We wanted a way to tell if the air was flowing in our data center but real air flow instrumentation can be expensive. Air handlers move air very fast and a simple wind speed gauge would fall apart in a matter of days. There are some proprietary solutions that are relatively cheap and would work for our needs but they wouldn't easily integrate into our current home built monitoring solution. This is my somewhat cheesy but working solution.
Step 1: Supplies
1) Computer cooling fan
4) General purpose diodes
Resistors + potentiometer (I used a 10k pot and ~2.8k resistor)
200-300 micro ferrads worth of capacitance
small perf board
small project enclosure (An Altoids tin would work fine)
some sort of connector (I used an rj45 and a keystone jack that I had disassembled)
Needle nose pliers
Total cost to build 2 units ran me around $40 US with parts to spare.
I bought the resistors, dioeds and capacitors in a "variety pack" and saved a couple of bucks
The fan that you use is the biggest factor, you want one that turns very easily preferably with large surface area across the blades. For my purposes I needed a fan capable of generating 0-10 volts DC
After quite a bit of experimentation I settled on a Radio Shack "Brushless 12VDC Cooling Fan" #273-238
The equipment that all this will plug into is a Veris Industries H8820 Acquisuite which in a nutshell is an embedded linux device for doing power and building monitoring. It has several a 0-10VDC pins to use. These are not cheap but we already have one so why not use it to its full capacity.
Another (cheaper) option might be a Z2TEN one-wire device and associated bus master like a HA7Net.
Because I'm using the H8820 as input I measured the impedance of the inputs and determined that its ~12.7K so my rectifier circuit will closely match that value (10k pot +~2.8k resistor)