[Solved] Anemometers, how do they work? My strange story.

[ballsystemlord]

Hello,
I wanted to measure the airflow of some computer fans I own, so I purchased an anemometer, this one: https://www.ebay.com/itm/195382463171 . I understand that to get a really accurate figure, I'd need a large machine that has a chamber. I think an anemometer would be good enough for practical usage.
  I held the top of my anemometer up to my fans output. I got an average reading of 2.4m/s. Out of curiosity, I held it right against the fan's motor such that the anemometer's tiny fan couldn't spin because there was no airflow. I got a reading of over 6 m/s. This seemed really strange to me, so I thought, "Maybe it's the vibration that is causing a reading." I pushed the anemometer against my fan and did not get a reading.
  Puzzled, I then covered the anemometer with my hand so that both the small and large opening couldn't get much, if any, air through them. I held it up to the fan and got a reading of 5-6m/s. I removed my hand and got a reading of 1.7m/s from the same distance.

I'm totally lost here. How do anemometers work?
Why am I getting higher airflow while the anemometers fan is not turning?

Thanks!

[Andy Watson]

The little fan in your anemometer rotates with the air speed. My guess is that this rotation is sensed magnetically. When you bring the anemometer too close to the big fan it is sensing the magnetic field of the fan instead.

[ralphrmartin]

Other types of anemometer work by the principle of measuring the amount a hot wire is cooled by the airflow. In practice, the means measuring the current needed to keep it heated to a fixed temperature. Compensation is needed for the temperature of the airflow, and would be important in the case of the flow produced by a cooling fan.

[mendip_discovery]

The vane type uses a sensor as such and counts the number of times the blade passes a point (often metal blades) using a hall sensor or they just measure the number of rotations. Then do some maths to work out the airflow, they have the advantage of being able to average out the readings due to it being a mass to rotate.

You can also get hotwire and pitot tube systems but they suffer issues of being very sensitive to position. Even more than the vane ones.

The thing is there are some magnetic things inside the anemometer that might suffer from being near a fan, the other is electronic noise from the fan especially if it's a PWM one. I doubt the anemometer is able to handle the noise given off and it is messing up its numbers.

[AndyBeez]

I certainly agree the hall effect sensor is measuring an external magnetic field.
For those playing at home, there is another type of  anemometer, the ultrasonic anemometer. These are expensive and targetted for use in automated met stations. But there is nothing to stop you mounting one on the roof.

More: https://r-p-r.co.uk/windsystems/anemometers.php

[MrAl]

Hello,
I wanted to measure the airflow of some computer fans I own, so I purchased an anemometer, this one: https://www.ebay.com/itm/195382463171 . I understand that to get a really accurate figure, I'd need a large machine that has a chamber. I think an anemometer would be good enough for practical usage.
  I held the top of my anemometer up to my fans output. I got an average reading of 2.4m/s. Out of curiosity, I held it right against the fan's motor such that the anemometer's tiny fan couldn't spin because there was no airflow. I got a reading of over 6 m/s. This seemed really strange to me, so I thought, "Maybe it's the vibration that is causing a reading." I pushed the anemometer against my fan and did not get a reading.
  Puzzled, I then covered the anemometer with my hand so that both the small and large opening couldn't get much, if any, air through them. I held it up to the fan and got a reading of 5-6m/s. I removed my hand and got a reading of 1.7m/s from the same distance.

I'm totally lost here. How do anemometers work?
Why am I getting higher airflow while the anemometers fan is not turning?

Thanks!

Hi,

If you can get away with using a tube the same diameter as the meter you can make it maybe 6 inches long.  That will keep the meter movement away from the fan.  If that doesnt help try a longer tube.

[rvalente]

I'd use a MAF (mass air flow sensor) from a CAR, which is based on the hot wire with a wheastone bridge.

[ballsystemlord]

Thanks again everyone.
I consider this question answered.