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I'm trying to deeply understand how the air-flow sensors mounted inside heaters works. They are composed of a U-shaped PCB with an NTC on each end. For example:

This PCB is inserted into a tube where the air must flow. If one of the NTC was shielded from the air-flow I can guess you can detect the movement due to the lowering of the temperature measured from the other sensor.

But because both will be affected by the air-flow it's not clear to how to measure such a movement.

Is this solution effective also in open air? I mean, placing this kind of sensor at the center of a room, could it detect any movement in any direction?

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    \$\begingroup\$ Search the interwebs for "hot wire anemometer" to see examples of what you want to do. \$\endgroup\$ – Elliot Alderson Dec 19 '20 at 16:43
  • \$\begingroup\$ Are they connected in a bridge configuration? \$\endgroup\$ – AJN Dec 19 '20 at 16:45
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    \$\begingroup\$ The basic concepts of a hot wire anemometer are the same as those of your sensor, even if the picture doesn't look the same. Maybe you should clarify what you mean by "this kind of sensor". \$\endgroup\$ – Elliot Alderson Dec 19 '20 at 16:51
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    \$\begingroup\$ "If one of the NTC was shielded from the air-flow I can guess you can detect the movement due to the lowering of the temperature measured from the other sensor." This sounds as though you think that you will read a lower temperature with moving air than with static. You won't unless the sensors generate heat. This is a common misconception due to people's misunderstanding of wind chill which is an attempt at putting a figure on increased heat-loss from the heat source - your body - due to increased convection cooling. \$\endgroup\$ – Transistor Dec 19 '20 at 17:28
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    \$\begingroup\$ So "this kind of sensor" really means "exactly this sensor", rather than a similar sensor with a different shape, or really any other sensor that uses the same principles. \$\endgroup\$ – Elliot Alderson Dec 19 '20 at 18:20
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There are two thermistors. One is operated at low current (or at least low power dissipation) and is there to measure the temperature of the air stream.

The other is operated at a relatively high current (or at least high power dissipation) and has significant self heating. That thermistor will have a resistance that is related to both the temperature of the air stream and the air flow, as the forced convection draws more or less heat away from that thermistor.

The first thermistor is used to compensate the second one, so as to separate the two effects.


In answer to your second question, that kind of sensor is not very sensitive to air flow direction, though there might be some effect from the hot thermistor heating the air going to the ambient sensor and the shown PCB design will block air flow in certain directions.

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