How Is Conductivity Measured?

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Conductivity is measured using a conductivity meter and probe. A voltage is applied between two electrodes inside the conductivity probe when the probe is immersed inside a water sample. The resistance of the water causes a drop in the voltage, which is used to calculate the conductivity. 

Conductivity measures the ability of a solution (usually water) to pass an electrical current. Electrical currents in water are influenced by the number of inorganic dissolved solids that carry an ionic charge.

Conductivity is also influenced by the temperature of the solution; the warmer the water, the higher the conductivity. As temperature influences conductivity measurements, conductivity is reported at 25 degrees Celsius (25 °C). 

What Affects Conductivity?

In streams and rivers, conductivity is affected by the geology of the area that the water flows through. For example, surface (stream) water that flows through granite bedrock typically has a lower conductivity level, as granite is made up of inert materials that do not dissolve into ionic components when dissolved in water. 

On the flip side, water that flows through clay soils picks up ionized components, which increase the conductivity. 

Depending on the bedrock the water flows through, groundwater inflows can also affect conductivity. 

Human factors such as a failing sewage system can increase the conductivity because of discharge (chloride, phosphate, and nitrate) that spills into the water. On the other hand, if an oil spill enters the water, it will lower the conductivity. 

Conductivity Units

Conductivity is measured in Siemens (S) per distance (microsiemens per centimeter (µs/cm)) or micromhos per centimeter (µmhos/cm). 

Typical conductivity levels include the following:

  • Distilled Water: 0.5-3 µmhos/cm
  • Rivers: 50-1500 µmhos/cm
  • Inland Freshwater: 150-500 µmhos/cm
  • Industrial Water: Up to 10,000 µmhos/cm

Conductivity levels outside the above ranges may indicate poor water quality.

Measuring Conductivity

Conductivity is an essential measurement of water quality in streams, and ones that have a relatively constant conductance level can be used as a baseline to compare a wide range of conductivity measurements. 

Large fluctuations in conductivity can therefore be used as an indicator that a source of water pollution like discharge has entered the stream. 

For accurate results, conductivity is measured with a conductivity meter and probe. A voltage is applied between two electrodes inside the conductivity probe when the probe is immersed inside a water sample. 

The resistance of the water causes a drop in the voltage, which is used to calculate the conductivity. The conductivity meter then converts the probe reading to µmhos/cm, further displaying the conductivity result to the operator. 

Some conductivity meters can also test the salinity and total dissolved solids (TDS) in the water. 

Conductivity meters can be used in the field or the lab, as they come in both benchtop and portable devices. However, water samples are usually collected in the field and tested in a lab. When water samples are collected in the field and transported to the lab, the bottles should be collected in a glass or polyethylene bottle which has been thoroughly washed with a phosphate-free detergent and rinsed with both tap and distilled water, so that the conductivity measurement is not influenced. 

If testing in the field is necessary, there are conductivity meters specifically designed for field use. There are also conductivity meters designed for industrial applications.

Reducing Conductivity Levels In Water

High conductivity is linked to the total number of ions in the water, therefore, to decrease conductivity levels in the water, the total dissolved solids (TDS) must be removed. You can reduce the TDS by lowering the conductivity via reverse osmosis, distillation, or neutralization using an ion exchanger.

Considerations When Measuring Conductivity

Conductivity measurements are usually very simple to take, but, mistakes can still happen. Therefore, it is important to familiarize yourself with the following considerations:

Always select a suitable conductivity sensor for your testing needs. 

Understand and anticipate how temperature affects conductivity measurements. 

Remember to also use the temperature compensation (TC) function, to calculate and display the correct conductivity measurement at the chosen reference temperature. If the TC is not switched on, the conductivity measurement will be displayed at the current temperature, not the reference temperature. 

Always allow time for the conductivity meter to equilibrate to the same temperature as the water sample. Temperature equilibrium must be achieved before taking the conductivity reading to achieve accurate results.

A one-point calibration of the conductivity probe cell constant is sufficient enough for accurate conductivity readings. However, if the sample overs a large range of conductivity levels, then you can use multipoint calibrations.

Avoid setting low calibration standards because they are more difficult to use, and they can easily become contaminated. For that reason, calibrating at 100 µS/cm or above is recommended. 

When working with low-level conductivity water samples, take extra care when handling them, as it can affect the accuracy of the sample reading. This is because low-level conductivity water samples are easily affected by contamination, degassing, and carbon dioxide (CO2) absorption. 

Finally, store the conductivity meter properly by following the storage and maintenance guidelines in the manufacture’s instructions.

Conductivity Applications

As mentioned, conductivity is used to measure the number of conductive ions in a solution. Many industries and applications rely on measuring conductivity, these include:

  • Water treatment and industrial applications (drinking water and wastewater)
  • Hospital equipment
  • Boiler water
  • Brewing beverages such as beer and wine 
  • Natural waters
  • Aquaculture applications
  • Environmental applications
  • Agricultural applications
  • Hydroponics applications
  • Oceanographers measuring seawater 

These types of conductivity measurements are often not ion-specific, but they can often be used to determine the TDS in the water if the composition of the solution and conductivity are known. 

For industries that are measuring conductivity to determine water purity, additional purity tests may be required, as these types of samples do not respond well to non-conductive contaminants.

Summary

Conductivity is measured to determine the number of dissolved ions in a sample, usually water, and therefore conductivity serves great importance in measuring water quality. 

The most accurate way to measure conductivity is with an electrical conductivity meter with a probe. 
If you have any questions regarding conductivity or what conductivity probe will best suit your testing needs, do not hesitate to contact our world-class team at Atlas Scientific.

Conductivity Probes & Circuits

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