April 30, 2020

Fast SFS and FS2 thermal mass flow sensors

Respirators and ventilators are currently in high demand, and accurate measuring results are of vital importance

Air flow meters in ventilators are used to control the airflow, which is an important factor. Adults, for example, have different breathing volumes than younger persons, including children. As an alternative, temperature sensors have been used in ventilators to perform a simple air flow measurement, but they have a relatively large thermal mass. An IST flow sensor enables the possibility to have a better and more accurate adjustment through a higher sensor sensitivity, which results in better user experience while reducing the occurrence of drying out the patient’s nose, nose bleeding over time or even a lung injury from overventilation. Considering a possible breathing cycle between 300 ms to 3 seconds, the faster response time on our SFS flow sensors down to a T63 of 5 ms, allows one to perform a detailed breathing profile while opening the possibility to detect very fine and subtle breathing disorders. Both sensors, the SFS and the FS2, are suitable to detect inhalation and exhalation with a single sensor, through a clear flow direction detection.

Choose the right material
An inline flow sensor can be applied to monitor the flow through a cannula or mask and can be used to control the oxygen feedback loop. A simple and low-cost implementation was in mind during the development of the FS2 thermal mass flow sensor. While the SFS is made of silicon substrates, the FS2 is made of ceramic and glass which are inert, even if used in direct contact with 100% oxygen.

Other sensors and materials, FS7 and MFS02
During the development of the FS7, a simple implementation in combination with a robust and versatile material was achieved. The FS7 is also made of ceramic and glass (just like the FS2), however comes with only two resistors, which again allows an even simpler CTA (constant temperature anemometer) configuration when performing a flow measurement. For specialized applications, our MFS02 is made from polyimide and glass sensor construction materials.

Most modern ventilators can deliver flow rates between 60 and 120 L/min (which is 7 m/s with a 19 mm ID tube at peak flow). All sensor chips, the SFS, FS2, FS7 and the MFS02, cover a wide dynamic range and flow rates beyond the required flow volumes for a respirator application.