Second, barometric pressure measurement is important in calibrating dissolved oxygen sensors. Lower barometric pressures imply lower equilibrium oxygen concentrations. Post-calibration changes in barometric pressure may be used to correct percent saturation readings. For instance, a storm event might lower ambient barometric pressure by 20 millimeters of mercury. The mercury is forced into the tube by the atmospheric pressure exerted on the pool of mercury.
Barometric pressure is measured directly as the height of mercury in the tube.
Changes in barometric pressure cause changes in the height to which the mercury rises in the tube. A more modern method for barometric pressure measurement is the electronic pressure transducer. In this device, the electrical characteristics of a silicon wafer bonded to a metal diaphragm change according to the shape of the diaphragm. An electronic circuit measures that change, and calibration converts the electrical signal to a pressure measurement in enginering units. It should be noted that barometric pressure is reported in two ways.