I’m working on an electronic design that includes a fan to exhaust warm air, and a filter on the air intake to keep the electronics from getting covered in dust. The finished project will be placed in a somewhat dirty room that is quite remote from where I will be monitoring the equipment. I want to sense whether the filter is dirty enough to restrict air flow. Of course I can (and will be) monitoring temperatures, but I’m looking for something that’s a more direct measure of air flow.
I had an idea: that there should be a difference in fan speed between a clean filter and a dirty filter. The fan speed should go down, I thought, if the filter was clogged up (as it would be working harder, kind of like a car going uphill). So I decided to test this thought out. I connected a small fan (an 80 mm “case fan") to 12 VDC, and monitored its RPM. Sitting on my desk, this fan rotated at just under 4,500 RPM. Then I put a piece of cardboard across the intake side of the fan, expecting the RPM to go down, though I didn’t know by how much. The RPM with the cardboard completely blocking the intake was 4,700 RPM — it went up! This effect was completely repeatable…
Does anyone know the explanation for this?
