The electronic racks in many of the older Physical Electronics / PHI surface analysis systems use fans to provide airflow to cool the electronic units and card rack boards. As part of the yearly preventive maintenance on these systems the fans should be inspected and replaced as needed.
A dead fan is easy to spot as the blades are not spinning at all. But for the fans that are spinning, how can you determine where the fan is providing good airflow?
There is an effect that we have all seen called the Wagon Wheel Effect. The phrase comes from seeing wagon wheels that appear to be spinning backwards or even not spinning at all in old west movies and TV shows. What it is really is just the frame rate of the movie or TV camera and how it syncs to the spinning wheel. You can see this effect on the card rack fans due to the 50 or 60 Hz fluorescent or LED room lights and even in outdoor steady sunlight due to the few milliseconds that the human brain needs to detect motion.
What this means is that you can’t be 100% sure of how fast a fan is spinning just by looking at it. An Anemometer is a way to measure the velocity of air and is a great way to get a relative answer as to how much airflow the fan is putting out. For this blog post I purchased a TOPTES TS-301 on Amazon for about $30.00
This air speed measuring device is equipped with easily rotatable fan blades and measures air velocity in 6 units: Feet per minute (ft/m), Feet per second (ft/s), Miles per hour (mph), Knots (knots), Kilometers per hour (km/h), Meters per second (m/s) Value of wind velocity x10.
For this blog post we used km/hour to get a relative fan air speed measurement. In the picture below we get a measurement of 15.3 km/h on a working fan.
Then comparing that to a non-working fan (the blades are not moving at all) we get a measurement of 4.1 km/h just from the air that is flowing past the defective fan.
All the good fans read close to 15 km/h and so the anemometer is a good way to confirm not only that a fan is working but that the airspeed is comparable on all good working fans.
Another way to get a relative airspeed indication is to use a small piece of paper such as a post it note and see how much the paper bends when held over a fan.
In the picture below we and see that a good fan bends the post it note by a noticeable amount.
The non-working fan does not move the post it note very much at all.
This method is not as quantifiable as using an anemometer, but it is still better than just feeling the air with your hand. You could also use a small piece of string.
The anemometer displays air speed, but fans are rated in CFM (cubic feet per minute). If you want to convert air speed to CFM you need to factor in the area of the fan blades. This blog post explains how to do that –
Finally, when testing the fans you should also inspect and clean the air filters, this blog post shows where those are located –https://www.rbdinstruments.com/blog/preventive-maintenance-rbd-techspot/
