Thursday, 28 May 2020

New Venturi tube for Mass Air Flow sensing

Some of my blog readers in the Sudan have asked me to help them in designing equipment for respirators.  At the moment everyone everywhere seems to be suffering with COVID-19 problems and I suspect certain countries are having a much worse time than others.

The readers have seen my efforts at designing a 3D printed venturi tube and believe it might help them in their efforts to design a easy to build respirator.  There are several open source respirator projects at the moment - here are two of the more promising ones:



I have looked for information on the air flow sensing part of the requirements and cannot find much information.  It would seem that this part of the problem is either being overlooked or resolved with off the shelf products.  I don't have any issues with using off the shelf products but they will only be available to those that can afford to pay for them...and they are only available...while stocks last.

The product most discussed is the SFM3300 Mass Flow Meter made by a company called Sensiron:


It looks to be a well designed product and is available here in case this is useful:



I'm looking to design my own version using the following electronic breakout modules:


The datasheets for these devices are here:


These components will need to be mounted on or near to a new venturi tube which will be 3D printed to ensure it is easy to manufacture.  The electronic signals from the breakout board will be connected to the analogue to digital converter (ADC) and the output from the ADC will connect to an STM32 microcontroller known as the 'blue pill'.


The microcontroller used on the 'Blue Pill' in this version is the STM32F103C8T6.  It's datasheet is here:


Here is the design for the venturi tube:

Latest Venturi Tube with indent for BME280 Breakout PCB

The principle dimensions for the tube are 105 x 36 x 31 mm and I will be 3D printing it very shortly to allow for testing.  Once I have printed and tested one I will be performing all of the calculations required to use it with the MPX7002 Pressure sensor.  I will be sharing the files once I have validated the design.

Internally the venturi tube has several cylindrical volumes removed in order to create the venturi apertures.

Side profile showing the internal volumes of the Venturi Tube

The next post will cover the calculations of the internal volumes of the tube ready for writing the firmware for the microcontroller.

That is all for now, take care - Langster!
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