Electronics > Projects, Designs, and Technical Stuff
Sprint Day 2: What developing world substitutions are there for flow meters?
chris_leyson:
A small turbine might be one approach as this wouldn't be effected by changes in air temperature or humidity, perhaps a small DC fan used in reverse. Small turbines were used in mines to measure airflow and here is a picture of an L. Cassella air meter No. 312. I think it's calibrated in cubic feet.
coppice:
--- Quote from: Simon on April 18, 2020, 01:20:52 pm ---All flow measurement tends to revolve around speed measurement. It's how car mass airflow meters work.
--- End quote ---
Mass gas flow sensors don't measure speed. As their name says they measure mass flow. For gases mass movement is a combination of speed and pressure, and for most gas flow measurements its the measurement you really want. The actual MEMS sensors typically used these days for mass gas flow measurement are very cheap. If BOM is your major issue, building the basic MEMS device, from someone like Memsic, into your own tube should be a winner.
You could also look at the ultrasonic gas measurement solutions being built for domestic energy meters. You can measure flow quite accurately for a few dollars with those, but the mass flow MEMS sensor is probably a lot quicker to design in.
Simon:
--- Quote from: coppice on April 18, 2020, 02:15:53 pm ---
--- Quote from: Simon on April 18, 2020, 01:20:52 pm ---All flow measurement tends to revolve around speed measurement. It's how car mass airflow meters work.
--- End quote ---
Mass gas flow sensors don't measure speed. As their name says they measure mass flow.
--- End quote ---
Have a look at vehicle mas air flow meters. They are air speed meters. I use them, we take the meter out of the tube it comes in and into a smaller tube to increase sensitivity because same mass in a smaller tube has to flaw faster. They work on the basis of air cooling a temperature dependent resistor whilst compensating for ambient.
Any flow measurement based on pressure differential is a speed measurement, again the cross section of the channel needs factoring in. If you measure pressure drop over a restriction of same mas flow but in different diameters you will get different pressure results.
coppice:
--- Quote from: Simon on April 18, 2020, 02:41:23 pm ---Have a look at vehicle mas air flow meters. They are air speed meters. I use them, we take the meter out of the tube it comes in and into a smaller tube to increase sensitivity because same mass in a smaller tube has to flaw faster. They work on the basis of air cooling a temperature dependent resistor whilst compensating for ambient.
--- End quote ---
The reason you have increased the sensitivity is that the MEMS sensors only sample the mass of the air close to the edge of the tube. If you reduce the tube diameter that becomes a bigger fraction of the total mass flowing through the tube. Its not a linear relationship, because the gas near the middle of the tube typically moves faster than the gas near the edge, but this is fairly consistent over time, so you can calibrate a good mass flow figure for any particular tube.
Simon:
--- Quote from: chris_leyson on April 18, 2020, 02:11:29 pm ---A small turbine might be one approach as this wouldn't be effected by changes in air temperature or humidity, perhaps a small DC fan used in reverse. Small turbines were used in mines to measure airflow and here is a picture of an L. Cassella air meter No. 312. I think it's calibrated in cubic feet.
--- End quote ---
you would have to account for "stiction" or startup inertia and friction that would bias results. I have just bought a peak flow meter to measure how strong my lungs are. The instructions state to test the freedom of movement of the marker, it's a very simple instrument that relies of free movement with low friction of the needle.
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