The Resistance Of Water

[nadiro]

I want to measure the resistance of water. How can I do it?

once I tried it by puting the probes of multimeter in a glass of water and it showed the resistance about the range of 50kΩ-800kΩ and it wasn't constant, it went up and down and didn't have specified amount!
What is the reason of this change?

Best regards

 

[andre_luis]

Take a look on the thread bellow on which this subject was already discussed, and alternative approaches were suggested to achieve this task:

https://www.edaboard.com/threads/314091/


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[Audioguru]

My tap water comes from a fresh water lake. But it has the salts of many minerals in it which causes it to have some resistance.
Distilled water is pure so it has no resistance.

 

[esp1]

Distilled water is pure so it has no resistance.

hi agu,
Do you mean a very high or infinite resistance.?

Eric

 

[Audioguru]

hi agu,
Do you mean a very high or infinite resistance.?

The opposite of a superconductor. Infinite resistance.

 

[albbg]

The opposite of a superconductor. Infinite resistance.

It's better to refer to resistivity.
To be precise, the resistivity of pure water is very high but not infinite. The accepted value is 18.2 Mohm-cm at a temperature of 25 degrees Celsius. However it depends from the temperature going from rouglhy 86 Mohm-cm at 0 degrees down to 1.3 Mohm-cm at 100 degrees

 

[andre_luis]

It would be important @nadiro mention for what kind of application intend to measure the resistance of the water so that we can evaluate alternative solutions to the original problem.




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[goldsmith]

I want to measure the resistance of water. How can I do it?

once I tried it by puting the probes of multimeter in a glass of water and it showed the resistance about the range of 50kΩ-800kΩ and it wasn't constant, it went up and down and didn't have specified amount!
What is the reason of this change?

Best regards

Hi
It depends on a lot of parameters ! is that fresh water or perhaps salt water ? or perhaps it deals with a lot of compounds ?
Any way , your application and the required precision are absolutely important . furthermore , temperature of the area which you're attempting to operate on is so important too .
Nevertheless , your measurement will be based on injecting a low value current into it and measure the voltage across it . that's all i can say with your low information .

Best Wishes
Goldsmith

 

[D.A.(Tony)Stewart]

All insulators are dielectrics and water is a highly polarized dielectric.

One of the properties of polarized dielectrics is that the +conductors attract charges by ejecting mobile electrons. The other property is that in fluid dielectrics with contamination, the chemistry changes. The contamination affects the flow of charges whether it is a dissolved gas, liquid or solid. Then it depends which types of each. For liquids, acids and bases allow ion exchange easily and thus allow the flow of charges easily, such as in batteries with acid and distilled water. With solids it can be an inert dust particle that still moves easily and thus causes current with an excitation Electric Field. With Salts, it can create additional chemical ion exchanges with the H2O and makes water boil faster in a microwave oven but not in an infrared stove. So the excitation frequency affects the resistance or lossy characteristics greatly.

Pure water conducts no electricity

By pure that means 100% pure and free of salt such as Sodium Chloride (NaCl) or particulate matter ( dust ) or metals ( iron ) or dissolved gas (Methane) or bacteria (1um) or viruses or microbial contamination.

Otherwise the resistance will change as a function of excitation voltage and DC or AC frequency.,

Generally water purity tests consist of high voltage constant current sources with DC and swept AC frequency to determine the conductivty and tan delta phase shift (and break frequency) and dielectric constant.

So before you try to measure water, decide what it is you wish to detect in the way of impurities and how much ( ppm or %)

 

[nadiro]

I just want to know what is the resistance of drinking water?

How much should it be? Why dosen’t multimeter show a specified amount and just keep changing and changing…

What dose the resistance of water depends on at a temperature of 25 degrees Celsius?
For example does it related to the probes be close to or far from eachother?

Is there any thing related to this formula: R=ρ l/A ?

An important question is that is it correct way to measure the resistance of water by multimeter? If so How does multimeter do this? What happens inside it?

