Conductance of water versus frequency

Hi

I'm working on a water sensor

The application is a pump which has a metallic surface connected to the earth wire. The sensor consists of a metal plate housed in a plastic enclousure mounted on the chassis of the pump. The capacitance measured is the one from the metal plate to the chassis - growing in value when the water is present. The sensor plate is in direct contact with the water

So with pure or low contaminated water we measure the capacitance between the electrodes taking advantage of the dielectric constant of

80 for water.

When the water is polluted or is a salt solution we measure the conductance of the water

But, I'm searching for theory/documentation for the conductance of water with a salt solution at different frequencies. I have measured the conductance using a network analyzer and have found that it seems the conductance is lower at higher frequencies. So it seems at very high frequencies (300MHz) the water begins to act as a capacitance again without significant conductance.

But I want to be sure and don't want to measure all kinds of extremes. So does anyone know of theory/books/links to descriptions of the properties of water? (conductance, dielectric value etc)

Thanks

Klaus

Reply to
Klaus Kragelund
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What are you trying to do? Are you trying to simply measure presence or absence of the water, or its composition?

If it's simply presence or absence, then use an insulated sensor, and trigger on capacitance increase at high frequency. This works if the liquid is fully conductive - as the plate spacing of the cap is now just the insulation, or if it's non-conductive - as the dielectric constant of the capacitor formed by the electrode and the motor case.

Reply to
Ian Stirling

The usual reason for measuring water with ac rather than dc is to prevent long term plating and electrochem corrosion effects.

The field is called EIS, short for electrochemical impedance spectrocsopy.

Intro at

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Don Lancaster                          voice phone: (928)428-4073
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Reply to
Don Lancaster

snip

This will confuse you

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martin

Reply to
martin griffith

The theory of the electrical conduction of water containing dissolved ions has been worked out - at least for ionic concentrations up to about 0.1 molar. The theory is good for higher concentration solutions, but the math doesn't work too well.

Search on "Warburg Impedance", which dominates the conductive impedance at low frequencies - up to about 10kHz IIRR. This has a resistive component which decreases as the square root of frequency, and a capacitative component which increases as the square root of frequency. The resistance and reactive impedance is equal at any given freuquency, so you get a constant 45 degree phase shift between voltage and current over the frequencies where the Warburg effect is dominant, which is to say where conductivity is dominated by the diffusion of ions.

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That web-site does recommend a book by Bard

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but I've never come across it. I have used an other electrochemistry text, cited here on the 6th March 2003

Bob Greef's "Instrumental Methods in Electrochemistry" from the Ellis Horwood series in physical chemistry published by the Halsted Press - now part of John Wiley and Sons - in 1985. ISBN

0-85312-875-8.

which was good enough for what I needed.

-- Bill Sloman, Nijmegen

Reply to
bill.sloman

At low frequencies the water molecules have time to realign dipoles in opposition to the electric field, thereby showing a very large dielectric constant. At frequencies higher than the reciprocal time needed for reorientation you will see a pronounced drop in dielectric constant. Then there's the Helholtz layer, ion clouds... stuff like

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Uncle Al
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Uncle Al

You f***in' ignorant, stoooopid, LYING bastard!

ROEMER,DOPPLER, MICHELSON, SAGNAC!

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"Cassini had observed the moons of Jupiter between 1666 and 1668, and = discovered discrepancies in his measurements that, at first, he = attributed to light having a finite speed."

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Einstein: "we shall, however, find in what follows, that the velocity of = light in our theory plays the part, physically, of an infinitely great = velocity."

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Get the f*ck out of the river of shit, you are the biggest TORD in it, = you lying fuckheaded cunt! Go and worship Nehemiah Scudder!=20 FUCK OFF and DIE!

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Dumbledore_

discovered discrepancies in his measurements that, at first, he attributed to light having a finite speed."

in our theory plays the part, physically, of an infinitely great velocity."

lying fuckheaded cunt!

PLONK

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Jim Thompson

"Jim Thompson" wrote in = message news: snipped-for-privacy@4ax.com...

discovered discrepancies in his measurements that, at first, he = attributed to light having a finite speed."

of light in our theory plays the part, physically, of an infinitely = great velocity."

you lying fuckheaded cunt!

*plonk*
Reply to
Dumbledore_

message news: snipped-for-privacy@4ax.com...

discovered discrepancies in his measurements that, at first, he attributed to light having a finite speed."

light in our theory plays the part, physically, of an infinitely great velocity."

lying fuckheaded cunt!

Aha! A first class turd... changes address at each posting.

No problem, I have my ways ;-)

TURD!

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Jim Thompson

Dumbledore_ wrote: [snip]

Nothing

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Behold little Fumblebore
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Uncle Al
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Uncle Al

Dumbledore_ wrote: [snip]

Nothing

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Behold little Fumblebore
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Uncle Al
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Reply to
Uncle Al

Conductance of salt water is nil at radio frequencies. At those frequencies, skin depth is generally the more relevant consideration. Submarines use radio frequencies below 30 kHz with usable penetration to about 20 meters. Attenuation, rather than conductance, may be the property you are interested in. Try a Google search on "attenuation radio waves salt water" and you should see some links of interest.

