Ultrasound sensor for gas concentration measurement
Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
In an anesthesia machine, there is a need to monitor the anesthesia concentration that is being delivered to the patient, mainly to stop and flush the system in case of fault but also to control the dosage of anesthesia. The infrared sensor with absorption spectrometry technology that Maquet use today is expensive and the desire for a cheaper solution leads to this master thesis. It is possible to determine the proportion of one gas in a gas mixture by using the acoustic properties in a sound wave. This thesis describes an attempt to replace the infrared sensor with an acoustic sensor using ultrasound technology. Modification on an existing acoustic sensor optimized for measuring O2 concentration was done in order to expand its functionality to measure concentration of N2O and different anesthesia drugs. It is observed with the binary gas analyzer developed by Maquet that it is possible to measure O2 concentration with a maximum absolute discrepancy from the built-in Control Gas Analyzer in the FLOW-i anesthesia machine of in air/O2 mixture and in O2/N2O mixture. The concept to measure anesthesia is based on two ultrasound sensors, one placed before the anesthesia vaporizer and the other one is placed after the vaporizer. The sensor placed after the vaporizer will use its measured value as well as the measured fresh gas concentration value from the first sensor to determine the anesthesia concentration. It will inherit the error of the first sensor. However, the error inherited from the first sensor is minimal, since the dynamic range of the sound speed for measuring anesthesia ranging from 0% to 100% is a lot higher than air, O2 and N2O which is the gas compounds that the first sensor measures. Thus the first sensor in the Dual Gas Sampling System has minimal influence on the measured anesthesia concentration. This also means temperature changes on the first sensor that occur because of pressure variation in FLOW-i?s working condition will have minimal effect on the measured anesthesia concentration. Measurement shows that the binary gas analyzer designed as semi side-stream can be fast enough to replace FLOW-i?s Control Gas Analyzer for measuring the fresh gas concentration. The reaction time of the sensor can further be improved if the ultrasound transducers are placed to directly measure in main stream gas flow instead of in a semi side-stream. A modification to the sensor by attaching a thin variable flow restriction that forces the gas flow into the measurement chamber gives an improvement with 10 times shorter rise time for flow rates up to 1 l/min. At higher flow rates, the variable flow restrictor will be pushed open to ease the pressure variation in the measurement chamber. This modification is equal as having the transducers placed in main stream for low flow rate.
Place, publisher, year, edition, pages
EES Examensarbete / Master Thesis, XR-EE-MST 2011:003
Engineering and Technology
IdentifiersURN: urn:nbn:se:kth:diva-91812OAI: oai:DiVA.org:kth-91812DiVA: diva2:511270
Sohlström, Hans, Docent