Mastech Multimeters

[i4004]

Re: Mastech Multimeters

who's too young for analog meters and problems of electricity in water?
<wink>

http://www.ecmag.com/index.cfm?fa=ar...articleID=5031

now something is fishy to me there
"The shocking currents were the result of potential gradients, or voltage differences between their feet in the water. The fault current traveled up one leg, flowed through the torso of a victim, and then exited out of the other leg back into the water."

but what potential gradient are we talkign about? what amount of voltage can you get there? 1v? how will that push current thru you?
other explanation: why the torso? it's shorter if you don't go thru(and reach heart) torso and electricity "knows" that.
but overall these explanations left me unimpressed.

i have simpler explanation; big portions of human flesh is water(up to 60%
http://ga.water.usgs.gov/edu/propertyyou.html ) so in "energized" water humans just become one more parallel resistor....
resistance is relatively low because of wetness and it takes only low current to mess up the heart...
i don't think we need to find a voltage there at all, because we effectively become medium for current to spread...not very good one, but good enough to kill us....

[willawake]

Re: Mastech Multimeters

yeah but he is talking about galvanic corrosion
on boats even we have galvanic corrosion of aluminium via carbon fiber galvanic coupling with other more noble metals
also have crevice corrosion of stainless steel or aluminium

basically means expensive and maybe dangerous rot

in some marinas they cant even give away some berths because of stray current



what happens when you coat a nice steel boat with copper based antifouling designed for GRP

[i4004]

Re: Mastech Multimeters

how much faster is that corrosion to normal corrosion rates?

[willawake]

Re: Mastech Multimeters

the one in the picture occurred in one year reducing steel that was in good condition to what must be replaced. a very expensive mistake to apply antifouling with this more noble metal

if the corrosion is fast then it is usually from current leakage. the above example is a special case and not due to current leakage.

so yeah if current leakage is bad then the propellers can be at a stage that they will need replacement in perhaps one year

in normal conditions sacrificial anodes are protecting the shaft and propeller or also the steel or aluminium hull if the boat has that. usually zinc in seawater or magnesium in fresh water. these are replaced when they are 50% consumed http://en.wikipedia.org/wiki/Sacrificial_anode
there should be an anode on the shaft or otherwise it should be connected with wire to the shaft. sometimes this wire is missing

with crevice corrosion it is not seen until the bolt or screw is pulled and if inspection is neglected it could fail with dangerous results.

stainless steel components and aluminium masts is a problem. should be separated with zinc chromate paste or silicone.

all this stuff is a major problem with boats and the only basis for diagnosis is the galvanic series and basic electrical knowledge. not so difficult...yet many builders screw up.

some info
http://www.yachtsurvey.com/corrosion.htm

http://www.yachtsurvey.com/corrosion_in_marinas.htm
http://www.azom.com/details.asp?ArticleID=1177
http://www.boatus.com/seaworthy/galvanic/default.asp

[arneson]

Re: Mastech Multimeters

Ah ha,
All those lessons taught you a lot Will. That stuff is a whole science to itself and really specialized.
The propellers you mention could cost more than my house.

[willawake]

Re: Mastech Multimeters



here is corrosion meter from http://www.pmariner.com/
it looks like a $3 multimeter. the interesting bit is the sex toy lookalike item. I believe this is a "half cell" AG/AgCl. It sells for $300

can anyone explain the half cell?

[willawake]

Re: Mastech Multimeters

ooh even fluke is into it

[i4004]

Re: Mastech Multimeters

>it looks like a $3 multimeter.

it IS 3$ meter. which means that sexy thing is 297$, then.
what's inside?
gold?
well, silver anyway
http://en.wikipedia.org/wiki/Silver_Chloride_Electrode

[willawake]

Re: Mastech Multimeters

well, apart from the material it has specific market (all instruments especially where boats are concerned so damn expensive) but yeah the price seems normal according to the other ones i saw. nice to hang $300 over the side of a boat.