How to measure resistance of water?

Thread Starter

sycolax

Joined Feb 16, 2020
3
Why dont you use a multimeter?
We want to design an electrolytic tank using tab water in-order to that we need to know the resistance of the tab water so we can design the tank dimensions, moreover, multimeter doesn't work it gives unstable reading between 250-350 ohms for 1 liter of water
 

jpanhalt

Joined Jan 18, 2008
8,725
I don't quite understand. When I do electrolytic rust removal, I use "tap" water ( not "tab" water). If I were plating shiny whatever, I would be more selective in the water I used, even if it didn't make a difference.

If the issue is simply measuring the resistance of water, there is plenty advice on the web. My advice would to perform the electrolysis at whatever voltage gradient (i.e., V/cm) is needed and add electrolyte as needed to maintain the current you want, which is usually determined by the area of an electrode. In an electrochemical cell, low conductivity water can do amazing things if you use constant current regardless of voltage -- particularly if the matrix is a gel.
 

Reloadron

Joined Jan 15, 2015
5,118
We want to design an electrolytic tank using tab water in-order to that we need to know the resistance of the tab water so we can design the tank dimensions, moreover, multimeter doesn't work it gives unstable reading between 250-350 ohms for 1 liter of water
It's about using two probes at a set distance. Note the use of centimeter in the descriptions:

Resistivity - the tendency of water without ions to resist conducting electricity.

The unit of measure is megohm-centimeter (MΩ-cm), often shortened to MΩ or "meg". It is generally used for high purity water. The theoretical maximum is 18.2 MΩ-cm at 25°C. The higher the ionic content - the lower teh resistivity and the lower the ionic content - the higher the resistivity (high resistivity is good!). In ultrapure water systems this value is determined using an in-line meter. Conductivity and resistivity measurements are inversely related to each other.

Conductivity - the tendency of water that contains ions to conduct electricity.

The unit of measure is microsiemens/centimeter (µS/cm) or microhm/cm. The measurement is used to measure feed water or lower quality treated water. The more ions present in the water, the higher the conductivity. This is measured by a conductivity meter.

Resistance and conductance are reciprocals. A conductivity probe is used. The electrical conductivity of a solution of an electrolyte is measured by determining the resistance of the solution between two flat or cylindrical electrodes separated by a fixed distance. An alternating voltage is used in order to avoid electrolysis. The resistance is measured by a conductivity meter. This is not as simple as shoving two probes from a DMM set to measure resistance into a glass of tap water. Tap water alone, here in the US will vary between cities and can change day to day. I assume where you use tab water you mean to say tap water?

Ron
 

SamR

Joined Mar 19, 2019
1,803
I used to demonstrate electrolysis to my chemistry students. Using tap water in the apparatus, it would only generate less than half a cubic centimeter each of Hydrogen and Oxygen in 24 hours due to the water's very low electrical conductivity. So I would add a couple of grams of NaCl (salt) to a couple liters of water to increase the conductivity and the volume would go up very significantly to a few cc's.
 

MrChips

Joined Oct 2, 2009
20,215
Resistance depends on the dimensions of the tank whereas resistivity does not.
Conductivity is 1 divided by resistivity.
It is common practice to quote conductivity.
Conductivity of water varies depending on dissolved substances and temperature.
A resistance of 250-350 ohms per litre is meaningless since it would depend on the shape and dimensions of the tank.
 

Thread Starter

sycolax

Joined Feb 16, 2020
3
It's about using two probes at a set distance. Note the use of centimeter in the descriptions:

Resistivity - the tendency of water without ions to resist conducting electricity.

The unit of measure is megohm-centimeter (MΩ-cm), often shortened to MΩ or "meg". It is generally used for high purity water. The theoretical maximum is 18.2 MΩ-cm at 25°C. The higher the ionic content - the lower teh resistivity and the lower the ionic content - the higher the resistivity (high resistivity is good!). In ultrapure water systems this value is determined using an in-line meter. Conductivity and resistivity measurements are inversely related to each other.

Conductivity - the tendency of water that contains ions to conduct electricity.

The unit of measure is microsiemens/centimeter (µS/cm) or microhm/cm. The measurement is used to measure feed water or lower quality treated water. The more ions present in the water, the higher the conductivity. This is measured by a conductivity meter.

Resistance and conductance are reciprocals. A conductivity probe is used. The electrical conductivity of a solution of an electrolyte is measured by determining the resistance of the solution between two flat or cylindrical electrodes separated by a fixed distance. An alternating voltage is used in order to avoid electrolysis. The resistance is measured by a conductivity meter. This is not as simple as shoving two probes from a DMM set to measure resistance into a glass of tap water. Tap water alone, here in the US will vary between cities and can change day to day. I assume where you use tab water you mean to say tap water?

Ron
Thank you! that was very helpful, yes (tap** water)
 
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Janis59

Joined Aug 21, 2017
1,057
Simply bought a 25 USD cheap device from China what is produced for conductometric control of distilled water. Works ideally. Only after that I found that the tap water of 100 KOhm/cm becomes to the 10 kOhm/cm after my rrussia made destillator. :) :) Thus I scraped off all the calcium and it raised near the 1 MOhm, whilst the spare bi-destillator made in Germany (glass everywhere) gives about 20 MOhm.
 

MrChips

Joined Oct 2, 2009
20,215
Please note that the units of resistivity would be Ω⋅m, not Ω/m.
Conductivity is the reciprocal of resistivity and the units are S/m.
 

