doesn't usually float up to the surface. They typically sink. The reason they float when brought to the surface and die is that their swim bladder has ruptured and thus they're positively buoyant.