Electronics > Projects, Designs, and Technical Stuff
Sprint Day 2: What developing world substitutions are there for flow meters?
coppice:
--- Quote from: Simon on April 18, 2020, 03:11:15 pm ---Yes, and guess what? you have to know the pressure to calculate mass!!!! I am sorry coppice but I work on life support systems using these types of sensor!!! we also take into account pressure. More modern MAF sensors contain guess what???? A PRESSURE SENSOR!!!!!!!
--- End quote ---
You have this backwards. The MAF sensor gives you the mass which has flowed. You need a pressure sensor if you want to derive the speed from that. For a ventilator you want to know the amount of gas which has flowed, and a MAF gives you that directly.
Simon:
actually no your right we measure pressure because we want to know the volume not the mass. But as the sensor presents itself for adaption it has to be considered as air speed if you want to recalculate on a new cross section area. But they still do not tell you what the mass measurement of the sensor, they don't tell you what amount of mass is actually going through the sensor and they don't tell you the cross section of the sensor so on the available data it is a speed measurement at nominal atmospheric pressure.
chris_leyson:
Good comment about stiction and inertia but once running you would have to overcome very little friction and the load imposed by the gear trains which I guess would be quite low for a well maitained and calibrated intrument. I think the Casella air meter might date from the early 1900's and perhaps eariler if it was purchased second hand. Somewhere around here I've got a much larger version and that only needs a gentle breeze to get the turbine spinning.
It's important to note that these sort of instruments were used in free air and the pressure drop across the tubine is tiny. In a CPAP ventillator the pressure range is somewhere between 15 to 25 cmH2O more than enough to overcome stiction and get a turbine started. Turbine flow meters have been in use for over 100 years so why change something if it isn't broken.
Simon:
i am not saying that the little turbine meters are a problem. an actual turbine meter is designed to be light and rotates very easily as it has no brushes. The suggesting though that a cheap DC fan be used as a metering turbine is not a good one. You have a "heavy" rotor to get moving and it has brushes on it that create a lot of friction.
A small permanent magnet synchronous motor with very low cogging maybe if it does not have a heavy rotor.
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