Sensing something?

Starting to wrap my head around sensors. The one i got in class is a fuel tank pressure sensor that fits almost any GM vehicle. The tank is supplied with a certain pressure, the sensor can recognize any change in that pressure. Things like the gas cap being open or the EVAP purge valve stuck open will tell the ECU the drop in pressure which throws the check engine light on. This is not as much of a drive-ability issue as it is a environmental issue. It is a 3 wire sensor, 5v reference, output and ground.Its quite easy to test this sensor by applying 5v to the reference and a 12v ground. Then you simply blow into the tube and see if your output volts fluctuate from 0.1v to 4.9v. I ran into a problem at work where the oil pressure gauge was not showing any pressure. So I started barking up the wrong tree when I saw only 5v going to the sensor, so after some pointless wire chasing I gave up and went to the internet and that's when I learned about the reference volts. The truck needed a new sensor and it was all good from then on. I also liked the trick we learned that by disconnecting sensors you can find out which ones they are on the scan tool.