Sprint Day 2: What developing world substitutions are there for flow meters?

[coppice]

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.

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.

Ventillators go back a long way. Before MEMS mass flow sensors there were larger versions of mass flow sensors (there still are in soma applications, like HVAC systems) which may have been used, but I assume if you go back far enough they used spinning paddles, The stiction is a definite drawback, but if you want cheap, and quick to implement it could be a reasonable way to go.

[Simon]

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.

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.

Which is air speed! the air in a smaller tube has to go faster for the same volume. A small unknown portion of the air goes through the sensor but the result is meaningless in terms of volume if you do not know the cross section area you are sampling. The sensor element on it's own will not tell you the mas airflow you have to calculate that yourself. Further back i uploaded the datasheet for a bocsh sensor, the same sensor can be purchased in different size tubes and each one will give different mas flows for the same output voltage. With great ease we took the curve of the smallest tube and recalculated what it would be for a slightly smaller tube. The change being so small that our recalculations were very accurate when we came to callibrate the setup.

Put it this way, if you take the sensor out of the tube and run around a room with it will you get a reading? Yes you will, What mass of air did the sensor move through? No one can tell you. Or what if you blew on the sensor, what mass of air did you blow? no one can tell you because the cross section area is unknown. At it's very heart yes t is a mass flow that causes the reading but the sensor itself outputs air speed,

Same reason why my peak airflow meter is a tube, it needs a known duct to tell me what the flow was or it would just be a dumb pressure meter. Unsurprisingly the further up the scale you go the closer the marks but then 700 and 800 are further apart again so clearly this was calibrated.

[Simon]

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.

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.

Ventillators go back a long way. Before MEMS mass flow sensors there were larger versions of mass flow sensors (there still are in soma applications, like HVAC systems) which may have been used, but I assume if you go back far enough they used spinning paddles, The stiction is a definite drawback, but if you want cheap, and quick to implement it could be a reasonable way to go.

I am not saying not to use that type of sensor but to re-purpose a fan is a bad, bad idea. My £15 peak flow meter off amazon is entirely mechanical. But it was designed for this purpose.

[coppice]

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.

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.

Which is air speed! the air in a smaller tube has to go faster for the same volume. A small unknown portion of the air goes through the sensor but the result is meaningless in terms of volume if you do not know the cross section area you are sampling. The sensor element on it's own will not tell you the mas airflow you have to calculate that yourself. Further back i uploaded the datasheet for a bocsh sensor, the same sensor can be purchased in different size tubes and each one will give different mas flows for the same output voltage. With great ease we took the curve of the smallest tube and recalculated what it would be for a slightly smaller tube. The change being so small that our recalculations were very accurate when we came to callibrate the setup.

Put it this way, if you take the sensor out of the tube and run around a room with it will you get a reading? Yes you will, What mass of air did the sensor move through? No one can tell you. Or what if you blew on the sensor, what mass of air did you blow? no one can tell you because the cross section area is unknown. At it's very heart yes t is a mass flow that causes the reading but the sensor itself outputs air speed,

Same reason why my peak airflow meter is a tube, it needs a known duct to tell me what the flow was or it would just be a dumb pressure meter. Unsurprisingly the further up the scale you go the closer the marks but then 700 and 800 are further apart again so clearly this was calibrated.

Try increasing the gas pressure, while maintaining the same gas speed. Something like a paddle sensor, which senses speed, doesn't increase its reading. Try it with a mass flow sensor, and the reading does increase, because more mass of gas per second is flowing past the sensor.

[Simon]

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!!!!!!!

[coppice]

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!!!!!!!

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.