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Volumetric measurement of liquid in transparent container

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Berni:

--- Quote from: tpowell1830 on June 26, 2018, 11:30:02 pm ---Some clothes washing machines have an open tube that goes to the bottom of the washing tank so that when you rotate the load size switch to different positions, the machine can sense how much water is in the tank. I have not examined one of these circuits or setups very closely, but there would be varying air pressure in the tube as the water rises, so I am assuming it has a pressure sensor at the top of the tube. Makes sense...

Hope this helps...

--- End quote ---

Actually yeah i remember washing machines from the old days already using pressure to determine water level. It was just a round pancake shaped device with a small tube running from the bottom of it into the bottom of the drum. When the water level got to certain levels the pressure caused switches inside to close and that signaled the mechanical program controller to advance to the next step in the program.

max_torque:
Why are you not just weighing the tank??  Simple, robust, sealed, repeatable.  You have to be mad (or have a VERY good reason) to do any other method!

pwlps:

I have recently built a capacitance probe to measure the level of liquid nitrogen in a dewar. But I have tested it with water too and it works fine.

I did not try the FDC1004 but I see two potential problems you should consider:

1) The capacitance sensor described in the documentation of FDC1004 uses bulky electrodes an the calibration seems to be extremely sensitive to any stray fields or small changes in geometry. I wanted to have a simple probe that I can insert through a small hole in the dewar and also a portable system i.e. a probe that I can withdraw and insert again without loosing the calibration. Therefore I'm using a simple coaxial capacitor made of two concentric stainless tubes (6mm/3mm diameters), with the inner tube held with teflon spacers. The coaxial geometry makes the capacitor completely shielded from the outside environment (the outer tube is connected to the circuit ground). There are a few small holes in the outer tube so that the liquid level inside the capacitor can adjust quickly to the level in the dewar.  The calibration is very easy: the change of capacitance is proportional to the liquid volume fraction and the dielectric constant of the liquid.


2) Beware of simple capacitance meters which are sensitive to the resistive part of the impedance : if there is a leak in the capacitance then you would need a true LCR meter to extract the capacitive part   (of course the constraints depend on the capacitance range, for my application it is of order of tens of pF).  The  commercial capacitive level meters I found for nitrogen are using simple circuits with either low frequency excitation or  time-of-discharge measurement and are rather sensitive to leaks, they usually specify that the leak should not be bigger than  10 MOhms or so. This is not always possible to achieve in an environment where the electrodes can be polluted by  moisture near the top of the dewar, some of commercial probes fail to measure anything if there are are water droplets making a leak of order of hundreds kOhms, this is why I designed my own circuit. 
The documentation of FDC1004 is not very clear about this point, as it only says:

"The FDC1004 can be used to measure liquid level in non-conductive containers."  (sec. 9.1.1)

What about the liquid itself? A pure non-conducting liquid does not exist, water can be considered as conducting or non conducting dependiong on the resistance range you are concerned with. 

My circuit (I can send you the schematics if you are interested) is a capacitance half-bridge working at 2MHz and using a phase-sensitive detection to discard most of the resistive part, it is insensitive to leaks down to a few kOhms. The circuit is in a small box mechanically attached to the top of the coaxial capacitor to suppress any stray capacitances.

Actually for water you might not need such precautions because its huge dielectric constant will give you much higher capacitances, so that simpler circuits might do the job (but I didn't do any numerical estimations).

speechsupply:

--- Quote from: max_torque on June 27, 2018, 10:47:45 am ---Why are you not just weighing the tank??  Simple, robust, sealed, repeatable.  You have to be mad (or have a VERY good reason) to do any other method!

--- End quote ---

Yeah it's absolutely a solution. The reason why I want to avoid weighing is primarily due to the "mechanical" aspects of building a sealed enclosure with weight cells. Tried it before, not impossible, but once someone other than me starts throwing this stuff around...


--- Quote from: DTJ on June 27, 2018, 03:19:03 am ---
--- Quote from: tpowell1830 on June 26, 2018, 11:30:02 pm ---Some clothes washing machines have an open tube that goes to the bottom of the washing tank so that when you rotate the load size switch to different positions, the machine can sense how much water is in the tank. I have not examined one of these circuits or setups very closely, but there would be varying air pressure in the tube as the water rises, so I am assuming it has a pressure sensor at the top of the tube. Makes sense...
Hope this helps...

--- End quote ---
I've seen industrial washing machines use this arrangement with an MPX style pressure sensor.

--- End quote ---
Absolutely, but due to contamination I would prefer to avoid direct contact with liquids within the container.


--- Quote from: pwlps on June 27, 2018, 02:53:57 pm ---
I have recently built a capacitance probe to measure the level of liquid nitrogen in a dewar. But I have tested it with water too and it works fine.

--- End quote ---
Have you shared this build on any website? Would love to see some pictures of it :D

jpanhalt:

--- Quote from: speechsupply on July 06, 2018, 11:57:19 am ---
Absolutely, but due to contamination I would prefer to avoid direct contact with liquids within the container.


--- End quote ---

Sorry, but that doesn't make sense.   The "container" is made of something.  You can make your probe out of the same material.   How will a probe made of the came material as the container add contamination that the container doesn't add?

How did your experiments with the Sharp detector go?

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