Blendtec says their blenders go up to 28,000 rpm. I want to know how my average blender stacks up.
But more than that, I’m interested in ways to measure high speeds using household objects. I’m not thaat interested in comparing blenders, so looking at wattage or just saying, “Most blenders spin at 10,000…” aren’t enough. Could I use a guitar pickup to detect the blades? I think a laser, detector and counter would work.
On bicycles we use a small magnet attached to one of the spokes and a pickup attached to the frame to measure how many times the wheel spins around. That way you can tell how fast the bike is going once you input the wheel/tire diameter.
However I don’t think a bicycle-specific computer would work. Even though the magnet is ‘small’ it would probably would weigh enough to throw a blender out of balance. Also Lance Armstrong couldn’t get a bike wheel to 10000 rpm no matter what drugs he was taking. 
Still you’d think that the scientific community would have such a device.
Another thought would be using an adjustable strobe light. Paint one of the blades and when you get it to ‘freeze’ (like a timing light on a car) you’d know how many times per minute the blades are spinning.
Found them! Go to amazon.com and search on “rpm meter”. They have a bunch of non-contact models for under $25 that go to 99,999 rpm.
I think that strictly, you’d only know that the blades are rotating at a multiple of the speed of the strobe light, although you may be able to take multiple readings to infer a higher speed – if it freezes at 500Hz and 700Hz, then it has to be going at least the least frequency that is a multiple of both: 3500Hz.
I think the simplest with consumer-level tech is a laser-pointer/photo-detector/oscilloscope setup, with the laser blocked by the blade at some point in its rotation. Then just watch the signal on the scope.
Only $25? Wow, that’s even cheaper than most DIY solutions, unless you already have the parts. Still hoping for innovative solutions, though.
It’s not all about RPMs though, it’s about torque. Who cares if it spins at 28,000 if it’s going to jam the first time you drop a chunk of ice in it. OTOH, if your’s spins at 15,000 but can grind up concrete…
Now, if it spins at 28,000 (and, say, regular ones spin at half that) it’s certainly going to have some momentum behind it that’ll help push it through some of the harder things, but that doesn’t mean the motor is strong.
We have a Blendtech blender. It will blend just about anything.
You tube has numerous videos of that thing grinding iPhones, iPads, soda cans, etc.
I’ve got a LEGO rotation sensor. I’d have to gear down, but I could measure a blender’s RPM with my RCX.
ETA- I also have a standalone LEGO rpm counter. But I’d use the RCX. 
Frankly, this sounds like an excellent way to cut your fingers off.
My first though was to attach a mirror to it, bounce a laser off of it to a distant retroreflector, and see how much the returning beam was deflected. Basically, the same way that Michelson measured the speed of light, except in reverse.
I make no claims about the practicality of this method, of course. But it would in principle work.
I wanted to tie 1 m of string to it and see how long it would take to wind it up.
I know this is an old thread, but I just finished writing up a description of how to measure blender speeds with two relatively simple DIY methods. The first method uses a simple stroboscope (like JerrySTL suggested) rigged up with an Arduino. The second method is easier, and all you need is to record the sound and then use software like Audacity to look at the frequencies. Here is the writeup: How to Measure Blender Speed: Methods.
Both methods worked surprisingly well for the Vitamix blenders that I tested.
Wow fascinating, if anything I didn’t think Arduino and LEDs would make a decent strobe at 380Hz.
I think a guitar pickup might also work…
The default Arduino clock speed is 16 MHz, and you can generally implement code with microsecond resolution, so I wasn’t anywhere near the upper limits. The one thing that I found to be slow/limiting is the serial-USB connection that I used to display the frequency. It’s not a problem as long as you don’t try to do a data transfer on every cycle. (My solution was to only initiate a serial transfer upon a button press on the board.)
Very interesting, arcticsilo. I hope you stick around the Board and continue to contribute.