Water detection

I've only done it with ultrasound but the cheapest method would probably be plates that are part of an oscillator. When water reaches that level the oscillator loses lock and quits oscillating. The latter is quite popular in industry (although the ones I've dealt with were all magnetic coupling). Power goes down, triggers a Schmitt gate, bingo. Some uC such as the MSP430F2xxx family have Schmitt action built into their ports.

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Regards, Joerg

http://www.analogconsultants.com

Here's an idea.. How about a trip to the local scrap yard and look at how cars detect fluid levels.. Windshield, brake and oil fluid may have an electronic sensors. Pullm off and maybe getm for cheap.

What do you mean by non-invasive? Some fluid level detectors have a magnet on a float. At low level the magnet reaches the hall sensor (or reed sw) and trips the "you're out" light.. Do you mean no floats allowed?

Clear water + clear plastic + optical level detection...mmmmmm :( Light conductance of air vs water .... ohh that's a toughy.... Light reflection due to plastic/water refractive index.... possibly.....

Take care..optical methods may have to compete with ambient light..

Capacitance method: A 25mm water dielectric.... ewwww D from BC

How about a bubbler tube and a dye in the liquid?

Coil around tube at the "threshold" point. Coil is part of an AGC'd oscillator. AGC voltage reflects losses in water when it reaches coil.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
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How clean is the water and tube? If its murky, or the tube is likely to become contaminated over time, then optical is out. What is the thickness (and composition) of the plastic?. And what is the required accuracy of the level detection? This will have to be considered if a capacitive pickup is designed to be applied to the exterior of the tube.

If optical sensing is possible, just aim a light source (cheap laser diode) through the tube offset w.r.t. the center of the tube. As the water level rises and crosses the beam, the change in index of refraction between the air and water will change the path of the beam. A photodiode placed appropriately will sense the shift in the beam path.

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Paul Hovnanian     mailto:Paul@Hovnanian.com
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If you're happy with discrete levels, and the water's clean enough, , just have an array of narrow angle LEDs on one side, and an array of phototransistors on the other side. Place the sensors at the focal point of the tube full of water, and use a comparator to dicriminate between the bright illumination of the full tube and the relatively dim illumination of the empty one. You could add a reference tube, or a pair above the maximum water level, to make the measurement ratiometric.

Paul Burke

If it's a closed system and you can plug one end, you could try measuring the air pressure above the column of water and calibrate it for whatever set point you need.

-mpm

Salvage a level sensor from an old washing machine. Im not sure what they use nowdays, but when I worked on the design of some of the first wasing machine controllers they were realy keen to use something cheaper than the pressure sensor types used for the cam wheel type controllers.

reading the oscillator agc voltage suggestion seems good to me. I gues if you made it a long coil you could even make it linear :)

Colin =^.^=

I designed a non-contact water level sensor for a large tank. It worked on the capacitive principle by changing the frequency of a multivibrator when water flooded the dielectric region of the capacitor. The detector was built inside a short piece of one inch PVC pipe arranged with an inner pipe and foil electrodes. The electrodes were on opposite sides of the pipes from the water. The PVC was machined to fit and accomodate the electrode foils. The water entered the region between the inner and outer pipes. The electrodes form the plates of the capacitor but do not touch the water. All electrical circuitry and electrodes weres sealed in the pipes and protected from the water and moisture. There is no electrical connection to the water.

Water has a dielectric constant of 80 so it increases the capacitance of the capacitor when present and decreases the oscillation frequency or pulse timing. When water is not present, the oscillator frequency is higher. A second fixed frequency multivibrator operating in sychronism with the detector multivibrator formed the reference. The operating frequency was about one MHz . A D flip-flop compared the timing of the two multivibrators. It was connected so that when one time was longer than the other, the output was high and when shorter, the output was low. The high/low condition of the D-flop indicated the presence of water or not.

I think you could do something like this for your application. Optical methods work but are more tricky and are easily affected by contamination, algae, or water deposits. The capacitive system is more fool proof and can be made very low power. Also avoid any system that makes contact with the water because corrosion and electrolosis can affect things and can contaminate the water. Bob

You could also hang the cap into the feedback path. Dimensioned right this would provide a digital signal because the oscillator cuts in and out depending on the water level. A sense resistor and transistor in the supply path could act as an OC digital port.

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Regards, Joerg

http://www.analogconsultants.com

On a sunny day (Thu, 12 Apr 2007 15:26:22 GMT) it happened "Bob Eld" wrote in :

Apart from pressure sensors and this method you describe, there is also 'echo', use a 555 timer and some piezo sensors and echo the open pipe from the top. This system is pretty good, but gets confused if there is foam or stuff floating on the water. Somebody mentioned to me once the following solution: Place mike and small speaker in opening pipe, amplifer feedback, thing will resonate at a frequency related to the area where there is no water. So sort of an organ pipe :-)

floating

resonate

And then the people wonder why their dog doesn't want to go outside anymore...

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Regards, Joerg

http://www.analogconsultants.com

why not make the tube part of a resonator in an oscialltor. As the column proportionately fills; the air is replaced by water and a dielctric constant shift experienced. The shift is proportionate to column fill level. you may obtain a continous water lvel to Frequency transducer this way!

Marc

Many thanks for all the replies thus far. To answer most of the questions:

Non-invasive means that I cannot get to the inside of the plastic tube. I have not yet seen the tube to be used, but think it's PVC or polythene, and has a wall thickness of 2-3 mm. The water is generally dirty, and flows fairly freely, so pressure sensors will be tricky. The application is subject to a fair amount of movement and vibration, so float switches may be a problem. I can control the ambient light in the application. EMI is a real concern.

Later today I can get to the site and determine the tube to be used, and hopefully get a sample piece. I'll then do some trials with "capacitor plates" either side of the tube; a laser diode or LED, and maybe a coil around the tube.

I'll report back when it all works.

Thanks Barry

IME a bubbler will work best. You bleed compressed air into it slowly and the back pressure operates your device or system or indicator. This will even work with flammable or opaque liquids.

capacitative could be hard, how thick is the tube wall?

Any colour will work, arrange it so that the beam is difracted away from (or towards) the sensor when the water is present,

Bye. Jasen

jasen wrote in news:evs3d6$gc5$ snipped-for-privacy@jasen.is-a-geek.org:

Sorry to jump in down-thread, but I missed the original post.

Single or two point capacitive level detection can be cheap and easy on non conductive containers and lines.

Take a look at Quantam Research Groups capacitive sensor chips. For example:

They have lots of good supporting technical data, and are available from Digikey.

I've used some of their chips for fluid level detection and control circuits. One was to control the level alcohol in a

1L ,6"daimeter plastic lab jar. Detection and comtrol was within fractions of a mm. In another forum someone wanted to detect the presence, and absence, of fuel in a plastic line on an aircraft. For fun I breadboarded a circuit that would do this:

This moght do the trick for you Barry.

Ken

small low signal narrow R.F. TX, RX on the side of the tubes, R.F. does not pass well through fluids.

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Ken, Thanks for the info, it looks quite promising.

Barry