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Devices to Measure Electrical Conductivity

There are many different devices suitable to measure electrical conductivity in water based systems. Conductivity of such systems is rather high, above 0.0001 S/m up to 10 S/m. If one wants to measure electrical conductivity at this high range, he has a wide selection of devices from which to choose.

There are several parameters that distinguish such devices. One of the most important one is the way of preventing electrode polarization at the rather high ionic strength of aqueous solutions. Many devices use black platinum electrodes for increasing electrode surface area and, consequently, reducing polarization effects. This method works well for typical dilute aqueous systems. It might have problems with concentrated slurries due to particles building deposit on the rough surface of such electrodes. It might be very hard to clean them.

Alternative way of eliminating electrode polarization is applying high frequency. We selected this way for our aqueous conductivity probe, Option OP004. It functions at 3 MHz. This completely eliminates electrode polarization. Design of the probe is very simply - it has flat polished surface. It is very easy to clean. It can be used on-line applications. 

The situation with non-aqueous conductivity is quite different. First of all, it is orders of magnitude are lower than in water-based systems. Usually it ranges from 10-12 to 0.0001 S/m. This low conductivity level requires a completely different approach to measure it, as compared to the water based systems, as it follows, for instance, from ASTM Standard D4308 [4]. .

Application of the AC electric field is still desirable for preventing electrodes polarization, but not at that high frequencies. Conductivity is completely capacitive at MHz range for low conducting liquids. In  order to sense ionic conductivity frequency should not exceed Maxwell-Wagner frequency [1,2,3], which is on scale of tens Hz for non-polar liquids.

Secondly, design of the probe should be very different. Cell constant must be much smaller because we want to measure much smaller current due to much lower conductivity for the same applied voltage. This is usually achieved by using coaxial cylinders chamber with particular cell constant. That is how sensor of DT-700 is built. Driving electric field is AC for several frequencies from 1 to 20 HZ, or even DC.

Our DT-700 has orders of magnitude higher conductivity level up to 0.0001 S/m, comparing to usually 0.00000001 S/m. Combination of our two probes allow covering complete conductivity range. This allows us a completely novel approach for calibrating non-aqueous probe.

REFERENCES

  1. Maxwell, J.C. "Electricity and Magnetism", Vol.1, Clarendon Press, Oxford (1892)

  2. Wagner, K.W., Arch. Elektrotech., 2, 371 (1914)

  3. Dukhin, S.S. and Shilov V.N. "Dielectric phenomena and the double layer in dispersed systems and polyelectrolytes", John Wiley and Sons, NY, (1974)

  4. ASTM D4308 - 95(2010) Standard Test Method for Electrical Conductivity of Liquid Hydrocarbons by Precision Meter