OxyProbes measure the partial pressure of oxygen. The partial pressure of Oxygen is dependent on the mixture of the gases and the pressure inside the vessel. Uncompensated variations in pressure can be a significant source of error. When reading % saturation or ppm, the operating pressure of the vessel should be entered into the transmitter if the process pressure is not the same as the calibration pressure. This number should account for elevation above sea level and/or any overpressure on the vessel. The pressure can be entered in mm Hg, inches Hg or bar.