Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/5/84; site reed.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!ihnp4!houxm!vax135!cornell!uw-beaver!tektronix!reed!swift
From: swift@reed.UUCP (Theodore Swift)
Newsgroups: net.analog
Subject: Re: How Dry I Am
Message-ID: <1454@reed.UUCP>
Date: Sat, 4-May-85 06:36:37 EDT
Article-I.D.: reed.1454
Posted: Sat May  4 06:36:37 1985
Date-Received: Sun, 5-May-85 23:40:30 EDT
References: <238@aluxp.UUCP> <4097@mit-eddie.UUCP> <90@cadtec.UUCP>
Reply-To: swift@reed.UUCP (ted swift)
Distribution: net
Organization: Reed College, Portland, Oregon
Lines: 35
Keywords: water level, strain gauge
Summary:  How about a strain-gauge pressure sensor? 


For something that's going to be 150' down, the Polaroid rangefinder
is definitely out.  Have you considered using a solid state strain-
gauge?  I've just finished a thesis which uses two of the suckers to
measure water level in a recording tide gauge.  SenSym in Santa Clara
(a spinoff of Nat'l Semiconductor) makes all kinds, starting at 
about $20.00.  The ones I used act like Wheatstone bridges, giving
an differential output voltage proportional to pressure*excitation
voltage.  You could put a sensor and a differential amp at the end of
a four-strand cable and drop it down the pipe (two strands for +-power,
one ground, one single-ended analog output voltage).  Upstairs you 
could read the voltage with a normal VOM or, better yet, have a similar
p-sensor/amp circuit reading barometric pressure and subtract air from
air-plus-water pressure either digitally or analog.  A cheap/semi-accurate
way of going digital would be to hang a voltage-to-freq converter chip
off of the diffamp downstairs, then use a freq counter upstairs (set
the V/F converter to some appropriate range.
SenSym makes gauges in all sorts of pressure ranges for all sorts of
benevolent and malevolent environments (for a price..) in three basic
flavors: absolute (refered to vacuum inside sensor), differential (two
pressure ports read difference in two pressures) and gauge (pressure at
port refered to ambient air pressure around sensor).
  A good discussion of the idea can be found in "Build a Solid-State
Barometer" by Gupta in _Radio Electronics_, around June, 1984. An even
better discussion can be found in _the Pressure Sensor Handbook_,
Sensym Corp, Santa Clara, Ca. (sorry I don't have the address/phone)
They sent me a copy without extracting a promise to order 10,000 units
with little or no hesitation.
  I apologize if I didn't present the basic idea clearly.  There are 
lots of different ways of doing it.
Oh yeah:  I guess I should say this:  I have no connection to SenSym,
NS, RE, or any other individual or corporation mentioned in the above
text, other than having bought two 06030 absolute sensors (0-30psi)
from SenSym, and having read Mr. Gupta's article in RE.
Happy dousing...