On Wed, Jun 24, 1998 at 07:14:16AM +0200, Tjaart van der Walt wrote: > Temperature and visibility and ambient light would > be easy to measure. How does one measure a river's > water level (2.5m change) without little contacts that Use a pressure sensor with a pipe going to the bottom of the river. I've got this set up with some water tanks - still some calibration issues, but the basic concept works well, and there is no electronics in the water. You do need an air pump to purge the tube. An alternative would be an absolute pressure sensor encapsulated and dropped to the bottom and referenced against a similar sensor in air. For a pump I ended up using a 12V automotive tyre pump - one pump supplies two tubes via check valves (fish tank type). Fish tank pumps have too low a pressure output to work for more than about 500mm of water. The plumbing hardware is basically 6mm irrigation tubing and barb connectors. There's a piccy of the setup at http://www.htsoft.com/images/tanksense.jpg > frequency sweeped from the low resonant frequency > to the high resonant frequency. I could detect the > peak amplitude with an electret microphone. This might work, but it sounds a little touchy to set up to me. You could probably measure the speaker impedance rather than using a microphone. > BTW what sort of a sensor would be able to measure > barometric pressure? I am going to pay for this, so it Honeywell have small pressure sensors - they're not too expensive. They are a bridge setup with a diaphragm and strain gauges, excited with 5-10V and output is differential, in the mV range. (Thanks to Bob Blick for the sensors, op-amps and ideas). Onya Bob! Cheers, Clyde -- Clyde Smith-Stubbs | HI-TECH Software Email: clyde@htsoft.com | Phone Fax WWW: http://www.htsoft.com/ | USA: (408) 490 2885 (408) 490 2885 PGP: finger clyde@htsoft.com | AUS: +61 7 3354 2411 +61 7 3354 2422 --------------------------------------------------------------------------- HI-TECH C: compiling the real world.