PaulS:
That is NOT all of your code. Quit wasting our time posting snippets.
On the contrary. I figured it would be wasteful of people's time to read all the graphical garbage I have filling the sketch prior to the sensor function being called. Also, I suggest you not allow yourself to be my slave, which is what you're doing here, if you think that I am somehow governing the usage of your time.
TomGeorge: I like your suggestion about the square wave test. I will have to see if the hardware I have can be set up to do this. I'm sure I can rig my laptop spk port with some effort. As for timing pulses, I'm not sure what the advantage would be, as I'd still need to keep track of the interrupt events and count them. If the interrupt hardware is twitchy or the signal is not clean, I suspect the count or the period will be wrong until I fix it. Am I understanding your suggestion correctly?