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.

 

[buzzp]

Glad I could be of help.

 

[Kido]

i would try dropping a piece of polly pipe down the well leaving both ends open, drop a piece of resistance wire down the center of the polly.
the polly will insulate the resistance wire from anything it might touch on the way down.

Then (presuming that you arent getting distilled water out of your well) the water will conduct sufficiant current to be able to mesure a resistance between it and the top of your wire.

the ammount of resistance will vary depending on the depth, and should be linear for the best part, and comply with:
Depth = (the oms/foot of the combination of resistance wire and water)*(the mesured resistance of the wire)

i can see this as being a cheap reliable and acurate way of mesuring the depth with a minimum of fuss

you could even pump a constant 100mA down the wire and use a comparator op amp set up to have a gain of the desired value (so that the output had say a variation of 1V/foot) and run the output to a digital voltmeter to read off depth directly as (as the voltage would be at a 1:1 ratio to depth in feet)

just a tought to ponder is all

 

[CarlRhino]

This will work well in theory... But, you will find that the metallic conductor will quickly oxidise due to the current and also due to time and the impurities in the water…

You may recall these experiments from high school chemistry…

 

[OperaHouse]

You can send a seperate tube down and pump air to it so it bubbles. This is basically like a low tech pressure relief valve. Pressure is a function of the depth of water from the bottom of the tube. Roughly 15 psi for every 32 feet. So just measure the pressure in the supply line. Simple and accurate. Just have to have a pressure source higher that what will be expected and have a small bleed oriface into the sense tube.

 

[barbarian]

I have to concur with Chuckles157. I currently am making a level sensor system (with LCD digital output inside my house) to measure level in my 15' well also. I have found the Motorola Sensors to be very cheap & there is tons of online help with circuitry etc. Can get it compensated & integrated (amplified signal) as well.

 

[alternety]

I have looked at the Motorola parts. The catch is a maximum pressure of 150 PSI. Even at 400 feet the pressure would be 173 PSI (ft/2.31). My static level is somewhere between 50 and 100 feet down with the pump at 500 feet down.

I will poke around some more. The sensor says it can do differential so I could potentially 'bias' it with a bit of plumbing and mechanical work. Then use two to get the range.

If I could get the range I need I think I could put down a plastic pipe. I would need threaded pipe which is hard to find. I could put the tube down then measure static level and adjust the pressure to match this water depth. If I could get that much air pressure.

Copper would have a couple of issues; galvanic activity and to get a continuous run you would need to use bendable tubing and if it is not straight it will be hard to put down the hole. The hole is not real smooth.

I put the pump down the hole yesterday and today I am plumbing and wiring. I should finish the power supply for the pump (48 VDC) tonight and try for water tomorror.

 

[nbucska]

Take a parallel TV cable -- cut of a long enough piece
and insulate the end with a blob of epoxy, asphalt, etc.

Build an oscillator with it as a tuned circuit and measure
its frequency. Calibrate it.


<nbucska@pcperipherals.com>

 

[alternety]

I thought I would add this to this to my original post since I was not sure where to ask.

Background: The controller for the pump was flakey from the start. Just replaced by the manufacturer. It is an entirely new design. I think the old one was some sort of analog implementation and this one is more digital. Circuit board is maybe 25% the size of the old one. The previous low water sensor was supposed to work with the new controller (current between a probe and the pump body), needless to say it does not. I am taking the opportunity of dropping a new sensor down the well to put a tube down to implement the previous suggestions about that approach. I have the following problem:

It has been too long since I had to do arithetic. The gray cells involved appear to have died.

I have 500' of 1/4" OD, 1/8" ID , 0.921 specific gravity, polyethylene tube going down a well. It will be used as a depth sensor by pressurizing with air to indicate water depth (around 200 psi max). The tubing weighs about 7.4 lb (probably includes the spool).

Roughly how many lbs of force will this exert on the weight pulling it down into the well. Positive (float) or negative (sink). I need to determine the proper weight of the device on the end of the tube and it is not handy to just add weight. Thre are some other issues with that.

