How to measure the conductivity of water with a multimeter

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Learn to measuring the conductivity of water as a test of its purity. When water conducts electricity, it is made possible by impurities in the water such as metals. The standard unit of measure for conductivity is microsiemens per centimetre. For example, most fish in the United States thrive in water with a conductivity between 150 and 500 microsiemens per centimetre, while rivers have a range of conductivity between 50 to 1500 microsiemens per centimetre. Conductivity is related to the resistance to current flow of the water.

Pour the water to be tested into the glass backing dish.

Turn on the digital multimeter and then switch its measurement dial to the resistance setting. Resistance is denoted by the capital Greek letter "omega." Omega is the symbol for the ohm which is the unit of resistance.

Plug the red and black leads of the multimeter into its positive and negative ports, respectively. Touch the leads to the water at opposite ends of the longest dimension of the glass dish. Note the resistance in ohms that appears on the screen. For example, assume a resistance of 33 ohms.

Measure the length, width and depth of the glass dish in centimetres. For example, use a length of 30cm, a width of 15cm and a depth of 3cm.

Multiply the width by the depth to obtain the area of the sides of the glass dish in square centimetres. Performing this step, for the example, leads to 15cm times 3cm, or an area of 45 square cm.

Divide the length by the product of the resistance and the area to arrive at the conductivity in units of siemens per meter. This step, for the example, yields 30cm divided by 33 ohms times 45 square cm, or a conductivity of 0.02 siemens per meter. The siemens units equals one divided by the ohm.

Convert the conductivity to microsiemens per cm by multiplying by 10,000. The prefix "micro" means one-millionth of a siemens. Finishing the exercise, you have 0.02 times 10,000, or a water conductivity of 202 microsiemens per cm. This is in the inhabitable range for some types of fish.

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