Use vertical water in pipe and vacuum gage to monitor bore hole water level?

[Tokiyata]

G'day, this is doing my head in and thought that someone here, much smarter than me, will probably know the answers to my questions. I have just sunk a bore for a small irrigation project and despite pumping it for several days to establish the bore, I'm not 100% sure of the bore yield capacity. I could only pump during the day as I had to power the borrowed pump with a portable generator which couldn't be left out overnight (theft). The borrowed pump has now been returned and I have bought a new pump to be installed in a couple days time and the power should be connected soon too.

I dipped the water level in the casing using my fishing rod and a stick which worked a couple times but then got hung up on the rising main. Anyway this was never going to be a long term solution. The problem I have is that I am going to have to push the yield to near capacity to achieve the irrigation goals and I need to closely monitor everything to determine exactly where the boundaries lie.

So, I am looking for an inexpensive method to monitor the water level inside the bore casing during pumping at different rates. I know I can use an air pressure line and gauge, or more accurately install a dip tube but both require additional input in order to give a reading. I am wondering if I can simply reverse the mechanics used in the air pressure line method and suck the tube full of water, using a vacuum gauge to measure the pressure (i.e. weight of water in the tube above the water level in the casing). The benefit is just that once it is charged with water and provided there are no leaks, it should be maintenance free and require no further input, just be read.

So question 1 - Is this theory sound?

question 2 - I assume the amount of vacuum pressure created will depend on the diameter of the tube and volume of water in it for any given length. Is this correct or is it like head pressure that is independent of pipe diameter (I still can't wrap my head around that one)?

question 3 - It would be nice if readings on the vacuum gauge translated easily into cm/m of depth to water inside the casing. Any suggestions about vacuum tubing and gauge combinations that would work well for this purpose? FYI, the bottom of the vacuum tube would be installed 0.5m above the pump head and at a depth of 30m. It should remain under the water at all times, if it doesn't I will have exceeded the bore yield.

Cheers!

EDIT: I just did some more googling and realised that it's not possible to draw water up into the tube any higher than 10.3m due to gravity, atmospheric pressure, etc. This doesn't necessarily mean that my proposal won't work though (or maybe it does?). The standing water level in the bore before pumping is between 8m and 9m from the top of the bore casing so it should be possible to charge the line before starting the pump.

EDIT 2: I keep googling, I should stop I know. I could be calculating entirely wrong here but my calculations indicate that the vacuum tube diameter is irrelevant as it works on force per unit of area. I am coming up with max -3 bar of force from the top of a sealed 30m vertical line filled with water. Is that right? Give or take a bit for elevation, water temp, purity, etc. If this is correct then would a small automobile type black plastic vacuum tube and 3 bar vacuum gauge do the trick? Oops, no such thing as a -3 bar gauge, bugger!

 

[BigInch]

You're not going to draw much water with a vacuum in the well.
Try an ultrasonic level measurement sensor.

http://www.stevenswater.com/water_level_sensors/othersensors.aspx

Otherwise you should have a small measurement tube installed.

http://www.oregon.gov/owrd/gw/docs/ground_water_level_measurement.pdf

 

[bimr]

No. It will not work.

Inexpensive water well transducers are used to determine the water levels in wells. The transducers measure the water pressure and the water depth is calculated from the pressure.

http://www.waterlog.com/productsdetail.php?Submersible-Pressure-Transducer-H-3123-21

 

[LittleInch]

Ok, I think you've answered some of your own questions, but to confirm some,

Q1, Yes the theory is partly sound, however practicality is low. The range is limited to around 5 to 6m and getting sealing over the long term is quite difficult.

Q2, the amount of air to be removed would be affected by the size, but the pressure wouldn't be affected

Q3, basically you could work on the basis of 1m= 0.1bar. However you can't get less than 0 bara, atmospheric pressure being approx 1bara at sea level.

If you insert a tube and know the water level then seal it and install a pressure and vacuum valve you could measure from this datum point.Beyond that the water might start to vapourise and allow water level to fall without registering any difference in negative pressure. However you would need to do some calculations based on how much distance is above that level full of air as a fall in water level will result in a smaller difference in air pressure. E.g. if you had 10m above the datum point and the water level fell 10m then the pressure should be approx 0.5bara. If you had 20m above datum and the water fell 10m, you would have an air pressure of approx 0.67bara.

I'm sure there are battery powered level detectors which you can drop down the casing....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Tokiyata]

Thanks for the replies fellas. After giving this way too much thought, I have concluded that LittleInch is correct, it is technically possible but not very practical. Transferring the downward force of the water weight in the tube into a form that can be measured from the top would require some sort of plunger or pistons, etc and would be impractical.

It would be more practical to drop a nylon 11 flexible tube down with the bottom capped off, fill it with water from the top and weigh it using a hanging scale. I understand that nylon 11 flex tubing has a specific gravity almost matching water. But anyway, I would still need a dip tube in order to chart the weight/depth ratio with confidence. So why bother.

I think I will just install an air line when the bore pump is fitted. It means I will have to bring a bicycle tyre pump and pump it up each time I want a reading but that's not the end of the world. At least I know now what capacity and resolution pressure gauge I require, so all the pondering hasn't been a total loss. 0-400kpa (300kpa gauge not available), with each 10kpa increment equal to 1m of water over the outlet of the air line. Give or take a small percentage of course.

 

[LittleInch]

How about

http://www.hydrokit.co.uk/107/borehole-dip-meter.htm

or

this

http://www.environmental-expert.com...battery-powered-data-logger-with-modem-415920

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[itdepends]

a)Determine the minimum level of water that you are willing to tolerate in the well (i.e. how much head does the pump need to work)
b)Work out how far that distance is from the top of your bore hole (assuming you know how deep you've placed your pump)
c)Cut a piece of 19 or 25mm irrigation poly to the length you worked out in b)+ a bit extra for you to hold on to, mark the poly so you know where your depth mark should line up with the top of your riser pipe, push the poly down the hole until the mark lines up with the top of your riser/liner
d)Start pumping
e)If you can blow down the tube and encounter resistance (without trying to blow your guts out)- the end of the tube is still submerged
f)Increase your pumping rate until you can blow through the tube easily (the end of the tube is now no longer submerged- you just found your well yield
While doing the above you can raise the tubing up in the pipe and continue to blow- when you feel the end is clear the length of tube you've pulled out will give you the excess water level you have before you hit your minimum.

I've used the above successfully in a home environment with a 30m (98ft) deep bore to determine its yield.

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"