 

[Icanfix]

I concur with Operahouse. A bubbler would be the simplest method that you can use. If you take the time you could make a new face for the pressure guage to read in feet of water. Or even purchase a guage that is calibrated in feet of water.

The second simplest method would be to use Metratape (sp). It is kinda like the old twin lead-in wire for TVs. It works by the water on the outside of the tape squeezing the tape and shorting out the resistance wire on the inside of the tape.

 

[Warpspeed]

Me too.

The bubble method is commonly used to data log water level in rivers, reservoirs and sewers. The usual method uses some sort of electrical air pump and a (dry) air pressure transducer. The pump only needs to start and run while making the actual measurement. Pressure can leak away, and water climb back up the pipe between measurements, the only disadvantage might be possible pipe blockage. Keeping it bubbling very slowly will prevent the possibility of blockage at the pipe entry by silt or debris.

By slowly I mean perhaps one very small bubble per minute.

Pressurizing the pipe could also be achieved with a cylinder of bottled gas, a pressure regulator, a needle valve (to set a very small flow), and a solenoid to shut off the flow totally when not being used. A small cylinder might easily last a year or more. That is better than a motor driven pump if the thing has to run from batteries.

These systems can be made very accurate, and they are simple and reliable. Commercial water level data loggers are available, and I once repaired one, but they are not cheap.

 

[itsmoked]

alternety;

If you have ALL five hundred feet under water and run the bubbler so the line is charged with water you will have:

2.66lbs of buoyancy because of the air
0.63lbs of buoyancy because of submerged polyethylene

For a total buoyancy of 3.29lbs

If you have 300 feet actually underwater then you will have (300/500) * 3.29lbs = 1.97lbs


As for greymatter fading... it was all those 12packs that you tried to match your kidneys with your well flow..

 

[itsmoked]

Cripes did I say charged with water? I meant air!

I will stop my 12 pack-kidney output test now...

 

[Warpspeed]

Archimedes had the right idea. Try sitting in the bath for a while.

 

[alternety]

Thanks itsmoked, and everyone else taking the time to respond. When I posted my message I got to thinking about the pressure. While this was going on I looked at shop air compressor specs. Oops. I won't be able to charge the line with a reasonable compressor. But a comment from warpspeed about small gas cylinders is a real idea. I am going to poke around and see if there is a cheap small cylinder that I can get with CO2 or something in it at the required pressure. If I can't I will probably have to scrap the tube idea.

Archimedes is involved in more ways than one. I believe the pump is one of his designs.

I need to go find my current well benchmark 12 pack and contemplate some more.

 

[itsmoked]

hehehe

You are welcome.

You really need to figure out your water column depth.

Normally I think most wells oscillate seasonally only about 100ft max. that would only be about 40 psi. You can certainly try a standard air compressor first, even a 12VDC car junky compressor. You will need a regulator because with that absolutely tiny tube ID 500 ft of tubing will have horrendous restriction to flow. This could cause a really large pressure drop of many PSI which translates to many feet of error. This is why they are called bubblers because you truely must have only an occasional bubble popping out. Otherwise that pressure drop will get you.

I have never ever heard of a bubbler running more than 10 or 20 feet. They are primarily used in low head situations in sewage or some gooey putrid material where the hassle of plugged pressure sensor orifices makes the hassle of providing air pressure the only route availble. To solve the flow problem, normally a pipe with a large ID is used so restriction is not an issue. Furthermore changing head pressure is not an issue over some 10 foot delta.

If it were me I would cut immediately to the chase. Buy a waterproof S.S. ABSOLUTE 4-20mA 200psi pressure transducer which by itself will sink. Tie it to some water proof cable and chuck it in the hole. Excite it with 24VDC. Put a resistor in series with it. The 4-20mA current will entirely negate voltage drop concerns of the 1000ft round trip of wire (499 Ohms will give 10Vs at 200psi). Lower the transducer while watching a cheapo DMM. As soon as the voltage stops rising on the DMM you have hit the bottom. Anchor the cable and watch your DMM. It will be giving you the instantaneous depth. Scale that resistor anywhere you want your computer to read/log and away you go.

This is what I do when monitoring water hammer in power plant surge towers during load dumps.

 

[alternety]

itsmoked - that is the same chase I initially cut to. That was until I tried to price those sensors. Way too pricey was all I could find.

The water column is 470'.

This level correlates extremely well with where the well casing ends and the well transistions from dirt to solid granite. I suspect that the water may actually be leaking out at this point when it is full. Maybe not. Just as an aside, I also have highly reflective/refractive clay with a less than 0.20 micron size and containing Mn and Fe. This could be coming from a leaky interface as well. I am on the side of a good sized hill with water running down that dirt/rock interface. It was also hydrofracted and I believe looking at the logs that the last pressure shot broke through to the surface of the rock. If is not a simple well. Sure not what I thought I was getting myself into when we bought the lot.

I don't think the pressure drop will be an issue. If I just pressurize the tube and monitor pressure it is essentially a static environment within the tube. The movement of the well level is, I believe, slow enough that the system could be treated as a static system. If there is no flow, will there be a pressure drop? I don't think so.

 

[itsmoked]

I'm sorry I'm still not understanding.. Is there 470ft of water standing in the well? If there is why would someone proceed another 400ft down? I'm not getting it.

You must mean the water starts 470ft down and goes down to 500ft? Tell me this is what's happening.. pleeeease!

 

[alternety]

Right the first time. 470' of standing water in the well. The well is actually 605' deep. We kept drilling to find water and finally called it off at 605. The pump is down 500' because it is a low productivity well. I have a pump that does about 3 - 3.5 GPM and it will empty the well. The pump speed is actually somewhat adjustable and when I find out what the actual recharge rate is I will set the pump so it will not empty the well. 500' worked out as a good comprimise with costs. Going further would have meant moving from #6 wire to #4 wire. A non-trivial difference. We think most of the water is entering at about 400' down from a video examination but are not real sure. It was examined before hydrofracting but not after.

The depth also gives some extra storage. If I did the math right it gives me 98 cubic feet and 733 gllons.

 

[itsmoked]

Wow... So you have some wild dynamics down that hole.

This is why you need a down hole pressure sensor...

If you have say 470ft of H2o standing above the bottom of your bubbler tube you need 203psi to just barely get a bubble out the bottom. This would be at the no (air) flow rate. Now the pump kicks on and starts dropping the well level. It drops 300ft. Now you would only need 74psi to still only barely have air flow. At 203 psi, air would now be flying down that tube. You would then have the pressure drop I mentioned.. Things would not be happy.

What you really need to make this work, is FLOW control of the air. You need a *flow* sensor and an electrically controlled orifice. You then use a computer to control the orifice to maintain next to nil air flow at all water depths.... Is this easier or less expensive than a single pressure transducer??!?! A transducer that provides your signal already in a form your controller can use?

Keep in mind the bubbler will also have some lag, possibly causing control dificulties.

Also I gare-un-tee you will have valve and air compressor headaches.

I just cannot see how your tubing, weight, flow meter, analog orifice/valve, flow controller, compressor, electricity(compressor), maintenance (all of the above), can come close to the price of a single pressure tranducer on the end of a couple hundred feet of 2 wires and a resistor..

BTW your idea to control the pump is a great one. I put a three phase pump down my hole then ran it with a VFD from single phase power. I could run the pump at any speed. I had it ramp up and down.

 

[BrianR]

A cheap pressure transducer such as Digikey 480-1926-ND, 0-30 psig at $30 seems simple enough to me. 5V powered, 0.5-4.5V o/p. 500" = 18 psi

Mount it at the top of a pipe pushed to the bottom of the well, with some air pumped in and measure the pressure of the head of air as this is not a waterproof sensor.