A novel technique of flow measurement using thermistor with real time graphical display for anaesthesia ventilator

J. Kaur, J. Kumar, P. Kapur, B.S. Sohi
Central Scientific Instruments Organisation, Sector 30 C, INDIA

Anaesthesia ventilator is a life saving device that is quite useful in clinical applications under general anaesthesia giving respiratory support during various surgeries in the operation theatres. When respiration takes place, flow at different points of the respiratory tract changes.

An inexpensive airflow monitoring system based around thermistor based circuitry, 80C31 microcontroller with RAM, EPROM, AD574, peripheral interface devices and graphical LCD etc., is a module of the indigenously developed instrument. The airflow changes over a respiratory cycle. This makes the anaesthetist to have a feeling of how flow in the tract changes with time. Anaesthesia ventilator uses thermistor for measurement of instantaneous flow rate of respired gas. An accurate and sensitive type of air flow sensor i.e. thermistor has been used outside the mouth and inside the piping. In application of thermistor for flow measurement, self heated region of V-I characteristic curve of thermistor has been used. What has been envisaged here is to develop a flow measurement system using graphical approach.

This paper describes development of an accurate gas flow measurement system (0-70 l/m), which is particularly helpful for airflow measurement. Main advantages of developed technique using thermistor are simplicity of design, dynamic stability and fast response time. Finished thermistors are chemically stable and not significantly affected by aging. Thermistor based hardware design associated with graphical approach has been developed successfully and incorporated in indigenous prototype of anaesthesia ventilator.

Keywords: Anaesthesia ventilator, Flowmeter, Gas flow rate, Mass flow controller, Thermistor