Can you pump air down the tube? If so, weight the end and slowly put air into the tube. As the pressure rises the liquid gets forced down. At some point the air escapes the tube end and the pressure in the tube then tells you the distance between the top of the liquid and bottom of the tube. You can also do this by creating a capacitor in the tube using long wires and create an rc oscillator using two I/O pins and a resistor. Depth of immersion will change the capacitance. I built this once with a four layer pcb (cap traces on the inner layers) and an avr to drive it. I could easily see a drop of water evaporate on a sensor of about 6 sq inches. On Aug 26, 2016 5:03 AM, wrote: Is there a way of having a 'stick' hit the container and some sensors listen for the 'ring' of the cavity inside? I'm thinking here of the way to check the level of gas in a gas bottle and see if it rings or is damped. You could have a couple of 'sticks' and a couple of sensors at around the critical point you want to measure. I'm thinking a sensor like a crystal microphone where you could arrange a pin from the diaphragm to be in contact with the outside of the container, yet still be a rugged sensor, and a spring loaded stick that could be pulled back by an arrangement similar to a hammer in a clock that strikes a bell. I'm guessing it probably only needs to operate about 4 times a day to get a sense of the content level. > -----Original Message----- > From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf > Of James Cameron > Sent: 26 August 2016 11:34 > To: piclist@mit.edu > Subject: [EE] external liquid sensing, through 4mm plastic? > > Living in the outback ... > > Looking for a way to estimate liquid levels on the other side of 4mm plastic; a > potentially flammable methane-air atmosphere, without necessarily cutting > holes in the plastic. > > Composting toilet collection bin has a coarse filter 50mm from the void at the > base, and a liquid drain. A DC fan pulls air through the bin contents, through > the void, into a chimney. > > Composting works best when the void has less than 20mm of liquid. > > Sometimes the drain becomes blocked. > > Liquid levels increase slowly, blocking the airflow, until the gas generation of > the bin contents exceeds the airflow, then the residents really notice. Pong. > > How to measure the liquid level? > > Float switches would become fouled. > > IR reflectance sensors would be great, but expensive, and if covered by > muck wouldn't reset until the bin was serviced. > > Stainless steel bolts through the plastic might work, but there's a conductive > path through condensate on the inside surface. > > "Can't you just point an instrument at the tank from the outside and have it > tell you the liquid level?" ;-) Magic is wanted. > > -- > James Cameron > http://quozl.netrek.org/ > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .