Depth of water sensing 1

[alternety]

I would like to monitor the water depth in well. Maximum depth is 500" and minimum is 0'.

I have been thinking pressure sensor but everything I have found is quite expensive packaged sensors.

Any ideas on other ways to monitor depth or sources of something I can use for a sensor in a 500' well.

 

[itsmoked]

Hi BrianR I think you misread the previous posts.... You are correct about 500inch system unfortunately we are talking about 6,000 inch system. (500ft)

 

[BrianR]

I guess I got confused with the original question which says 500" on line 1 and 500' on line 3.

500' = 217 psi but it's still do-able using eg a 19C500PA2K 0-500 psia sensor costing $73

 

[Warpspeed]

I have been thinking about how to monitor the very low air flow through the well bubbler, and the obvious way is to fit a small bubble chamber into the above ground airline.

That could easily be built to run at the required 200 psi static pressure, and have a window through which flow could easily be observed.

The bubbles might even be counted or measured automatically with some sort of optical detector.

 

[itsmoked]

Howdy Warpspeed.

I'm not sure I follow you...

Do you realize that *normally* you have your pipe going down into your tank.

You screw a TEE onto the top.

You screw a pressure sensor into one leg of the TEE.

You screw a needle valve into the other leg of the TEE.

You put anohter nipple and another TEE after the needle valve.

You screw a pressure gauge into this second TEE.

You screw a regulator into the other side of this TEE.

You hook an air source to the regulator.

Then you set the regulator to just a high enough pressure to force the tank material out the bottom of the pipe.

Then you reduce the flow to the lowest possible flow with, the tank to be measured full, using the needle valve.

Now as the tank level drops the flow will go up because less pressure is required but because you are only talking 10 or 20 feet *normally* and you normally used a 3/4" pipe, there still won't be any appreciable flow related pressure drop to screw up the depth reading.

But in alternety's well case with a huge dynamic head and a miniscule pipe that is hundreds of feet long... Pressure Drop City.

But I digress.

You probably know all this Warpspeed but I don't understand your chamber idea. There would never be bubbles at the well head. The bubbles would be at the top of the water column a hundred feet down.

Help me out here.

 

[alternety]

One of my posts disappeared. What I said was that I have not been clear. I am not going to run bubbles. I am going to put the tube down the hole, build stuff on top, fill the tube with the little 12 tire pump until it runs out the bottom. Close off fill valve, remove pump, and just monitor air pressure to see depth of water.

Additional stuff. I tried looking at the sensor suggested above but the Honywell site is not doing much more than bitching about SQL server when I try. What I did see from Digikey seems to show no internal electronics (just a raw bridge) and I am not sure how electrical connections are made to the package.

Bubble counting - just drop a video camera down to monitor the bubbles. Use a PC with sufficient performance to do video analysis of field of view and do OBR (Optical Bubble Recognition). Given the depth of my well the software would have to be configured to ignore scans of Sampan bottoms.

 

[Warpspeed]

itsmoked,

I understand how a bubbler works, the problem is to regulate the airflow to some suitable very low known value, as the remote pipe end (down the well) cannot be seen.

The idea was to place a small water chamber fitted with a window into the main air supply line. Air enters this chamber, and air leaves it, but the flow should create identical bubbles in the water chamber as it does at the base of the well. You can then just twiddle your flow control needle valve to get the desired rate of bubbling.

I feel that the system absolutely must bubble to work properly. A static "pumped up once" system will gradually depressurize and water will slowly climb back up the tube. If the open end is sitting in silt or mud, it will very easily block. The bubbles keep the end open, and keep the point of measured hydrodynamic pressure right at the end of the pipe. That is vital.

If this is not done, when the well empties, the high pressure air stored in the pipe will all escape. When the well fills up, the water will climb back up the pipe as all the stored pressure has been lost. You will not then be measuring the true water height. It is vitally important that the whole pipe length remains completely filled with air. But the pressure will vary up and down with water level. A source of replacement air will always be required whenever the water in the well rises, or a true depth measurement will simply not be possible.

 

[BrianR]

alternety

It will be necessary to force enough air into the pipe to bubble out the bottom as that is the only way you can be sure the pipe is full of air, meaning the air pressure now balances the water pressure. Who cares how many bubbles come out the bottom, just as long as some do.

That pressure gauge is a calibrated temperature compensated bridge, 0-500 psi = 0-100mV out. It has 4 pins to solder into a circuit board or directly to 4 small wires. Power it from regulated 10VDC and read it directly on a high impedance DVM or data-logger on a 0-100mV input (54mV = 500ft) or amplified by a simple differential amplifier to 1V or 10V.

http://rocky.digikey.com/WebLib/Sensym/Web Data/19C Series.pdf

 

[alternety]

I do not understand why the tube needs a constant supply of bubbles. Please explain where my thinking goes astray.

If you seal the top end of the tube and drop the other end into the water, water will enter the tube until the pressure of the compressed air just balances the water pressure. The pressure of the air at this point should represent the weight of the water column + 1 atmosphere.

The air/water interface in the tube is quite small and air disolving in the water should be a very slow process so accuracy ought to be stable for quite some time.

If the well level goes below the tube it will simply resume operating when the level rises again.

Doing it by pressurising the tubing after installation would reach the same end state after starting with a tube containing only air. As the water goes down the bottom of the tube bleeds air to maintain proper pressure.

 

[BrianR]

When you seal the pipe at the top the air pressure will be the water pressure at the water depth in the pipe. If the water rises up the pipe then it reads a lower pressure. Only when the pipe is full of air does the air pressure equal the water pressure at the bottom.

 

[itsmoked]

Warpspeed thanks for the description. That is a clever idea rather like a hash water pipe or something. So the pressurizing air enters the system at the bottom of your little sight tank. The bubble shoots up past the little window interupting an LED/detector. Feedback the bubbles/sec to a valve controller and you'd have it. Now about the algae on the window.. Heck the tank could have Chlorine in it; no algae. I like that.

alternety; I think you are confusing a mercury or h2o barometer with what you are doing. In that case the tube would be filled with h20 sealed then upended. The water column would drop to where the vacuum matched the weight of the h2o still in it.

In the well case that doesn't work. Just as Warp and Brian are saying. If you had enough air pressure to force out all the water when the water column is highest, as the well is drawn down the trapped air will be under reduced pressure.

It will expand and lots of it will bubble out. Now the water column rises again. There isn't the original amount of air in the column. So now water rises up the tube. For every inch that rises up that tube your measured depth will now be off 1 inch. This doesn't bode well for pump control... :{

 

[Warpspeed]

A bubble sight tank will need to handle 200psi safely, so glass may not be ideal. Thick wall clear acrylic tubing may be better, and it is very easy stuff to work with.

Entering air could be via a small bore J pipe with the lower end submerged. Some bleach in the water will kill the nasties. Lots of fairly simple ways to do this.

 

[alternety]

I unserstand what you are saying itsmoked. I had indeed thought of a barometer at one point - but incorrectly.

A sensor seems a better alternative. I will put the low water assembly down now and do sensor later. I need to get the well safe now.

With my new understanding I would need to fool with continuous compressed air to make a surface sensor work and the submersible ones are two expensive. Maybe I dould seal something like the one brianr referenced and drop into the well. Or I could put something with a flexable membrane at the down hole end, presurize it, and put the sensor up top.

Thanks everyone for this discussion. It has kept me from going off hunting wild geese.

 

[itsmoked]

My pleasure alternety!

If you really have to go "cheap" then dhwilliams solution really is the best. It is far, far, far, simpler and nearly fool proof to the bubbler. It is also cool! As you could actually watch the "signal" ,as it were, with your own eyes. I would simply hang a stainless steel rod for the weight and a ballasted cylindrical plastic float, (to keep it pointed vertically like a bouy). Use monofilament fishing line with 7 or 8 times the breaking strength you need. It would be small and slippery, not caring if it touched the sides. Get the whole thing working using a five gallon pail or maybe something deeper (a lake) then when satisfied replace the the monofilament with the correct length. Lower the float down the well followed by the weight. It would work well.

 

[Yashu]

How about infrared distance sensing to a platform floating on top of the water.

 

[itsmoked]

Yashu; hard to have a platform target floating on the surface when the surface is only a 6" dia with a violently wiggling pipe somewhere in it.


Hey alternety; I have another idea! Maybe someone can poke a hole in it.

Take a SlowSyn stepper motor. You feed them 120Vac and they just turn with huge amounts of torque. They generally turn at a leisurely 72RPM. You take a slender weighted bouy as mentioned before. Use the same monofilament line.

Set up two microswitches and a small little jig. Set the jig up so as the water drops the line tension increases. When the line tension reaches a certain point of tension the related microswitch turns on the motor in the lowering direction until the tension drops and the switch opens.

As the water rises the line gets more and more slack. When the line reaches a certain slackness the other switch closes. This runs the motor in the other direction winding up the line. The line is on a spool so only one side goes down hole. You just need to keep track of the turns of the spool to accurately know the depth.

Now is that not brillant?!?

 

[VEBill]

The simplest solution goes back to the beginning of the thread. Attached a suitable pressure sensor to a weight, add a long shielded cable, seal it up and toss it in. Obviously the pressure at the bottom of the well is a function of depth.

Ultrasonic pinging from the well cap to the surface of the water is a close second. The only reason that this technique is on the short list is because there are off-the-shelf modules that would make it a relatively simple project.

 

[ScottyUK]

Just a thought:

How straight are boreholes typically? Straight enough to use a laser sensor to detect the water surface? Nothing in the hole to go wrong, although I imagine initial alignment would be tricky.

I think the ultrasonics would suffer from multipath reflection problems because of the small diameter of the borehole compared to its length.



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If we learn from our mistakes,
I'm getting a great education!

 

[Warpspeed]

The problem with a lot of these ideas is the power cable and pipe already snaking down the well. That will be moving around quite a lot and tend to cause problems with optical or acoustic systems reflecting back off the water surface.

Could the displaced air be measured ? As the water column rises and falls, so air must move out and into, the top of the well. It would not give absolute depth, but it may reliably track changes ??

Commercial gas meters measure flow very accurately at low pressure drop, but I am not sure if they are bi-directional. If they are, maybe the little dials that show cubic feet of gas may be useful.

I really don't know, but maybe someone else can come up with a better way to accurately measure air volume flow in two directions. That would have the considerable advantage of not needing anything to go down the well that might tangle, jam, or drop off. Six hundred feet is a very long way down.

 

[itsmoked]

The air volume method, ugh, very tough, I would think. And lack of the absolute would leave you/system always wondering.

How about microwave? Water is a MAJOR difference to well casing and plastic. I would expect you would get a very strong return from the water surface that could easily be wrung out from the rest of the returns. (Radar)

One of my college profs helped some students make a radar for mapping below sand. They took it to Egypt to hunt buried pyramids because it could tell the difference between sand and stone at depth.

 

[alternety]

Right now I have bigger problems. I tried to put the low water sensor down today.

Short result - probe is stuck at about 100'. Won't go up, won't go down.

Ask me if I am amused.