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measuring water depth in a tank

Posted: Tue Mar 30, 2010 7:43 am
by Ricky
Hello,

This is a similar question as post viewtopic.php?f=3&t=2274 and I'm wondering if your recommendation is the same.

My water tank is an above ground galvanized steel tank about 8 feet tall by 7 feet in diameter holding about 3000 gallons water. Basically I want to know what percent the tank is filled. It has an inlet/outlet valve at the bottom which feeds a 1" PVC pipe. Water passes in both directions through the pipe according to whether the wells are pumping or not.

I have the phidget SBC board. It is about 100 feet from the tank to the SBC.

Is your recommendation the same. You recommended http://rocky.digikey.com/weblib/Measure ... Series.pdf

What about this device: http://www.meas-spec.com/downloads/LM_Series.pdf?

Could either of these pressure transducer devices be installed with the same PVC pipe by using a T fitting and having the transducer off to the side? I don't want to go inside the tank and install it at the bottom and have to maintain it there. I guess I could drill a hole in the side of the tank and install the transducer there but I would have to empty the tank. Putting the transducer in the line below the tank has the advantage that there is a shut-off valve at the tank that the PVC pipe goes into.

Do the wires from the transducer feed directly into the SBC sensor input, I mean there is not an additional phidget sensor I need, right?

I have 12 V DC available and the PDF link you gave says this should work.

How many wires do I run to the tank for the sensor and electrical needs and what kind of wire should I use?

Thx

Re: measuring water depth in a tank

Posted: Fri Apr 02, 2010 2:36 pm
by theTroll527
How accurate do you need your sensor to be? And can you can tell me what the resistance value per inch of your water is that would be very helpful.

theTroll

Re: measuring water depth in a tank

Posted: Sun Apr 04, 2010 3:43 pm
by Ricky
Accuracy is not that critical, I would like to know if the tank is 90% full versus 80%. I suppose to the nearest inch or two in terms of height of water in an 8 foot column.

I do not know the resistance of my water. Do you mean for me to measure it with an ohm meter? If I do that and put my electrodes about 1 inch apart there is a lot of variation between measurments. One time it is 40k ohms, another time 120, then 80, 160. And the measurement bounces around in this range while the electrodes are held in the water.

Re: measuring water depth in a tank

Posted: Sun Apr 04, 2010 5:07 pm
by theTroll527
If you just need to know 90% then it is easy.

Put in a rod with two terminals about an inch apart. Hook up 5-9 volts to it (depending on the resistance of your water). Put the top terminal down to the 90% mark. Use use a standard analog input. What will happen is that when the water covers both terminals it will complete the circuit and you will get a reading on the analog input. Once it drops below the 90% the circuit will open and it will turn off.

Now if you want other ones, you can spread them around the tank, I can tell you how far apart if you tell me what your resistance is per inch (yes you can use a standard meter to test it). If you design it right you could use all 8 analog inputs and be able to test 8 different levels.

What we really care about it is the LOW resistance reading. That way we don't burn out the analog ports.

theTroll

Re: measuring water depth in a tank

Posted: Mon Apr 05, 2010 7:49 am
by Ricky
OK, I'm getting the idea. I'm not sure I like the idea of using up all my ports as I would like to know if there is water every foot in an 8 foot tank. In addition to this well water tank, I have two rainwater tanks I would like to measure. Currently, I just have 2 ports left in a SBC but I could get an interface kit just for this application, so lets keep talking.

There is a lot of variation in the measured resistance. When I put the meter electrodes in a cup of still water about an inch apart they read anywhere between 100-1000 k ohm and the measurement keeps bouncing around in that range. The most common measurement is say 0.6 mega ohm.

Why the analog ports? Can the discrete ports report whether a circuit is open or closed?

Thx

Re: measuring water depth in a tank

Posted: Mon Apr 05, 2010 10:08 am
by theTroll527
The problem with the digital ports is that you need 3.75 volts to trigger an on and less then 1.75 to trigger an off. With the wide range of resistances it would be hard to set up a circuit that would not risk over power to the board.

The nice part about the analog is that is you get a continual signal of just about any level you can say that you have a water level at the height.

I have an idea for being able to use a single sensor, will post the idea in a bit. Need to see it is going to work.

theTroll

Re: measuring water depth in a tank

Posted: Mon Apr 05, 2010 11:32 am
by theTroll527
Okay think I have it figured out.

Make a 6' 6" boom with a float on it. I would suggest using 3/4 or 1 inch PVC pipe. Mount the boom just above your maximum high point on a pivot. Use the 1018 Rotation Sensor and attach the knob to the boom and the base to the the base point. For each degree it is the sensor value * 360/1000. If you use the 0 for the vertical value it will make the math pretty easy. That way 90 degrees will be 250.

I would make sure that your float will keep the boom about 5 degrees off the wall so that you don't get it jammed up.

You can then use the sensor value and a little geometry to calculate the level of the water at any time.

If you want a little help with the geometry, let me know.

theTroll

Re: measuring water depth in a tank

Posted: Thu Apr 08, 2010 7:32 am
by Ricky
Ok, thanks, let me think about that idea.

Is there a problem with using pressure transducers in your opinion such as

http://www.meas-spec.com/downloads/LM_Series.pdf

Can that be interfaced with the phidget sbc and how?

Re: measuring water depth in a tank

Posted: Thu Apr 08, 2010 8:01 am
by theTroll527
I can't see a reason why that wouldn't work. It should be able to be hooked up directly to the analog port. I foot of water gives you 0.43 psi. So the math should be pretty easy.

theTroll

Re: measuring water depth in a tank

Posted: Thu Apr 08, 2010 4:38 pm
by Ricky
Which one of the choices below should I get? Am I correct that I can just plug either into the analog sensor port. Do I need to apply externa power? The sensor port provides 5VDC, right? The tanks are between 15 and 75 feet from the sbc board, is that an issue? How many wires do I run to each sensor, 2?

Thx!

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=356-1073-ND&itemSeq=84814559&uq=634063445336569116

or

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=MSP6025P4-4-ND