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Resistivity Of Water

What is resistivity in water?

Resistivity in water is the measure of the ability of water to resist an electrical current, which is directly related to the amount of dissolved salts in the water. Water with a high concentration of dissolved salts will have a low resistivity, and vice versa. Resistivity is measured in Ohms. When salts dissolve into water, free ions are created. These ions are able to conduct an electrical current. Examples of dissolved salts that can be found in water, and that lower the resistivity of water, are: calcium, chloride, magnesium, potassium, and, of course, sodium. The table below shows some example levels of resistivity for different types of water.

Type of Water Approximate resistivity in Ohms (Ω)
Pure Water 20,000,000
Distilled Water 500,000
Rain Water 20,000
Tap Water 1,000 – 5,000 Ω
Brackish river water 200
Coastal seawater 30
Open seawater 20 – 25

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The resistivity of water is the reciprocal of conductivity, so if a water sample has a low resistivity it will have high conductivity. Either measurement can be used to indicate the purity of a water sample.

It can be said that the less dissolved salts in a water sample, the purer that water is. This purity of water is one factor in water quality monitoring. Natural factors that affect the resistivity of water include rain, geology, and evaporation: human factors include urban and agricultural runoff, landfill leachate, and road salt.

Why use a resistivity meter for water testing?

A water resistivity meter is a useful tool for resistivity testing. Resistivity is a vital measure when ultra pure water is required, for example for use in a growing number of laboratory and industrial processes. Out in the field, a water resistivity sensor is used as part of the water quality testing process, alongside other parameters such as dissolved oxygen and pH. Monitoring resistivity in water can also be used for testing groundwater, to check for contamination from landfill leachate, for example, or to investigate contamination in lakes, rivers and tidal estuaries.

Once you have been monitoring resistivity in water for a while it is possible to build up an understanding of the normal range of resistivity in a particular body of water. Using this knowledge can assist in the identification of anomalies, that may represent a pollutant entering the water. Within aquatic ecosystems each organism has a range of tolerance, and if the resistivity of the water moves outside this range it could devastate the ecosystem and take a long time to recover.

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Aquaread’s resistivity testing equipment

Aquaread design and manufacture water testing equipment and water resistivity meters suitable for both portable and fixed applications. We make multiparameter water quality monitoring equipment, of which resistivity is a standard parameter. Resistivity is measured using electrical conductivity and temperature measurements. Aquaread’s water resistivity meters are just one function on our multiparameter water quality testing equipment.

Our equipment consists of probes that have a range of electrodes, which measure various parameters. The dissolved oxygen and electrical conductivity electrode is combined into one. A voltage is applied between electrodes. Depending on the resistance of the water there will be a drop in voltage. This drop is measured and, using the temperature reading, is displayed on the Aquameter that is used in conjunction with our Aquaprobes, as resistance.

To allow for the fact that resistivity is greatly influenced by temperature, Aquaread’s resistivity testing equipment adjusts the resistivity value based on the temperature of the water sample in order to display a value that is normalised to 25 degrees C. Resistivity can be measured in the range of 5 Ωcm – 1 MΩcm, with an accuracy of +/- 1% of reading or 1 Ωcm if greater.

For portable applications we have the Aquaprobe AP-2000, Aquaprobe AP-5000 and AquaPlus Optical DO/EC system. These water testing devices have been designed to be highly durable and portable, to allow for spot monitoring across multiple locations. The Aquameter allows geo-tagging of data so that it can be viewed in Google Earth. For long term deployment, we have the Aquaprobe AP-7000 with it’s innovative method of self-cleaning that allows fixed monitoring over extended periods of time.

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Get into monitoring with Aquaread

For more information about our water resistivity monitor and other water testing equipment, or water quality monitoring in general, please feel free to get in touch. Click here to see our product range.

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