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A glass of drinking water is expected to be clear. That’s because US government regulations determine the acceptable level of turbidity—or volume of particulates suspended—in the water. Untreated stream water that contains clay, plankton, or organic compounds may appear as a murky brown. Aside from appearing unappetizing, this high degree of turbidity may not be safe to drink.
Water clarity is expressed as level of turbidity (NTU or nephelometric turbidity units) and measured with a turbidity meter. Beyond drinking water and manufacturing water for other beverages, turbidity is helpful in determining water quality for fish and aquatic life. High turbidity can impact water treatment costs, which in turn affects production in industries such as food processing, pharmaceuticals, and many others.
Tools for Measurement
Turbidity meters, Secchi disks, and turbidity tubes all measure turbidity. While Secchi disks and turbidity tubes are lower cost options, users may sacrifice accuracy and face other limitations.
Secchi disks have a long tradition in measuring water transparency. They are mounted on a pole and lowered into water to indicate the level at which the disk is no longer visible and the water is no longer transparent. While these disks are highly economical and efficient, they cannot deliver the accuracy of a turbidity meter.
Turbidity tubes (also referred to as transparency tubes) are sometimes preferred for use in smaller community water supplies because of their low cost and low maintenance. Designed for field use, they take samples of water, which are then drained through the tube to reveal a black and white (Secchi) disk. The remainder of the water is then measured in centimeters. Tubes are not recommended for very low turbidity levels.
Turbidity meters are digital devices that may be handheld for field or lab use or they may be benchtop models primarily for lab use. They operate by reading a beam of light which is passed through the water sample. Meters require a greater financial investment than either disks or tubes, but they deliver the best accuracy and can measure even very low turbidities. Because meters run on a power source (either batteries or electricity), they do require some maintenance.
Advanced turbidity meters can offer advantages such as push-button calibration, selectable readout modes, diagnostic error messages, and signal averaging. For those seeking to simplify routine laboratory work, the consistent performance of a turbidity meter may be worth the cost. EPA- and ISO-compliant models are available for regulatory monitoring.
Best Practices—Keeping Measurements Accurate
Several practices can help ensure that turbidity readings are not faulty:
View our selection of turbidity meters .