Calibration of O2 and CO2 Sensors
The Q-CPX systems uses a unique method for calibration that requires only a calibration gas (such as CO2:O2:N2 at 5:16:79), and room air. Temperature, Relative Humidity and Barometric Pressure are measured and the data used for automated standardization. The calibration procedure is automated, and the GUI prompts the operator to procede via a series of mouse clicks. Delivery of room air sets the low CO2 and high O2 input values, and delivery of the calibration gas sets the high CO2 and low O2 input values. Calibration gas is vented in a controlled burst of pressure through the pneumotach tube attached to the front of the gas analyzer. The burst, simulating an exhaled breath, allows precise automated alignment of signals from the gas analyzers and the pneumotach pressure sensor.
VCO2 and VO2 from breath gas concentrations and breath flow is calculated every 10 milliseconds. Each calculation requires that gas concentration be multiplied by the corresponding breath flow rate at each sampling time. The flow signal, which is a pressure wave travelling at the speed of sound, is detected instantly by the differential pressure sensor in the poneumotach. However, there is a ~1.5 m displacement of the gas sensors from the pneumotach flow meter. Due to the pumping rate of 300 ml/min (~1.9 m/sec in the 1.59 mm ID sampling tube), the signal from the CO2 sensor is detected ~800 msec after the flow signal. The calibration procedure corrects for the delay in gas analyzer signals relative to the pneumotach signal to ensure sensor synchronization.