Methods and systems for calibrating a sensor for measuring an analyte in a patient monitoring system are disclosed. The method includes calculating sensor drift; calibrating the sensor using at least one calibration fluid; and periodically updating the sensor calibration based on the sensor drift calculation. The system adjusts a gas concentration in a fluid. The method in one of several variations includes providing a calibration fluid; setting the analyte concentration in the first fluid to a first concentration with an adjustment mechanism; measuring the first analyte concentration with the sensor; setting the analyte concentration in the first fluid to a second concentration; measuring the second analyte concentration with the sensor; and determining the calibration coefficients for the sensor from the measured first and second analyte concentrations. Some embodiments ensure that the period between recalibrations of the sensor is extended, thus reducing the number of disruptions to the sensor monitoring process.