Re: [Paddlewise] SPOT - Yes, it floats but....

Pamvetdr_at_aol.com wrote:

> I would speculate that we can force failure of the seals by using 
> excessive pressure when washing or rinsing. Also if we are using wetsuit
> shampoo on other equipment, the surfactants may enable the water to get
> past seals on electronic gear.  I am being more careful about these
> things. Do other people have any suggestions?

Ah, use of "surfactant" in a sentence ... go to the head of the class, Pam! 
  You must have been well-educated by some O-chemist back in the dim past. 
  [grin]

Seriously, perhaps the surfactants (hand soap/dish washing detergent, etc.) 
help in passing those seals, but I think the prime culprit is likely a 
pressure difference caused by temperature cycling of the unit.  Goes like 
this:  you check your gear in the warm confines of your home, making sure 
the batteries are secure, and seal up the battery compartment (perhaps 
anointing the O-ring with a small smidge of zipper lube).  Having a nice 
day, you journey the 300 miles from where you live to the BC coast, where 
the real paddling lives, pop into the motel overnight, which is overheated, 
and re-check the batteries and gear, opening and closing the compartment 
again.  Next day, you load boat and strap the GPS, the SPOT, and the 
digital camera into position on deck, whereupon while paddling they get wet 
with cold, cold water.  The water, aided by traces of those nasty 
surfactants, infiltrates next to the seals, and as the unit cools, the air 
inside also cools.  And, voila! Gay-Lussac's law comes into play, 
generating a low pressure inside the unit, most likely low enough to 
suuuuuck water into the unit.

But wait, gets worse!

Now you have moisture, maybe only a little bit, inside, and the temperature 
cycling continues as the unit sits on deck in the sun for a while, and air 
(mainly) is pushed out through the seal while you are enjoying munchies on 
a sunny beach somewhere in Clayoquot Sound.  The vapor pressure of water 
being a positive function of temperature, some of the liquid inside 
vaporizes, increasing the positive pressure inside, removing more air.

And, worse, yet!

Back in the water you go, perhaps snapping off one of Pam's patented 
layback rolls, and the unit gets immersed again, in that cold, cold BC 
water, the air inside cools, _and_so_does_the_water inside.  Its vapor 
pressure tracks downward, reinforcing the Gay-Lussac's law effect, and now 
the reduced pressure inside is even lower than it was before.  More liquid 
water intrusion occurs.

And, worst of all!

The salty water forms a bridge across one of the positive supply contacts 
from the battery supply to a ground near it, the battery says, time to push 
electrons through that low resistance path, and it heats up the inside of 
the unit more, perhaps frying its innards now, or maybe not  Maybe later 
after a couple more cycles.

In short (hehe!), these units should be rated for _temperature_cycling_ as 
well as pressure differentials.

I'd bet dollars to surfactants it is the former which kills the majority of 
the units which die.  My Garmin GPS Map60C has a battery compartment with a 
separate seal from the remainder of the unit.  The remainder of the unit 
has a little Goretex-backed seal hole somewhere (I think) to allow venting 
of the unit.  But, unlike its more expensive cousin, the Map60CSx, it will 
survive.  The cousin has a data card, behind the battery, and access to the 
data card is achieved by breaking that seal into the inside of the GPS. 
Two friends say they have lost a 60CSx from water intrusion, and claim they 
did not do anything wrong.

YMMV.

And, yes, I am waiting for the sun to come out so I can go out and work on 
my boat!

-- 
Dave Kruger
Astoria, OR
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