# The Theoretical Principles of Measuring Conductivity in Liquids

Measuring electrical conductivity in liquid substances is a highly powerful diagnostic and analytical tool in a range of application in spite of its simplicity.

Figure 1.

The latest thin-film conductivity sensor element (figure 1, above) is a suitable alternative to the bulky, classical conductivity sensors of the past.

## Theoretical Background

An electrolyte is a liquid that contains ions. The ions behave as charge carriers and a current flows when a voltage is applied. Hence the liquid quality can be assessed by determining the conductivity. The liquid conductivity is based on two temperature-dependent parameters that include ion concentration and their mobility. A temperature sensor is placed directly at the measurement point for improved accuracy.

Figure 2. Liquid conductivity based on two temperature-dependent parameters.

## Selected Conductivity of Electrolytes

The electrical conductivity of electrolytes is provided in the table below:

Electrolyte Electrical conductivity
pS/cm S/m
Ultra pure water 0.05-0.1 5-10*10-6
Tap water 300-800 0.03-0.08
NaCl (0.2 g/l) 4'000 0.4
NaCl (2 g/l) 38'600 3.86
Seawater ~56'000 ~5.6
Bulk silver (for comparison) 62.5*106 6'250

## Measurement Principles

The conductivity (using electrodes) is shown in Figure 3.

Figure 3. Measurement Principles.

In order to minimize degradation of the electrolyte and the electrode, AC excitation is recommended.

## Conductivity and Cell Constant

The conductivity value which is obtained as a result of the measurement additionally depends on the cell geometry. The impact of the cell geometry can be eliminated by introducing the so-called cell constant. The electrical conductivity κ using the following formula can be obtained at a particular temperature:

The exact cell constant value can be obtained because of calibration measurements in standard solutions. In order to avoid additional measurement errors, it is essential to use a solution having electrical conductivity values close to the values of the intended application solution.

## Conductivity

Cell constant is influenced by:

• Boundary effects
• Planar geometry of chip layout

where κ is the electrical conductivity, l is the length and A is the area.

## Major Parameters for Customer Specific Designs

The key parameters for customer specific designs are:

• Measurement liquid characteristics (stability of platinum electrodes): Customer testing is mostly required and samples can be provided.
• Measurement range(cell constant) can be adjusted by the electrode geometry.
• Read out value is AC 300 to 3000 Hz, 1.6 VPP or less.
• The assembly method is encapsulated wires and fixation.
• Customer expectations include only chip, assembly and electrical readout (under development)
Conductivity Sensors - Innovative Sensor Technology IST AG

This information has been sourced, reviewed and adapted from materials provided by Innovative Sensor Technology.

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