Good luck.

Chuck

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Reply to
chuck

I've only heard about such technique in soil water content sensing or when they wanted to gauge water content in crude oil. This contains some of the basics:

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A little more indepth:

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A lab meter, don't know how good it is though:

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An article about contactless sensing of sea water with different salinity:

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Let us know what the goal of the sensing is. Just the presence of water? Bottomline I believe anything that is in contact with the water won't last long. I just had my dose of reality this weekend. Part of our irrigation system broke and I thought it was only due to frost damage. Well, when ripping it out frost must have indeed torn the stem of an electric valve. However, I also found that the chlorine in our water had seriously eaten away at lots of metal and rubber that it got in contact with.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

basics:

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ensors/whatistdrfdr.html

though:

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salinity:

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I think you need to go back to the basics. Conductivity is measured in (micro) siemens (S) older units were micromhos per centimeters. deciSiemens per metre are used also. Resistivity of water is measured in ohms/cm. Conductivity meters are commonly used in hydrology, hydrogeology, soil sciences, chemistry and a host of other fields. There are also various geophysical methods for measuring different kinds of conductance and or resistance of water or soils and rock. What kind of instrument you would want to use depends on what you want to measure and for what purpose. For meters that use capacitance see the links below for examples

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JL

Reply to
Jean

basics:

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though:

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salinity:

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The sensor is made of stainless steel, so it won't corrode. We will only be detecting the presence of water or not. But since the sensor is in contact with the water we have the time constant of the capacitance and the conductance of the water to fight. With the present sensor size (about 20mm dia), the capacitance is about 60pF if in contanct with water and a 5% NaCL (salt solution) has about 10ohms of conductance (expressed in ohms)

So the timeconstant is very low demanding a very fast sensor. We are however investigating if the 5% solution is likely, we don't want to design something to be way overspec'ed.

We have two signals coming from the sensor. The capacitance and the conductance. So when we have high capacitance we say we have "water", and also when the conductance is high we also have "water". The only problem is if some wet paper shorts the sensor (sensor plates about

5cm apart), the the conductance will be medium and we have to have a well defined limit there. So therefore we are making the sensor so it can measure capacitance in very high conductance water

Regards

Klaus

Reply to
klaus.kragelund

basics:

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though:

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salinity:

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If it's good stainless it may not corrode but what can happen over a period of a few years is that mineral deposits build up on that sensor plate. Water almost always contains some solids and they'll slowly roughen up the surface even of polished stainless steel. That can speed up the mineral deposit process.

Even on our pool pump we have that happen. It only sees rather clean water but every 5 years or so the presure sensor quits and I have to scrape off the mineral deposits. Now I do that on a regular schedule so I don't have to go out there when it's freezing and come back with blue hands.

If you'd pulse it in bipolar fashion you should always get a signal. Probably you have to arrive at some compromise with the rise times and the level, for a balance between capacitive effect and conductivity. Plus some amplitude margin in the comparator to accommodate the wet paper situation. I'd start with a Schmitt version where the threshold would determine the amplitude limit.

There are some more esoteric approaches here. One that I have seen in proximity sensors was based on magnetic coupling but it should also work with a combined capacitive/conductive situation. The sensor would become part of a resonant section or the feedback path of an oscillator. The oscillator is set up so that it quits oscillating beyond a certain limit. That would become your output signal. In our case its current was used which dropped to a lower limit (set by a parallel resistor) when the oscillator quit. Kind of like a 4-20mA loop. So we always had a minimum current to be able to detect a broken wire and stop the unit when that happened.

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Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

Hah. You ought to be about 30 miles north of where you live. We've got the white stuff fairly thick on the ground and more coming down.

Is the only question whether there is or is not water in a particular vessel? Drop a diode into the vessel and run some significant current through it. Monitor the voltage across the diode. When the water hits the case of the diode it will cool it significantly and remove some of the 2.5 mV/°C drop with temperature. An identical diode outside the vessel with the same current ought to take care of the ambient temperature problem.

Jim

Reply to
RST Engineering (jw)

basics:

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though:

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salinity:

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Try

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they have worked all this out.

Also search the web for EIS as in electrochemical impedance spectroscopy.

intro at

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--
Many thanks,

Don Lancaster                          voice phone: (928)428-4073
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Reply to
Don Lancaster

No snow here. But it is rather chilly, every winter gets colder. In fact, we are slowly becoming concerned that what's left of the four cords of firewood ain't enough :-(

Two cords of wood used to be sufficient. Global warming? Hah, not here. This spring I'll order five cords...

A diode is rather slow if encapsulated. If not it'll get ripped. And it would require the water to be below a certain temperature.

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Regards, Joerg

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Reply to
Joerg

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