Tuesday June 18th

I brought a fan from home that actually has a spec sheet and decided to test it out.

The spec sheet says the RPM should be 2600. Since I didn't have a strobe light, and since the photogate wouldn't work at the speed we were operating on, I hit upon the idea of matching the pitch to obtain the speed. The fan blades whooshing through the air create a pitch that can be matched with a frequency generator. Any musician (like me) can match the note.  Matching the pitch gave me either 302Hz or 604 Hz (one octave apart, which can be tough to tell. With 7 blades on the fan, 302Hz divided by seven is 43rotations per second, or 2589RPM. As the spec sheet says 2600RPM, this is probably correct. Probably is not good enough for me...
I knew I could strobe the fan to get the speed but my strobe is at school, so I wrote an easy arduino code to blink an LED at a high frequency to act as a strobe. Since the arduino is limited to whole numbers of milliseconds, this made it tough. The code I wrote would blink a light about 43 times per second (really 45). The arduino would flash the light every 22milliseconds.

To get the measurement, we had to bring the voltage of the fan up to 13V, but we got confirmation that the 2600RPM on the spec sheet was probably correct. Based on the bust yesterday from the pressure sensor, we needed to find a way to measure the CFM (cubic feet per minute) that the fan could pull when attached to the box.        

We took a trash bag (very thin) and taped it to the exhaust port on the box. We then ran the fan and timed how long it would take to fill the bag. It took about 33 seconds. OK, now to measure the volume of the bag......hmmmmmm. Can't submerge it and do water displacement. "I do have a giant 60mL syringe! I'll just take the air out 60mL at a time," I said stupidly. After 2100mL of air had gone out and the bag hadn't changed shape, I decided to go with another approach. Since the method of determining how long it took until the bag was "full" was sketchy at best, we didnt need measurements to 4 sig figs. I refilled the bag and squashed into a pumpkin shape.

The measurements on the bag were roughly 22.2 cm tall and 34.5 cm in diameter. This puts the volume at 21000mL or cm^3, which is .73 ft^3. This gives us a CFM of 1.3, far from the 30 on the spec sheet. But at least we have numbers!

The rest of the day was spent practicing arduino, and coming up with ideas for tech fest. More later.