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Salt Water Resistance
Dear List,
I got bored last night and did a simple experiment with salt water and a
cheap digital multimeter (DMM). I dipped the DMM probes into about a
quarter cup of tap water and measured it's resistance at around 1600k. The
resistance was about the same whether the probes were 1/4" or 2" apart (the
width of the cup).
Next, I stirred a dash of table salt into the cup, and the resistance
dropped to about 865k. Hmmm.
Finally, I added salt until no more would dissolve. This solution ohmed
out at around 26k. It wouldn't go any lower. I got inspired and poured
the brine into a stainless steel measuring cup. The dipped leads still
read about 26k.
I pressed the point of one probe against the metal bottom of the cup and
dipped the other in. Resistance fell to 5k. I pressed the opposite lead
to the bottom and allowed the other one to dip, checking to see if there
was a favored polarity--nope. Still read about 5k. That's a pretty
dramatic improvement over 26k!
Conclusions? (1) Salt water is a pretty lousy conductor. (2) At least at
1.5vdc, salt water resistance seems largely to be a function of contact
area. (3) I've got to find something better than plain salt water--NaCl
plus epsom salts perhaps? Copper Sulphate?
I don't know how relevant my experiment is at 15kvac. For my own part, my
next generation beer bottle caps will take contact area into account. The
"wet" electrodes will be something with lots of surface area, like 3/8"
threaded brass rod or even 1/2" copper pipe, and long enough to reach to
the very bottom of the bottle.
Greg