Janis59

Joined Aug 21, 2017
1,057
RE: Mr chips
Yes, in general resistor wire it is sacred truth. However for case of water tin, the surrounding volume is large as much one may want, thus the eternally diminishing sum is convergent to certain value what is not much dependant of cross section area at all, therefore in praxis who cares about. But yes, more correct is multiply.
 

Reloadron

Joined Jan 15, 2015
5,118
Please note that the units of resistivity would be Ω⋅m, not Ω/m.
Conductivity is the reciprocal of resistivity and the units are S/m.
This is a little off topic but like many here in the US I grew up with the unit of conductivity being the Mho which everyone knows is Ohm spelled backward. Thought that was cool right till Siemens came into my life. :)

Ron
 

jpanhalt

Joined Jan 18, 2008
8,725
Is there any other SI unit that is simply the reciprocal of another SI unit? Why should it be an SI unit when something as useful as an inch or psi or torr aren't?
 

Reloadron

Joined Jan 15, 2015
5,118
At around 13? :p
"The siemens (symbolized S) is the Standard International (SI) unit of electrical conductance. The archaic term for this unit is the mho (ohm spelled backwards)".

The archaic term? Archaic is things like The Pyramids, Sphinx, West Wall or Rome Coliseum. I learned Mho and I am getting tired of people screwing around with naming conventions. I learned Ohms Law as E = I * R but no, we had to change that and make V in there for E. They never consulted with me about that. Then if all of that wasn't enough trauma in my life Oh yeah and then I learned enough to get my first amateur radio license at 13. All my radio dials simply said Kilo-Cycles or Mega-Cycles and even some said Kilo-Mega-Cycles and we just had to screw with that. Now for those who needed it further defined we needed Hertz. Note how again the SI (System International) had to change my life. The hertz (symbol: Hz) is the derived unit of frequency in the International System of Units (SI) and is defined as cycles per one second. It is named after Heinrich Rudolf Hertz, the first person to provide conclusive proof of the existence of electromagnetic waves. All we did was change the name again. What's with that anyway?


Is there any other SI unit that is simply the reciprocal of another SI unit? Why should it be an SI unit when something as useful as an inch or psi or torr aren't?
Yeah, and what's with that? I think it's part of a large plot or movement. I have noticed those damn black helicopters and now drones around my house, just hovering. I think they are watching me. I flipped one off the other day and await the consequences. :)

Keep in mind this little rant is all done in humor just in case anyone actually would believe I was serious. I am beginning to feel my age a little though.

Ron
 
Buy one. Here is a meter schematic, minus stuff that makes it work like bypass caps, layout etc. https://www.analog.com/en/analog-dialogue/articles/fully-automatic-self-calibrated-conductivity-measurement-system.html

Everything makes sense. I built systems that could measure the resistance/conductance of paper and watch it be affected by humidity.

The resistivity is high, so it' s better to use and I-V converter and a voltage source. When it's low it;s better to pass a current between the outer pads and measure the voltage of the inner pads. All normal stuff for bulk resistivity of a solid. Thin films are a lot different.

Big differences here is:
1. you have to use a "nice electrode".
2. You have to use an AC voltage and hope that the plating and removal happens at the same rate.
3. DC offsets cause plating of your electrodes over time.
 

MrChips

Joined Oct 2, 2009
20,215
"The siemens (symbolized S) is the Standard International (SI) unit of electrical conductance. The archaic term for this unit is the mho (ohm spelled backwards)".

The archaic term? Archaic is things like The Pyramids, Sphinx, West Wall or Rome Coliseum. I learned Mho and I am getting tired of people screwing around with naming conventions. I learned Ohms Law as E = I * R but no, we had to change that and make V in there for E. They never consulted with me about that. Then if all of that wasn't enough trauma in my life Oh yeah and then I learned enough to get my first amateur radio license at 13. All my radio dials simply said Kilo-Cycles or Mega-Cycles and even some said Kilo-Mega-Cycles and we just had to screw with that. Now for those who needed it further defined we needed Hertz. Note how again the SI (System International) had to change my life. The hertz (symbol: Hz) is the derived unit of frequency in the International System of Units (SI) and is defined as cycles per one second. It is named after Heinrich Rudolf Hertz, the first person to provide conclusive proof of the existence of electromagnetic waves. All we did was change the name again. What's with that anyway?


Yeah, and what's with that? I think it's part of a large plot or movement. I have noticed those damn black helicopters and now drones around my house, just hovering. I think they are watching me. I flipped one off the other day and await the consequences. :)

Keep in mind this little rant is all done in humor just in case anyone actually would believe I was serious. I am beginning to feel my age a little though.

Ron
That's nothing like radiation units. I work in nuclear physics and we've had to learn and then drop curie, rad, rontgen, rutherford and replace them with gray, sievert, becquerel.
 

Reloadron

Joined Jan 15, 2015
5,118
That's nothing like radiation units. I work in nuclear physics and we've had to learn and then drop curie, rad, rontgen, rutherford and replace them with gray, sievert, becquerel.
See what I mean. Hey on a radio active note I just read where my company I retired from just got another US One Billion Dollar contract with the US Navy. I like solvent, keep those retirement checks coming. :) Anyway, yes, see what I mean? People just aren't happy unless they screw with things. Pluto is still a planet in my solar system! So There!

Ron
 
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