Conductivity Sensor circuit interface to ADC

[a8811323]

Hi,
I bought a commercial conductivity sensor (LFS 117 from IST). I am trying to build a circuit for water conductivity measurement. According to equation sigma (conductivity) = k*I/U, where k is a cell constant and I is the applied current and U is the voltage measurement by using 4-point probe.

Initially I am thinking to build a fixed ac current source and a voltage sensor. However, as the conductivity relationship is inversely proportional to the voltage, as conductivity increases the voltage becomes so small and the error will become significant. Then the best way to measure the conductivity is by fixing the measured voltage, and sense the current. Is there any good way to do so? I am thinking using dual-op amp to fixed the voltage across the two measured voltage pins. However, I can not think of a good way to sense the current.

Can anyone give me some advice or is there any existing commercial product can do the job with data acquisition system?

Regards
Chenjin

 

[FvM]

Then the best way to measure the conductivity is by fixing the measured voltage, and sense the current. Is there any good way to do so? I am thinking using dual-op amp to fixed the voltage across the two measured voltage pins. However, I can not think of a good way to sense the current.

It's in fact much easier to apply an AC voltage and measure the current. To support all possible cell resistance values up to short, you'll supply the voltage by a voltage source with output impedance (series resistor) and measure AC voltage and current at the cell terminals. Then calculate I/V. That's how most impedance (LCR) meters work.

Current can be measured by simple transimpedance amplifier for a not grounded cell, or by the series resistor voltage drop with a differential amplifier for a grounded cell.

Conductivity sensors with analog output are made by many major vendors in industrial measurement, e.g. E+H or Krohne.