Kitsos, Vasileios; (2019) A Novel Multi-Modal Thermal Flow Sensor for Biomedical Applications. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Heat and moisture exchangers (HMEs) are basic breathing system filters used by patients that have undergone tracheostomy and breathe through an airway hole at their neck called stoma. Their main function is to replace the normal humidification, warming and filtering of the inhaled air that used to be performed by the upper airways. Clinical evidence show the beneficial operation of the HME, however, there are patients who complain about difficulty in breathing while using it. Although efforts in clinical environment to assess HME’s performance in terms of humidity, temperature, and filtering have been already conducted, the market currently lacks of a portable solution that can effectively measure the breathing pattern of the patient in situ. This thesis proposes novel methods and circuits towards thermal flow sensors intended for the in situ monitoring of the tracheostomee’s respiration. This work addresses the two main restrains for a flow sensor of that purpose: power consumption and, safety concerns due to potential elevated temperatures at the sensor. Thermal flow sensors can operate in different modes, such as constant power (CP), constant temperature (CT), and constant temperature difference (CTD). While other published or commercial flow sensors operate in one of the pre-defined modes, the proposed sensor is able to toggle between modes avoiding high-temperature overshoots at low flow rates by using CP, and avoiding high power consumption at high flow rates by using CTD. Specifically, an overtemperature reduction up to 9.5% is achieved, and a heater power reduction up to 13.6%. This is the first dual-mode thermal flow sensor to operate in CP/CTD. Moreover, mechanisms for the accurate control of the CP, CT, and CTD are introduced to ensure better accuracy and reproducibility of the measurements. The issue of power consumption and output sensitivity are also addressed by modifications on the transducer; specifically, the effect of the temperature sensing elements’ location on flow sensor’s performance is investigated. This work proves that optimisation of the distance between the heater and the temperature sensing elements is required to achieve optimal sensitivity, and provides evidence of the interplay between power consumption and optimal distance. Based on the findings, a novel figure of merit that can be applied to any mode is proposed. Finally, this thesis provides experimental evidence that the output sensitivity for flows greater than the turn-over point can be increased by placing the temperature sensors asymmetrically. For the current setup, sensitivity has been increased up to 6 times. That is an important breakthrough since flow rates greater than turn-over point can now be included in the measurement range without any increase in power

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