Electronics > Projects, Designs, and Technical Stuff

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

<< < (11/12) > >>

coppice:

--- Quote from: Simon on April 18, 2020, 02:47:29 pm ---
--- 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.

--- End quote ---
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:

--- Quote from: coppice on April 18, 2020, 02:46:30 pm ---
--- 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.


--- End quote ---

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:

--- Quote from: coppice on April 18, 2020, 02:55:00 pm ---
--- Quote from: Simon on April 18, 2020, 02:47:29 pm ---
--- 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.

--- End quote ---
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.

--- End quote ---

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:

--- Quote from: Simon on April 18, 2020, 02:58:36 pm ---
--- Quote from: coppice on April 18, 2020, 02:46:30 pm ---
--- 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.


--- End quote ---

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.

--- End quote ---
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!!!!!!!

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod