need help in designing a Cnductivity meter

[priyesh p p]

Hi all,
I am planning to build a conductivity meter. I am using a conductivity cell of K=5 and i want to measure conductivity in the range of 4mS/cm to 6ms/cm .While doing a back ground study i came to know that using DC voltage for cell excitation is not a good approach as there is a chance of polarization and hence we should use ac voltage.Is there any other method for exciting the cell? because using ac may make my the circuit complex.
I am planing to use a voltage divider for the cell. Is this a good method ?
Hope some one will be there to help me.

 

[chuckey]

Use a two transistor multivibrator as your AC source. 4mS = 1000/4 =250 ohms but K = 5 therefore the resistance is 50 ohms?, So you are measuring between 50 and 30 ohms.
The easiest way to do it is to use a Wheatstone bridge. Use 2 X 60 ohm resistors for one arm, the cell and a calibrated 100 potentiometer for the other. Use an opamp as a differential amplifier/ rectifier, powering a LED which goes out at balance. Or just measure the voltage across the cell, feed to an opamp rectifier which feeds a meter which you need to calibrate.
Frank