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The turbidity tube was developed by the GLOBE Hydrology Team at the University of Arizona. It was designed to measure the transparency of water bodies that are not conducive to secchi disk transparency measurements. The turbidity tube can be used to measure the transparency of streams or shallow waters.

Since the tube is called a "turbidity" tube and it actually measures transparency, this has caused some confusion about the purpose of the measurement. In the GLOBE manual, we require students to measure the height of the water column in the tube-- the point at which the decal on the bottom of the tube cannot be seen anymore. This is a measurement of transparency. In order to find the turbidity of the water (which is not required for GLOBE measurements), students can use the following graph to compare height in the tube with turbidity of the water (NTU units).

Note that this is a log graph, so that each axis is the log of the actual value! See below for an explanation of how the turbidity tube was calibrated.

Explanation of Calibration Procedure

The turbidity tube was calibrated with kaolin, a powdery white substance that is easily suspended in water and is part of a group that includes kaolinite clay. Distilled water was added to kaolin powder to create diluted solutions with different turbidity values. Once the solutions were produced, an instrument called a turbidimeter was used to measure the turbidity of each solution. Each solution was then poured into the turbidity tube one at a time in order to measure the height at which the decal at the bottom could not be seen anymore. This process provided a corresponding height in the turbidity tube with a turbidity value for each solution made.