Indoor cycling fan upgrade and speed check

I recently bought a Cleva Air Mover centrifugal fan as an upgrade to my indoor cycling setup. 

It came highly recommended on several TrainerRoad forum posts (see here).  Compared with my old fan, it seems to be a lot better.  I wanted to measure exactly how much better it is, and this blog post describes my attempts to measure the air speed that the fan generates.  As usual, it's all a bit nerdy and over-the-top, so sorry about that.

Airspeed measurement methods

I bought a cheap anemometer (an anemometer is air speed measurement device) from AliExpress for an incredibly cheap £13.05.  It seemed good, but given that it was so cheap, I wanted to check its accuracy with a second source of airspeed measurement.

Having recently bought a 3D printer, I decide to make a pitot tube that I could use as a 2nd instrument to measure the air speed generated by the fan.  It was quick to create in CAD (<1 hour) and printed in less than an hour.

A pitot tube is a device that measures the total pressure of the flow, by having tube that is orientated in the flow direction.  When the flow enters the pitot tube is comes to rest within the tube, and when it does so the pressure will be the total pressure. Total air pressure is the ambient (static) pressure of the air plus the dynamic pressure of the moving air.

Hence, the difference between the static pressure and the total pressure is the dynamic pressure, which is a function of the air speed and the air density.  Hence, but knowing the air density and the by measuring the dynamic pressure (via the difference of total and static pressures) the air speed can be obtained.

I measured the difference between static and total pressure using a device called a manometer, which is the U-shaped device shown in the sketch on the left.  The manometer tube is filled with a liquid, shown in purple in the sketch, and could be any liquid such as water or mercury.  I used water because it was easy and its relatively low density helped with measurement precision.

I bought a manometer from Amazon for about £20.  A quick calculation in Excel allowed me to calculate the airspeed that corresponds to various manometer water height differences.  I also checked how sensitive the results would be to small changes on pressure and temperature, which affects the air density.

Results

I measured the airspeed at 0.5 metres from the fan nozzle and also at 1.0 metres from the nozzle.  These are distances that are representative of how far away the fan would be located from me when I'm cycling.

0.5 metre distance

The AliExpress anemometer gave a maximum airspeed, in the centre of the air stream, of 11.2 m/s.  The reading fluctuated by approximately +/-0.8 m/s.

My pitot tube gave a water height difference of 8 mm in the manometer, with a manometer measurement precision of about +/-1 mm.  This corresponds to an airspeed of 11.32 m/s +/- 0.8 m/s.  The nature of the pitot tube and manometer meant that any air speed fluctuations are damped, although the +/-1mm manometer measurement precision results in a +/-0.8 m/s uncertainty. The two measurement sources, the pitot and the AliExpress anemometer, therefore agree very well for this 0.5 metre case.

1.0 metre distance

At a distance of 1 metre, the AliExpress anemometer gave a maximum airspeed of 8.8 m/s, with +/-1.0 m/s fluctuations.

My pitot tube gave a water height difference of 4 mm in the manometer, +/-1 mm.  This corresponds to an airspeed of 8.0 m/s +/- 1.0 m/s.  Again, the two measurement sources therefore agree very well at this distance of 1m, and well within their measurement precision.

Speed directly at the nozzle

The Cleva website says that the maximum airspeed of their Cardio 54 fan, which is mechanically identical to their Air Mover Fan, is 54.0 kph, 45.0 and 34.2 kph for the three speed settings.  These speeds correspond to airspeeds of 15.0 m/s, 12.5 m/s and 9.5 m/s.  It's highly likely that these speed quoted by Cleva are the maximum speeds, directly in front of the nozzle.

I checked the speed using the AliExpress anemometer, and that device gave speed recordings at the fan nozzle of 15.4 m/s, 12.9 m/s and 10.7 m/s for the three settings, all with a variability of about +/-0.4 m/s.  These values are therefore consistent with Cleva's quoted airspeeds, and actually are slightly faster.