waterproof wire

[Stormin Mormon]

Yeah, but that thin stuff has a plastic jacket.

[Stormin Mormon]

Gold or platinum.

[Stormin Mormon]

I"m having a hard time put this into words. But some how I don't think that will work. The current flow with the water level over a resistor or under it would be about the same. Since the current is flowing from wire to wire. I doubt your project will work.

[barry]

Important variable here is the conductivity of the water- this varies greatly depending on the concentration of electrolytes. You're assuming that the water will "short out" the resistors; if this is so, you will have 0 voltage across the supply wires regardless of water depth, provided it's non-zero. Of course, the above assumes that you're talking a bunch of resistors in parallel; if in series you still have to determine water conductivity.

Time for plan-b.

HTH, J

[Bob F]

Unfortunately, the resisters will corrode. And they will not "short out" when water contacts them. You could use two conductors, somehow spaced equally, and measure the resistance between them, which will vary depending on the length immersed in the water.

Better - use a small tube and a pressure guage. Pressurize the tube until air comes out at the bottom. The pressure indicated on the guage will indicate the water level above the end of the tube.

Bob

[modervador]

This reveals a large enough misunderstanding of circuit theory (as well as the properties of conductors, resistors, and water) that I doubt you will be able to design this system on your own. A "storebought" solution may save you some serious angst.

Now, here's something you could probably do, which is a variation of the level-sensing switches in dishwashers. Solder a string of resistors in series. Run a conductor parallel to it which we'll call the traveller. At the junction between each resistor and the next, solder a reed switch will short the resistor junction to the traveller when a magnet is nearby. All components should be insulated from each other where necessary to eliminate unwanted shorts, of course. Encase this assembly in a tube which can be sealed against water. A magnetic float with a hole in it (like a donut) can ride up and down the tube. To eliminate "dead spots" make sure the magnet is big enough to always activate at least one switch (if you understand circuits, you'll know why it's OK for two or more switches to be closed at a time). The resistance measured between the traveller and the lead going into the first resistor will be proportional to the position of the magnetic float along the resistor "string." Use a cheap digital multimeter to measure the resistance.

%MOD%

[modervador]

This reveals a large enough misunderstanding of circuit theory (as well as the properties of conductors, resistors, and water) that I doubt you will be able to design this system on your own. A "storebought" solution may save you some serious angst.

Now, here's something you could probably do, which is a variation of the level-sensing switches in dishwashers. Solder a string of resistors in series. Run a conductor parallel to it which we'll call the traveller. At the junction between each resistor and the next, solder a reed switch will short the resistor junction to the traveller when a magnet is nearby. All components should be insulated from each other where necessary to eliminate unwanted shorts, of course. Encase this assembly in a tube which can be sealed against water. A magnetic float with a hole in it (like a donut) can ride up and down the tube. To eliminate "dead spots" make sure the magnet is big enough to always activate at least one switch (if you understand circuits, you'll know why it's OK for two or more switches to be closed at a time). The resistance measured between the traveller and the lead going into the first resistor will be proportional to the position of the magnetic float along the resistor "string." Use a cheap digital multimeter to measure the resistance.

%MOD%

[glenn P]

No such thing.

Standards require you use conduit to isolate water.

[Tekkie®]

glenn P posted for all of us...

Huh, what standards would that be? Yours?

[Matt]

thanks. [e-mail] . i like that design .the only issue is the device would have to fit in a 1/2 " ID pipe. also reed switches are quite expensive.

[Don Young]

The idea from Bob F to use air pressure seems to me to be the very best one proposed. It can be simple, reliable, inexpensive, and accurate. I did not notice if you said what the range and accuracy requirements are but these would determine the gage and air source requirements.

Don Young

[mm]

Yeah, there is a wide variation in the conductivity of water, but it might be fairly constant in one place, depending onwhat else is happening there. But before trying it with resistors, I'd just put the two wires into a test water tube and see if you can actually vary the measurable resistance between the two wires by varying how deep into the water it is. You may have to raise the voltage, don't know, or maybe your voltmeter will be sensitive enough not to need that.

AFter you see if this part works you can decide what would be a useful value for resistors. It has to be more than an inch of water, otherwise "shorting out" will only lower the resistance by 1/2, but if the resistance is too high, like someone said, even a little bit of water at the bottom might basically short out all the resistors.

I guess the resistors would move on the voltage scale the range of possible voltages seen, but maybe the resistors don't really help even if they worked as you are thinking.

For spacers between the wires you can use popsicle sticks, which will last a couple years or more, or plastic sticks which will lastlonger, with two holes drilled in them, and some way to keep them from falling to the bottom.

[mm]

BTW you didn't say if it is distilled water, pure water, fresh water, or salt water.

[Matt]

I agree that measuring the amount of pressure required to produce a bubble would be easier but this requires a lot of power. The piezometers are remote and do not have a power source nearby. i am going to implement a sensor network to transmit the readings. the resistor method would allow me to use the power from the two AA batteries as the source for the meter. The water is ground water. Current methods of reading the meters manually are capable of 1/10 th of a foot accuracy. I believe i can improve on that. The idea of using reed switches is good and i will not have to worry about corrosion of the wire. Its just squeezing all that wiring in a tube and still allowing room for a float will be tricky in a 1/2 " ID pipe. Also keeping the tube straight for accurate measurements and not jamming the float into the pipe will be another issue. Thank you everyone for your ideas except the ones who insulted my superior intelligence.

[BobK207]

if you supposedly posess "superior intelligence" why are you asking for help from the mere mortals on a newsgroup?

keep track of the time you spend on your solution & let us know the results if you get it working

this has got to be some academic MM project where time spent seems to have zero cost I hope you're not being funded with tax dollarsi in my state or fed dollars for that matter

sorry if you felt insulted but that's the price one sometimes pays for free advice from newsgroups

btw you'll get better answers if you define the problem fully from the "get go" without adding more constraints & information as the discussion unfolds

like....oh, it's got to fit......oh, it can't be......oh, i can't supply AC power.....oh, it's got to be.

cheers Bob

[Matt]

You are right on the money Bob. this is my senior design project for college. sorry about the bad posting edicate this is my first post to a newsgroup.

[Phisherman]

Copper, stainless steel, aluminum, etc.

[Phisherman]

Not knowing specifically what you are doing nor the conditions makes it difficult to make specific recommendations. Electricity with 2 different metals may cause corrosion. Metals that resist corrosion include gold, stainless steel, and Nichrome.

[Goedjn]

What's wrong with shining a laser down the pipe, pinging with sonar, or shoving a capped hollow tub down the thing and measuring bouyancy, or just let the pipe ride up and down and measure physical depth?

[mm]

etiquette?

They have things near my house, and all around here and on other streams, that measure the height of the stream that runs behind my house. (There has been a lot of flooding downstream, even in Baltimore-- well, a lot for Baltimore; not much for the gulf coast -- during hurricanes. I was told one of the original townhouses hear was washed away during construction, but I don't think I believe it anymore. They might have underground AC power, I don't know. In NYC there are loads of manholes that lead to power lines and phone lines, but in the suburbs I get the feeling most lead to sewers. OTOH, there are no telephone poles near my local water level meter, but plenty of aparttments and houses, so maybe that means it gets AC underground.