I use an Oxycheq. It's not that fancy, but does the job, and I think it cost me less than $180. Considering a sensor is about $75, that's not too bad.
If you're poor but good at basic algebra, you can just buy an O2 sensor and use a voltmeter. You'll need a calculator, I bet your cell phone has one.
The sensor puts out voltage in mV. The voltage will skew as the sensor ages, but typically a brand new sensor will spit out a number like 11.2mV when you calibrate it with air. Calibrate first, then run the sensor in the flow of a nitrox tank and take the new reading. Use the following formula to figure out the voltage:
(cgFO2 ÷ cgmV) • ngmV
cgFO2 is calibration gas FO2.
cgmV is calibration gas voltage.
ngmV is nitrox gas voltage.
So.. If you calibrate in air, and get 11.2mV as the calibration voltage, and when you analyze your nitrox mix you get 17.4mV your nitrox % would be..
(0.21 ÷ 11.2) • 17.4
32.6%
Ta da!
BTW -- the Oxycheck kit is basically just a panel volt meter with some electrical doodad wizardry to essentially start the mv output somewhere close to 21%, and then a potentiometer to adjust the output during calibration. They even have a "do it yourself kit" that is all the parts and you can assemble it yourself.
---------- Post added February 13th, 2015 at 05:43 PM ----------
And someone posted the link to the build it your own kit while I was typing my book.