Gas monitoring system using ultrasound sensors

Abstract: This thesis reports a collaboration between KTH Microsystem Technology Labs and Maquet critical care. Maquet is a company that produces medical ventilators and anesthesia machines. In an anesthesia machine it is important to monitor the anesthesia concentration that is delivered so that the delivered anesthesia concentration does not deviate from the desired concentration. Furthermore, in case of fail function there is a need to stop the delivery of anesthesia to the patient and flush the system. The anesthetic agent concentration is presently monitored with an infrared spectrometer. By using ultrasound technology it is possible to determine the volume concentration of a gas mixture by knowing the sound speed in the gases. Maquet has an ultrasound sensor that is developed to measure the oxygen concentration in air. This sensor was modified in order to measure nitrous oxide and anesthesia. The anesthesia concentration was measured by placing sensors upstream and downstream from the vaporizer. Using this ultrasound sensor system one can observe that the average discrepancy of the entire concentration range is ±0.84 % for Desflurane and 0.17 % for Isoflurane in relation to the infraredspectrometer sensor that is presently used in the anesthesia machine to monitor the anesthetic agent. Measurements show that the rise time of the ultrasound sensor varies when placing the sensor in different orientations with respect to the airway flow. It also show that by placing a flow restrictor that is used to force the airway flow in to the sensors measurement chamber reduces the rise time to a tenth of its previous value.