# Measuring conductivity for wheatstone bridge

#### chaitannya

Joined Apr 10, 2018
15
I have the following circuit to measure the conductivity of water ( aim is to measure dissolved minerals concentration), I have taken this circuit from this research paper. I have checked the solutions for which I want to measure conductivity & the range is roughly from 800k - 1.5M ohms.
My aim is to increase the range of output from the wheatstone bridge, which right now is almost flat & non-linear. The graph shows sweep output of RC resistance from 500K - 1.5M with the at IP1 & P1 respectively.
How can I do that? I have observed this same behaviour in the actual circuit that I had built.

Regards,
Chaitannya Mahatme

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#### MrChips

Joined Oct 2, 2009
20,245
Firstly, before you go any further, you need to get your definitions and units correct.
The unit of conductance is siemens (S), not ohms.
Ohm is the unit for resistance.

The units of conductivity is S/m, in contrast to resistivity which is measured in units of Ω-m.

#### chaitannya

Joined Apr 10, 2018
15
Firstly, before you go any further, you need to get your definitions and units correct.
The unit of conductance is siemens (S), not ohms.
Ohm is the unit for resistance.

The units of conductivity is S/m, in contrast to resistivity which is measured in units of Ω-m.
Is there a way to define conductivity in the simulation, instead of resistor? If not I would stick with resistance for time being.

Regards,
Chaitannya

#### MrChips

Joined Oct 2, 2009
20,245
I have measure electrical conductivity of water successfully using a different approach.
Your first concerns are the material used for electrodes and how to avoid polarization and electrolysis.
You have to perform measurement with an AC method in order to prevent polarization.
For electrodes, carbon or platinum are common materials used.

For AC measurement, I used an RC oscillator and measured the frequency of oscillation. The capacitance C is fixed while the resistance R is the resistance of the solution between the electrodes. Conversion to electrical conductivity is done by calibration with standard solutions.

#### chaitannya

Joined Apr 10, 2018
15
I have measure electrical conductivity of water successfully using a different approach.
Your first concerns are the material used for electrodes and how to avoid polarization and electrolysis.
You have to perform measurement with an AC method in order to prevent polarization.
For electrodes, carbon or platinum are common materials used.

For AC measurement, I used an RC oscillator and measured the frequency of oscillation. The capacitance C is fixed while the resistance R is the resistance of the solution between the electrodes. Conversion to electrical conductivity is done by calibration with standard solutions.
For electrodes I am using gold plated rings, I have to measure conductivity of flowing water.

Can you share the range & accuracy parameters of your schematic?
It would be great if you can share schematic.

#### MrChips

Joined Oct 2, 2009
20,245
Sure thing.

Here is my circuit schematic:

#### Danko

Joined Nov 22, 2017
870
Can you share the range & accuracy parameters of your schematic?
It would be great if you can share schematic.
For example - how this circuit changes frequency with water resistance change from 1.5M to 500k:

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