is it possible to use an ultrasonic distance sensor inside a tube to
detect water? tube is 1/2" plastic.
I would sure think so -- but the reading will be a bit different than
expected because sound travels more slowly in a tube than it does in
free space.
Your round-trip travel time for 150 ft will be almost 300 mS so
you'll surely have to modify the rangefinder elex or build your own.
Putting the transducer down the pipe a ways would eliminate that
issue, just have it a couple of feet above highest expected water
level.
National used to make a little sonar chip, LM1812. It's long obsolete
but it might work nicely if you can find one.
Ultrasonic transducers are easy to find surplus. See, e.g.,
http://www.allelectronics.com/cgi-bin/item/XDR-24/search/ULTRASONIC_TRANSDUCER_.html
You can use the same 'ducer for xmit and receive, just as fishing
sonars do.