Water Resistivity (RES) Sensor

What is Resistivity (RES)?

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 salt in the water. Water with a high concentration of dissolved salts will have a low resistivity, and vice versa. Resistivity is measured in Ohms.

Aquaread RES Sensor

Aquaread’s RES measurement is directly measured using the electrical conductivity (EC) sensor, in fact, RES is the inverse of the of EC. RES is the waters ability to resist electrical current and is linked to the amount of dissolved salts within the water.

Key Features

Parameter that’s always included on every Aquaprobe and Aquasonde.
Uses the Corrosion resistant gold coated stainless-steel EC sensor.
Measurement range 5Ohm/cm – 1MOhm/cm
Resolution 2 auto ranging scales: 5 – 9999Ohm/cm, 10.00 – 100.00Ohm/cm.
Accuracy of 1% of reading.
Requires no calibration – this is taken care of when you calibrate the EC sensor.

Our RES sensors come as standard on many of our probes, below are some examples.

To see all of the probes that feature RES please visit the Products section.

AP-5000

Advanced portable multi-parameter Aquaprobe.

AP-700 & AP-800

Basic Water Monitoring Packages

Aquasonde-2000

The Aquasonde-2000 has 2 spare ports. They can house either 1x ISE sensor and 1x Optical sensor. Or it can house 2x ISE sensor.

Always included

The RES parameter is included on every Aquaprobe and Aquasonde, it is also included in the AquaPlus optical dissolved oxygen sensor.

More on Res testing

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.

How to use the RES sensor

Using the Aquaread RES sensor is very simple. As soon as the probe is lowered into the body of water the Aquaprobe will begin taking readings every 2 seconds. The reading for RES can be viewed on the Aquameter using the right button. There is no need to calibrate the RES parameter, this is all taken care of when calibrating electrical conductivity (EC)

When to use the RES sensor

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.

Typical RES values for various water types

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