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. Here he uses a Dissolved Oxygen Meter (D.O. meter) to measure the concentration of oxygen gas present in the water. The D.O. meter is first calibrated and then 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 clean, healthy stream, such as this one, should give readings around 95 or 96 per cent.
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