Conductivity Meters

A conductivity meter measures an aqueous solution's ability to carry an electrical current.

(Copyright Hach Company, Loveland, CO)

General Information

Distilled water is a poor electrical conductor. The substances (or salts) dissolved in the water determine how conductive the solution will be. As the number of dissolved ions increases, so does the solution's ability to carry an electrical charge. This electrical charge is what allows a conductivity meter to measure the conductance of a solution.

The conductivity meter reports conductance as the inverse of a resistivity measurement. Resistivity is measured in ohms/cm, so conductivity is measured in mhos/cm. A mho is the same as an ohm, and is defined as the Siemen (S), a unit of conductivity.

(Copyright Hach Company, Loveland, CO)

Equipment Design

As seen in the animation, a conductivity meter consists of a probe that measures conductivity. A small electrical current flows between two electrodes set a certain distance apart, usually around 1 cm. If there is a high concentration of ions in the solution, the conductance is high, resulting in a fast current. The electrical current is slower and gives a smaller reading when a lower concentration of ions is present.

Many manufacturers produce different probes to measure the conductance of a solution. It is difficult to measure the exact conductance of a solution with an amperometric probe. Conductivity meters must be calibrated to provide accurate results.

One of the most common designs for a conductivity meter is the 4 ring probe system. This is a potentiometric system that provides an alternating current across all four rings. As seen in the animation, the probe is placed into a solution and the current flowing from ring 1 to ring 4 produces a voltage across rings 2 and 3. The amount of current is directly related to the ionic concentration of the solution, which means the voltage is also dependent on the concentration of dissolved ions in the solution. A voltmeter in the probe registers this voltage and sends the result to the conductivity meter, where it is translated into the conductance of the liquid.

A conductivity meter has the ability to measure the amount of totally dissolved solids (TDS) in a solution, in units of parts per million (ppm) or milligrams per liter. The standard correlation between the TDS measurement of a solution and the conductivity measurement is: TDS (ppm) x 2 = Conductivity (µS).

Note that a conductivity meter only infers the actual number of ions in a solution by measuring the electric charge of a solution. A conductivity meter is not a direct measurement of the actual number of ions contained in the sample.

(Copyright Hach Company, Loveland, CO)

Usage Examples

Conductivity meters are used heavily in agriculture to measure the salinity levels of surface water and of soil samples. Shown here is a conductivity meter being used to measure the quality of water in a wastewater treatment facility. In addition to conductivity, this particular meter can be used to measure pH and dissolved oxygen.

(Copyright Hach Company, Loveland, CO)

Shown here is a conductivity meter that can measure the conductance of up to 100 samples of plant material and analyze the measurements with available data processing equipment.

use2.jpg

(Copyright Reid & Associates, Durban, South Africa)

Damage to plant cellular membranes is a common response to a stress. This damage leads to leakage of electrolytes from the cells of stressed tissues. By measuring this leakage with a conductivity meter, an operator can reliably indicate the severity of the stress on a plant.

Advantages

Disadvantages

  • Available as a controller or an analyzer to be implemented into process situations.
  • Able to make a sampling measurement in less than one second.
  • Cannot distinguish between different types of ions.
  • Conductivity meters are temperature dependent; conductance increases approximately 2% per °C.
  • Does not measure the number of ions in a solution directly.

Acknowledgements

Hach Company , Loveland, CO

Reid & Associates , Durban, South Africa

References

Hanson, B. R., and K. Kaita. "Response of Electromagnetic Conductivity Meter to Soil Salinity      and Soil Water Content." Journal of Irrigation and Drainage Engineering 123 (1997):      141-143. Print.

McPherson, Lori. "Correlating Conductivity to PPM of Total Dissolved Solids." WATER Engineering & Management 142 (1995): 31-33. Print.

McPherson Lori. "How Good Are Your Values for Total Dissolved Solids." Chemical Engineering Progress 91 (1995): 58-59. Print.

Developers

Jonathan West

Christy Charlton

Kelsey Kaplan