Best regards

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Nevertheless , your measurement will be based on injecting a low value current into it and measure the voltage across it .
Goldsmith

Hi
good idea...
but how I can inject current into water and measure the voltage?
By just putting the probes into a glass of water?
Thanks a lot

 

[esp1]

How much should it be? Why dosen’t multimeter show a specified amount and just keep changing and changing…

hi nadiro.

The DVM probes will have the internal DVM battery voltage across them, so consider electrolysis at the probes, build up of hydrogen and oxygen bubbles, which will effect the measured resistance.

E

 

[andre_luis]

Why dosen’t multimeter show a specified amount and just keep changing and changing...

You are dealing with a material that has a fluid nature, and certainly is not under mecanical static conditions, but moving, which prevents the progressive evolution of a concentration of ions near the probes due to electrolysis, becoming measurement somewhat cahotic.



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[FvM]

As already explained, the resistance of water must be measured with AC voltage to avoid disturbance by electrolysis effects. A multimeter can't do this without additional equipment.

The "resistance" of water is usually specified as conductivity or resistivity to be independent of an electrode geometry. See https://en.wikipedia.org/wiki/Conductivity_(electrolytic)

The Wikipedia article also gives an expectable range of water conductivity. It varies over several decades depending on mineral content.

 

[very question]

Hi ...
I have done an experiment by a multimeter.
In it I faced with an increasing number(up to 2M ohm) that I get from the multimeter that was put in a glass of water.
you said that we can't measure the resistance of water by a multimeter .
now I wont to know what is this number?
and also how I can measure this actual resistance with AC voltage?
( I want to know the resistance of a glass of water.)

 

[nadiro]

Hi
Thank you for your participating in this topic.

Interesting…
A new question is coming to my mind,actually I know this question isn’t related to this topic but it comes to my mind suddenly after thinking about the properties of water.

The question is:
Why is intensity of electric shock greater when hands are wet?

 

[andre_luis]

Why is intensity of electric shock greater when hands are wet?

Because water is a universal solvent for a wide range of substances, partucularlly the salt that is the most abundant ion present on the human body, making the aggravating effect by the sweat glands in the superficial layers of the skin.



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[goldsmith]

Hi
good idea...
but how I can inject current into water and measure the voltage?
By just putting the probes into a glass of water?
Thanks a lot

Hi again
Simply design or use an oscillator and then inject it via two probes and then measure the voltage across the probes and measure the current too .

I suppose you know how to measure AC current and voltage isn't it ?

Good Luck
Goldsmith

 

[nadiro]

I suppose you know how to measure AC current and voltage isn't it ?

I just know for measuring AC voltage and current we use multimeter or oscilloscope which adjusted in AC mode,is there any thing more than this?

My question is, for example if we want to apply voltage across a resistance there is two junctions which we can connect the probes to them,but about water there is no junction so is it right just putting the probes in a glass of water to measure the resistance or voltage and current?

Thanks...

 

[chuckey]

You need a proper probe, this is because if you move your probes closer together, the apparent resistance will fall. Also the probe must be reasonabley insensitive to depth of immersion. So what you after is something like two discs of copper/brass/stainless steel with an insulating spacer between them to keep the distance the same fastened onto a plastic "handle " so the probes can be dipped into the water.
Put a resistor in series with one lead which must be fed with AC. With your DVM measure the supply voltage and the volt drop across the resistor, the apparent resistance of the water plus probe can then be worked out.
The "resistance" of water is expressed as a conductance in Mhos per cu Cms. Tap water is about 600µ Mhos, de-ionised water 2µ Mhos. Imagine a little cubic tank, one Cm cube, its the conductance across two opposite faces.
So if you build a test tank with bits of copper stuck to two opposite walls, fill it with your water, then you can accurately measure the conductance of your water and hence the calibration factor of your probe (K).
Frank

 

[RajivConfident]

I am confused,if the water contain different salt and minerals then that water is good condctor of electricity then the distilled water.So the currend flow more in salt and mineral water then pure water.Thats why distilled water have more resistance then salt and mineral water,isn't it?