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

 

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.

 

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

 

Your input amp is not setup as a diff amp, because your R values
do not seem correct so the CM rejection very poor....

http://www.analog.com/en/analog-dialogue/articles/deeper-look-into-difference-amplifiers.html

Plus you might want to consider a 3 OpAmp IA due to z levels
in bridge.

More than you want to know about IAs -

http://www.analog.com/en/education/education-library/dh-designers-guide-to-instrumentation-amps.html


Regards, Dana.

 

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.

 

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.

 

Sure thing.

Here is my circuit schematic:

 

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:

 

For example - how this circuit changes frequency with water resistance change from 1.5M to 500k:
View attachment 150998

Thanks, this circuit works in simulation. I will have to check it, I will buy components & check it out on bread board.