Magnetic Flow Sensor

A unique flow sensor designed for applications in acute care.

Siare Engineering International Group in 1993 decided to design a dedicated sensor for spirometry applications in lung ventilation and created a flow sensor which is made of a steel foil with high
magnetic permeability (magnetic permeability is the ability of the material to create a magnetic field). This steel foil is connected to a metallic support on one side to form an interlocking shelf.
The force created by the gas flow moves the steel coil from its equilibrium position. This movement creates a variation in the inductance of the 2 coils and a corresponding signal is detected and converted into voltage.

Finally, the signal, once transformed into electric signal, is elaborated by a microprocessor and sent to a display based on calibration tables.

Monitoring gas flow and volume exchanges is crucial in critical care to minimize ventilator-induced lung injury. Ventilators, use flow sensors to gauge amount of gas delivered to the patients and adjust the desired amount of gas to be delivered. It is crucial to identify considerations in measurements of flow to determine appropriate design of the flow sensors for ventilators:

  • The flow sensor should offer the least amount of residual pressure during respiration (i.e. the PEEP – Positive end respiratory pressure)
  • It should not generate resistance to the inspired and/or expired flow (the American Thoracic Society suggests for volumetric flowmeters used for the forced expiration tests a resistance to the flow of ≤ 1.5 cmH2O/(l/s))
  • It must have a series of performance capabilities such as accuracy, precision, resolution, reproducibility, static sensitivity, controlled zero drift, linearity, wide range, and adequate inlet impedance)
  • With respect to expiratory flow, one of the major contaminant is water or water vapor; the sensor should not get affected by water vapor
  • The temperature and gas composition changes can vary sensor calibration. The sensor should vary as less as possible across the range of temperature & gas composition that the system is likely to encounter.