Abstract: Apparatus for measuring the dissolved oxygen content of water comprises an open-ended duct immersed in the water in an upright position, an oxygen-detecting probe in the lower end portion of the duct, and means for injecting a continuous stream of air into the upper portion of the duct, the air causing water to rise in the duct and thereby induce a flow of water into the duct and past the probe. A valve in the upper end of the duct is operable by compressed air to shut off the top of the duct so that air builds up inside the duct and back-flushes the liquid through a strainer on the lower end of the duct. Displacement of the water around the duct by air enables the probe to be calibrated. The upper section of the duct into which air is injected may be connected to the lower section containing the probe by polyethylene tubing of a length to enable the lower section of the duct to be lowered to any desired depth.