Maximum Overhang?

[harpo99999]

the main catch as I see it is the interfaces between differing materials ie salt water to resin to air and each having differing speed of propigation of ultrasonic sound, and assuming you are placing the sensor in the salt water to get the 'z' height then there is the corrosion issue for the sensor, and if my understanding is correct the higher frequencies are used for the higher accuracies/smaller distances

[amoose136]

Why not stick two long copper probes close together and held vertically in the water and then measure the resistance? It should drop as an inverse function of fluid height as the resistivity of the salt water should be constant if it doesn't heat up too much from the resin hardening (which is exothermic) but the fluid wire crossection would be a linear function of height.

[Synchron]

(sensor)
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/ (semitransparent mirror) - - - - (laser)
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_ (floating mirror)

[jsondag]

I think a capacitive sense would work best. Resistance is relative to salt concentration, and would require re-calibration every time. I could easily rig up a test with an arduino. you just need to tape a conductive strip to the outside of the container vertically, and submerge a corrosion resistant lead into the water. Gold coated PCB trace would be ideal, and cheap/easily salvageable. You can measure the capacitance which should increase linearly as the water level rises. You can measure it with an arduino using 3 pins, and two resisters, or using a 555 timer, to generate a digital modulated output which could be measured by your drip input on the peachy.

[CescoAiel]

Why not use the laser and a floating mirror on the resin. Messure the time, the laserlight needs to a sensor.

Hardest there would be to maintain the floating mirror's position!

I think a capacitive sense would work best. Resistance is relative to salt concentration, and would require re-calibration every time. I could easily rig up a test with an arduino. you just need to tape a conductive strip to the outside of the container vertically, and submerge a corrosion resistant lead into the water. Gold coated PCB trace would be ideal, and cheap/easily salvageable. You can measure the capacitance which should increase linearly as the water level rises. You can measure it with an arduino using 3 pins, and two resisters, or using a 555 timer, to generate a digital modulated output which could be measured by your drip input on the peachy.

Now there's a new and novel solution I hadn't thought of! Will it be accurate enough tho?

[jsondag]

Now there's a new and novel solution I hadn't thought of! Will it be accurate enough tho?

That's a good question. One that I think more relates to the parts used, and the resolution of said parts, than the method itself. I can test this maybe in the next day or two, just with some foil tape, and my multimeter, and see if that provides decent resolution. for a micro, the test involves charging the capacitor through a known resistance, measuring the time it takes, draining the cap, then repeating. The resistor value, and build of the "Capacitor" will need to be experimented with a bit.

[Synchron]

Why not use the laser and a floating mirror on the resin. Messure the time, the laserlight needs to a sensor.

[amoose136]

Capacitance would depend on the dielectric constant which also depends on saltwater concentration so I don't think we've saved much by measuring capacitance instead of resistance.

Laser interferometer with a floating mirror is better but as soon as you add another laser and electronics that can measure time delays that small you significantly increase monetary costs.

[jsondag]

Capacitance would depend on the dielectric constant which also depends on saltwater concentration so I don't think we've saved much by measuring capacitance instead of resistance.

Laser interferometer with a floating mirror is better but as soon as you add another laser and electronics that can measure time delays that small you significantly increase monetary costs.

The saltwater is the conductor, not the dielectric. The dielectric is the container wall. It will vary from build to build, but remain constant on a single container.

[jsondag]

I tested this with just a multimeter as a proof of concept. I just dangled one probe in the water, and touched the other to the foil. Seems to work, and level of salinity has no effect whatsoever. I went from a weak salt water, to more than double the salinity, with 0 change on the meter. My meter clearly doesn't have enough resolution for fine detail, I never expected it to. A RC circuit on an arduino should give very high resolution, as you're converting capacitance to charge time, and time is something micros are very good at measuring.

I used HVAC foil tape, in both a thin strip and wider one. I've found that a wider strip gave me greater movement in readings. I also found that the surface tension plays a role, as you move up and the walls get wetted, if you move down there is backlash, because you're measuring how far up the wall is wetted basically. No big deal for measuring only one way.

I'm attaching a graph of readings at half centimeter intervals. (nF on the left, cm on the bottom) and a pic of my test container.

My meter is not high quality or very precise, so the little variation from linearity is most certainly from that.


capvswat.jpg2016-03-21 00.53.58.jpg