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  #1  
Old 02-04-2009, 02:10 AM
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whale whale is offline
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Default resistance of water

do water have a constant resistance ?or it is allways variable?
when i mesured the resistance of water in my water tank, at first it showed a very large resistance above 2MΩ,after when i bought the tips of multimeter inside the water close togather and took it away to earlyer position,the resistance is rapedly keeps on decreasing.
the value reached 10Ω,5Ω,3Ω,....
even it reached 0 Ω,and after it started to go to negative value.
so i confused a lot.what is the resistance of water?
in fact i need it to design a water level circuit.
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Old 02-04-2009, 02:19 AM
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http://www.newton.dep.anl.gov/askasc...0/phy00787.htm

and from http://en.wikipedia.org/wiki/Water_(molecule)


Electrical conductivity

Pure water containing no ions is an excellent insulator, but not even "deionized" water is completely free of ions. Water undergoes auto-ionisation at any temperature above absolute zero. Further, because water is such a good solvent, it almost always has some solute dissolved in it, most frequently a salt. If water has even a tiny amount of such an impurity, then it can conduct electricity readily, as impurities such as salt separate into free ions in aqueous solution by which an electric current can flow.
It is known that the theoretical maximum electrical resistivity for water is approximately 182 m/m (or 18.2 MΩcm/cm) at 25 C. This figure agrees well with what is typically seen on reverse osmosis, ultrafiltered and deionized ultrapure water systems used, for instance, in semiconductor manufacturing plants. A salt or acid contaminant level exceeding even 100 parts per trillion (ppt) in ultrapure water begins to noticeably lower its resistivity level by up to several kilohm-square meters per meter (a change of several hundred nanosiemens per meter of conductance).
Pure water has a low electrical conductivity, but this increases significantly upon solvation of a small amount of ionic material water such as hydrogen chloride. Thus the risks of electrocution are much greater in water with the usual impurities not found in pure water. (It is worth noting, however, that the risks of electrocution decrease when the impurities increase to the point where the water itself is a better conductor than the human body. For example, the risks of electrocution in sea water are lower than in fresh water, as the sea has a much higher level of impurities, particularly common salt, and the main current path will seek the better conductor. This is, nonetheless, not foolproof and substantial risks remain in salt water.) Any electrical properties observable in water are from the ions of mineral salts and carbon dioxide dissolved in it. Water does self-ionize where two water molecules become one hydroxide anion and one hydronium cation, but not enough to carry enough electric current to do any work or harm for most operations. In pure water, sensitive equipment can detect a very slight electrical conductivity of 0.055 S/cm at 25 C. Water can also be electrolyzed into oxygen and hydrogen gases but in the absence of dissolved ions this is a very slow process, as very little current is conducted. While electrons are the primary charge carriers in water (and metals), in ice (and some other electrolytes), protons are the primary carriers (see proton conductor).
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Old 02-04-2009, 02:21 AM
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Water does not have a constant value of resistance but its resistance depends on the substances (salts etc) diluted into it.
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Old 02-04-2009, 12:18 PM
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yeh,i can understand the nature of water but,for me the resistance is not varying between huge levels,it varyes between -ve ohm value and +ve very high(above 2 MEGA OHM).is ther any wrong in my multimeter.
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Old 02-04-2009, 12:46 PM
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yeh,now i can understand its problem.i'll analysis it accourding to your angle alberto,thanks.
ok,give me an idea,
i want to build a water level indicator.i thought of using conductivity of water for my project,but now the conductivity varies in vast quantity.what can coincide my problem?
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Old 02-04-2009, 01:35 PM
russ_hensel russ_hensel is offline
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Two insulated plates dipped in the water form a capicator. As the water level changes on the plates the cap. changes. You can measure this. It is not so dependentent on tempature, purity .....

Just one of many many ways. Probably more info. via google.
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Old 02-04-2009, 04:44 PM
KL7AJ KL7AJ is offline
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Hey...wanna see a really cool experiment? (Not for folks unfamiliar with safe handling of VERY high voltages!)

Get a high voltage power supply, such as an insulation tester, (20 KV or so....minuscule current necessary) Fill a beaker with distilled water. Put a negative electrode (ground) in the bottom of the beaker. Suspend (you might have to build a little jig for this) a metal spike (this can be either a highly sharpened nail or, even better, a sharpened tungsten rod) about 1/4 inch above the surface of the water. Connect the high voltage lead to the spike. Turn on the power. The top of the water will DEPRESS, just like a magic finger is pushing down on that. If you REVERSE the polarity of the power supply, a little haystack shaped bump will rise above the surface of the water. Above a certain height, the haystack will collapse, creating a vapor, which can sustain a very stable arc to the top of the water. This will actually create a rather clean radio frequency oscillation!

Something for the mad scientists in our midst.

eric
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Old 02-04-2009, 07:03 PM
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Quote:
Originally Posted by KL7AJ View Post
Hey...wanna see a really cool experiment? (Not for folks unfamiliar with safe handling of VERY high voltages!)

Get a high voltage power supply, such as an insulation tester, (20 KV or so....minuscule current necessary) Fill a beaker with distilled water. Put a negative electrode (ground) in the bottom of the beaker. Suspend (you might have to build a little jig for this) a metal spike (this can be either a highly sharpened nail or, even better, a sharpened tungsten rod) about 1/4 inch above the surface of the water. Connect the high voltage lead to the spike. Turn on the power. The top of the water will DEPRESS, just like a magic finger is pushing down on that. If you REVERSE the polarity of the power supply, a little haystack shaped bump will rise above the surface of the water. Above a certain height, the haystack will collapse, creating a vapor, which can sustain a very stable arc to the top of the water. This will actually create a rather clean radio frequency oscillation!

Something for the mad scientists in our midst.

eric
That's cool. Is the frequency of the RF near constant or does it vary based on other variables?

Lefty
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Old 02-04-2009, 08:53 PM
KL7AJ KL7AJ is offline
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Quote:
Originally Posted by leftyretro View Post
That's cool. Is the frequency of the RF near constant or does it vary based on other variables?

Lefty

It's based on a lot of variables, but once it's "lit" it stays in the general ballpark. It's actually a very crude approximation of the Poulson Arc.

eric
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