ADAM HART-DAVIS / SCIENCE PHOTO LIBRARY ADAM HART-DAVIS / SCIENCE PHOTO LIBRARY
A biologist checks the pollution level in a stream by monitoring both the animals living in it and the concentration of dissolved oxygen. A Dissolved Oxygen Meter (D.O. meter) is used to measure the concentration of oxygen gas present in the water. Here the D.O. meter is calibrated before being lowered completely into the water. The reading is presented either in milligrams per litre or as a percentage of the saturated concentration (the concentration of oxygen compared to the maximum concentration possible given the water's temperature). A polluted stream, such as this one, gives readings of around 70 per cent.
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