We have all kinds of telemetry in our data center to help us prevent and diagnose problems. One of the major areas we watch is the environment. We can tell how cold the air is coming in and going out, but thats only part of the picture. in order to keep machines cool you have to move lots of air continuously. The temperature under the raised floor could be 40 degrees but it does you no good if you aren't pushing it into the room.
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

Heres the items you will need.
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
some wire

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
Jewlers screwdriver
Soldering pencil
Rosin Flux
Super Glue

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)

If a computer fan doesn't seem quite right for the particular application, you could try a brushless model airplane motor. This has the advantage of no circuitry to disconnect. And much design flexibility, if you know how to interpret the specs. The following is an example, but there are MANY sizes and many choices in rpm's per volt. Some of these motors have &quot;cogging&quot;, which means that they probably won't turn below a certain air speed, unless you give them a little kick. <br>http://www.bphobbies.com/view.asp?id=B2552587 <br>A disadvantage is that you'd have to supply your own prop. I suspect the usual two and three bladers won't work very well. Unless the airspeeds being measured are very high, the prop should probably be larger than the one specified. <br>Possible suitable &quot;propeller&quot;: <br>http://www.bphobbies.com/view.asp?id=B1098407&amp;pid=T178449 <br>Another alternative, probably best for high air velocities: <br>http://www.bphobbies.com/view.asp?id=B1098403&amp;pid=T786626 <br>It's brushed, so you might not want to leave it running all the time.
A clever idea. I've a box full of these fans and am always on the lookout how to use them. Your idea of using just the AC is intriguing. <br> <br>Had you considered a simple sail switch available at AC supply houses? Very inexpensive. It would just indicate whether air is moving or not, but that seems to be your main concern.
The rising and falling of the voltage would indicate that your system is cycling on and off for either your heating or cooling. Even if you're running the blower continuously, it may use a different speed for heating or cooling. Interesting project. I've been looking at using one of these fans as an anemometer. If I actually get to it, I'll post an instructable about it.
If you'd eliminate the capacitor (filtering the DC) and measured the pulse frequency instead of the output voltage you could possibly get a more useful indication of speed. This might work better too if you used just a half-wave rectifier (1 diode instead of 4), this would give you just one diode voltage drop instead of two on each detected pulse from the coils.
I agree that measuring the frequency is probably a better way. Reading and reporting on the frequency would be harder to do with the equipment we have. The Aquisuite does pulse counting but I think it tops out at 10 Hz. This way was fast, cheap and easy and does what we need. Done right, done fast, done cheap; Pick 2 we picked fast and cheap :)
switching from full wave to half wave would drop the output signal to half the frequency, so if it was currently outputting 20Hz, the new output would be 10Hz. If you could buffer this to digital logic levels, you could feed it into a divider to further reduce the pulse frequency.
Do you have an idea of the working range of this device (wind speed)?<br/>I'm inclined to think that you should be measuring the <em>frequency</em> of output, rather than rectifying and smoothing?<br/><br/>L<br/>
We have a <a rel="nofollow" href="http://www.nkhome.com/ww/4200.html">kestrel 4200</a> that we use to measure the air flow out of the vented tiles. I'd be afraid to see what the air flow in front of the air handler would do to it. I have no idea what the wind speed is coming out but it feels like sticking your face out the window of a moving vehicle at 60+ mph, without the bugs of course. <br/><br/>I had considered using the pulse output (it does work) but as I said it would require opto-isolation. Originally I was in your court, knowing the frequency being generated and the surface area of the fan blades I could possibly calculate CFM, but the long and short of it is we don't care how fast its moving we just want to know that it is. It's really all the same the faster it turns the more voltage is generated and the higher the frequency goes, they are all proportional.<br/><br/>The first one of these I did was about 2 years ago. Since then we have added 3 more and they have helped us to avoid bad situations that would not have been acted upon until the room showed signs of getting warm, which in my opinion is too late.<br/>
Nice instructable, I wonder if placing a barometer in the raised floor would yield similar results, pressure drops=unit shutting off etc. like the following unit:<br/><br/><a rel="nofollow" href="http://www.hobby-boards.com/catalog/product_info.php?products_id=36&amp;osCsid=d2a56ebdd3f17c1607230bf8422b0002">http://www.hobby-boards.com/catalog/product_info.php?products_id=36&amp;osCsid=d2a56ebdd3f17c1607230bf8422b0002</a><br/><br/>which could then be monitored using standard 1wire technology..<br/><br/><a href="http://www.instructables.com/id/ELLX0VCBDTEYF8LQMQ/">http://www.instructables.com/id/ELLX0VCBDTEYF8LQMQ/</a><br/>
A barometer was discussed as a possible option, however this is a working datacenter, that is we do work quite frequently under the floor, running or removing wire or fiber. Sometimes it is necessary to remove quite a few tiles which would drop the floor pressure significantly creating a false positive. Nothing sucks more than listening to the guy from facilities belly ache about how he had to drop everything and rush out to replace the floor tile. Also from time to time its necessary to add or remove vented tiles to get the cold where you need it. Which in turn would require recalibrating your norm.

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