Freeflow flow sensor

100% open hardware/software flow sensor.

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This flow sensor is being developed in response to the current Covid-19 pandemic.

The sensor itself has two primary purposes:

It can be used stand alone to monitor the output of a mechanical ventilator. In this mode the small OLED display can be used to display pressure, flow rate and breath frequency. Limits can be set on any/all of these parameters to monitor the patient. If any parameter exceeds the set limits, an on board alarm will sound to alert medical personnel.

An alternative use is as a component to a larger ventilator system. The current version of the PCB includes an RS232 serial port which can be used by the ventilator controller to read current pressure and flow rate from the sensor. The sensor also includes a USB port which can be used for the same purpose.

The sensor itself consists of a 3D printed venturi tube with a PCB mounted on top of it.

A venturi tube is a simple type of flow sensor.  It's basically a tube with a constricted center portion.  As air flows through the tube it is forced to speed up as it passes through the constricted throat of the tube.  This increase in speed comes with a decrease in pressure.  By sensing the air pressure both at the entry to the tube and at the throat of the tube the flow rate of the air passing through the tube can be determined.

A PCB is mounted on top of the venturi tube.  This PCB holds the following main components:

  • Processor.  I'm using an ARM processor made by ST (STM32 family)
  • Pressure sensors.  There are two sensors mounted to the bottom of the board which sense flow in the venturi tube
  • OLED display.  This is used to display current pressure/flow rate information when the device is being used in stand alone mode.  The data is represented both in text form and as a moving graph at the bottom of the display
  • Rotary encoder.  This provides a knob that can be used to switch between screens of the display and enter limits for monitoring.
  • Buzzer.  A magnetic buzzer is used to alert medical staff in the event that the flow rate or pressure exceeds the limits entered.
  • Communication interfaces.  Both RS232 and USB ports are available for connecting the flow sensor to other equipment.

All design files are available in the github link below.

Additionally, a video is linked below which shows the results of some initial testing of the sensor.  Results have been very promising.  

  • 4-19-20 update

    Steve Glow04/19/2020 at 12:18 0 comments

    I haven't been doing a very good job of keeping this site up to date, but have been working hard on the flow sensor.

    At this time I've seen a lot of interest from people who are designing ventilators and would like to use this sensor as a component in their designs.  That's great, it's the reason I first started the project!

    I've shifted focus from making a stand alone monitoring device to making a simple accurate sensor that can be easily integrated into other projects.  I'm still planning to pursue the stand alone version as well, but that's a secondary task at the moment.

    I'm currently in the process of evaluating different pressure sensors.  The ones I'm using now are great, but I have concerns about their availability, there isn't much stock out there and what is there seems to be going quickly.  I've got a few different boards being built right now which will allow me to test with a couple different models of pressure sensors which are both inexpensive and abundant.  Stay tuned...

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