Whistling. We will come back to that.

A project log for VISP - Ventilator Inline Sensor Package

Bidirectional flow (volume), pressure, and temperature sensing in a single inexpensive I2C/SPI package.

Daren SchwenkeDaren Schwenke 05/23/2020 at 09:310 Comments

The whistle was still there even after my attempt to mitigate this problem.

The idea was that I could duplicate the path of the sound, L/R style, and have it cancel out where the paths combined.  This did not work.

This may be my fault.

In the original mitigation design I had two equal length/volume paths for the sound.  In other words I was borrowing from automotive ICE exhaust system design where they put an H shape between the two banks of cylinders to selectively add and cancel out particular frequencies of sound.   Everything being equal (which it was in the original re-design), the waves from both sources should have been identical and canceled out.  From that point within the ports, the internal path to the actual sensor went off at a 90 degree angle.

Then, I moved stuff around to generate a smoother transition to the sensor tube from both ports.  This resulted in a now unequal path length to the sensor, and I believe that is why the whistle still exists.

Besides being annoying, I think this may actually be contributing to the noise I see in the data at high flow rates (when the whistle sound actually happens)

I have not decided what to do about it yet.  I suppose the first change should  probably be to go back to the equal length paths I had and see if that fixes it.  If it does not, I could always move the whisling to below audible by increasing the 'dead space'  for the cam position to sort it out, bor, but I really don't want to do that.

The whistle sound wave, is a pressure wave.  It is definitely currently hitting my pressure sensors.  My pressure sensors are capable of reporting at a good fraction of the apparent frequency of the whistle sound.  This probably means whenever I do hear a whistiling sound that I am not getting the best data that I could be.

In any case, what we have works for testing... with the above potential caveats.  In the interest of moving forward, I am currently willing to ignore this part.