The Evolution of Conductivity - Geometry
Two-electrodes, two-pole design
The simplest sensor is based on the traditional two-electrodes, two-pole design. Here the current entry and voltage measurement are realized using the same pair of electrodes. Since at high conductivities the voltage drop at the electrodes (ZTransition) and in the cable (ZCable) becomes relatively big compared to the voltage drop in the medium (Rmedium), the two-pole design is only suitable for low conductivities (typically 1 µS up to 500 µS).
Figure 1 & 2: magnetic field line around a 2 pole sensor and the equivalent circuit diagram
Four-electrodes, four-pole design
The separation of current-inducing and voltage-measuring electrodes almost completely eliminates polarization issues and the influence of the cable resistance. However, the capacitive influence of the cable remains and has to be compensated. Due to the inhomogeneity of the electric field at the voltage electrodes, the cell constant of a four-pole system is strongly influenced by the conductivity of the solution. I.e., transmitters working with a four pole cell offer the input of different cell constants to cover the whole conductivity range.
Figure 3 & 4: magnetic field line around a 4 pole sensor and the equivalent circuit diagram
Six-electrodes, four-pole design
In a six pole design, the current entry is realized using two electrodes per pole. While the corresponding electrical circuit is identical to the four-electrode, four-pole design, the big difference lies in the strength and homogeneity of the electrical field present at the voltage measuring electrodes. The more homogeneous field distribution in the area of the voltage electrodes results in a much less variable cell constant, since the influence of the fringe effect is drastically reduced. In addition the 6-electrode 4 pole design allows for a much higher field density and less sensitivity to field distortion.
ACF60 conductivity sensor, 4-poles and 6 electrodes
Figure 5 & 6: magnetic field line around a 6 electrode 4 pole sensor and the equivalent circuit diagram
The Evolution of Conductivity 2.0
Six-electrodes, four-pole design + integrated electronics
While standard sensors for conductivity have a cable connection between sensing elements and measurement electronics, this new design has advanced electronics integrated into the sensor. This eliminates the influence of the cable (resistance and capacitance), allowing for a highly accurate response over a wide conductivity range. Having 4 electrodes for the applied current and 2 electrodes measuring the resulting voltage, the electrical field lines have an optimal distribution between the poles and minimize effects on the voltage measurement.
Inside the new 6 electrode 4 pole sensor an improved electronics with a 16 bit A/D converter ensures a stable, reproducible and highly accurate measurement of conductivity in a wide range of 0…10 µS/cm up to 850 mS/cm. Due to high quality electrode materials, degradation of the electrode surface is negligible. Both leads to a long lasting cell constant (typically 0.35 cm-1). Finally, with this 6 electrode sensor design only one calibration point is necessary, whereas other sensor designs require several calibration points.
From conductivity signal to output value
A Pt1000 temperature sensor integrated in the tip of the sensor is used to compensate temperature based conductivity changes during calibration and measurement. Although the sensor gets a factory calibration, it is recommended to perform a user calibration in the final installation setup, preferably using a calibration standard having a conductivity value within the desired measurement range. This ensures the best sensor performance and measurement accuracy at the final installation location.
Sensor accuracy optek ACF60 and ACS60
Following are the measurement range dependent accuracies with corresponding reproducibility:
|0 - 10 μS/cm||± 1 % of measurement ± 0,2 µS/cm
|0 - 250 mS/cm||± 1 % of measurement ± 0,2 µS/cm
|250 - 500 mS/cm||± 2 % of measurement ± 0,2 µS/cm
|500 - 850 mS/cm||± 5 % of measurement ± 0,2 µS/cm
Example: Measurement result is 20 µS/cm. 1% of measurement is +/- 0.2 µS/cm plus +/- 0.2 µS/cm. The measurement result accuracy is therefore 20 µS/cm +/- 0.4 µS/cm.