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I am not a smart person, but I do like performing a bit of science

cstanton

It's too warm in the United Kingdom, so I figured I'd invest in a fan/humidifier combo. 

image

The downside to this hardware, is that it will burn itself out. There's nothing detecting when the water reservoir doesn't have water in it. It's convenient enough, 3 modes to the fan, 3 levels of humidifier, battery, and USB rechargeable. 

So I figured, why not try to detect how much water is left in it with the hardware I have available? I couldn't wait for looking up too much detail or ordering parts from an online store.

I've no idea what I'm doing, so I figured, let's stick a couple of paperclips in there and connect it to an arduino uno.

image

So I read a bunch of values, do some buffering with an array, average them out. I get a very rough sine wave into the analog pin.

And then I notice the pattern, the water's low, the values are higher, the water's high, the values are lower. Setup some ranges, great.

Then I realise what I'm measuring when I turn off the humidifier, the paperclips are picking up the electrical noise from the humidifiers in the water

image


Not quite how I expected to measure if there's water left, but it appears if it's working harder, it's easier to pick up the noise, water isn't the best electrical conductor after all.

Still, I'm sure I should find a better method, but it is a small reservoir, I considered 3D printing something to be able to screw a bottle in to expand it a bit more though.

What do you think?

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  • I suppose it will take a mechanical engineer to solve this :-)

    Make a capacitor by twisting about 600mm of 26 AWG insulated wire together for use as the sensor.  Or whatever small diameter wire is at hand.  A hand drill works great for this.

    image 

    Now coil it around something like a pencil to the right length so that it will all fit into the reservoir.

    image

    If you have an LCR meter it can be tested before setting up the Arduino.  Here the capacitance is shown to be 61pF with the sensor out of the water.

    image

    Sticking the sensor into the water results in a measured capacitance of 123 pF.  Note that the end of the sensor needs to either be insulated or turned up so as to be kept dry as shown here.

    image

    Recall that in an RC Low Pass Filter the time constant is given by Tc = R x C.  Using that relationship and the ability of the Arduino to measure voltage and time a simple Capacitance Meter can be created as described here: https://www.arduino.cc/en/Tutorial/Foundations/CapacitanceMeter

    The Arduino code in the link above was modified by substituting micros() for millis() in the time measurement and the use of a 10M Ohm resistor since the DIY sensor has small capacitance and charges up fast.  The output to the serial monitor is shown below as the coil is inserted and removed from the water.

    image

    I didn't change the print statement so it says nanoFarads instead of picoFarads.  Note that it varies from 134 to 224 dry to wet which is more than what the LCR meter showed.  But it shows a clear and easily measured difference between dry and wet that could be calibrated.  Here it is plotted.

    image

    For those that require video, here you go:

    1321.IMG_7301.MOV

    This is the first thing that came to mind using the criteria that Andrew set out.  I've not tested repeatability or drift.  Another thing that might be done is just measure the temperature out as compared to the environment.  If it is cooler then presumably there is water evaporating and cooling the air if humidity is low enough. 

  • in reply to fmilburn

    Nice one Frank.  Even as I wrote my last post I started wondering if the water would make a useful dielectric and if so could we take a capacitance measurement.  Clever.

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