Detect conductivity through water, measure aprox. water level on different dots? - Page 1
Forum section: electronics / projects

author: nardev
author_rank: Unknown
timestamp: June 08, 2018, 07:23:30 pm
content: There is few ways to "detect" water level in big tanks. Probably the best one and most expensive is to use pressure sensor. And as far as i saw it, those are quite expensive.Some people experimented with distance measure top/bottom through water but again, it's not as handy.I'm just wondering is it possible for example to have 8 wires, all packed in single thread and every wire ending on different height from the bottom, and to have another line at the very bottom of the tank, and than check conductivity through the water between each wire and the bottom one. Like a simple switch.I'm wondering what current/voltage would be good enough for that? Is it possible at all? Does it exist actually?p.s. or perhaps resistance?

author: SeanB
author_rank: Super Contributor
timestamp: June 08, 2018, 07:29:16 pm
content: Yes it does, it is the basis for the most common level control used in industry, using a series of electrodes and an AC excitation to measure the tank being full or empty. Typically they use around 2VAC mains sine wave and measure the voltage with a simple rectifier circuit, limiting the current with a high value resistor.

author: nardev
author_rank: Unknown
timestamp: June 08, 2018, 07:38:48 pm
content: Thnx, one more thing worries me, does that wire get corroded or covered with plaque?

author: jmelson
author_rank: Super Contributor
timestamp: June 08, 2018, 07:51:39 pm
content: Another method is self-heating thermistors. If the thermistor is above the water, the current through it heats it up. If under the water, the water carries away the heat. You just have to find the right current to get the best effect for the specific thermistors you choose.Jon

author: EmmanuelFaure
author_rank: Regular Contributor
timestamp: June 08, 2018, 08:48:32 pm
content: Measuring a liquid level using "n" wires, capacitance measurement is actually a way more reliable than resistance measurement. The permittivity of water is ~ 100 times the permittivity of air. The capacitance between 2 submerged wires don't care about plaque, algua, surface oxydation, and so on.Ti have a nice chip :http://www.ti.com/product/FDC1004

author: tpowell1830
author_rank: Frequent Contributor
timestamp: June 08, 2018, 10:41:53 pm
content: I have done this before with a single pair (one level). The problem with copper is that it will corrode and if there is salt/sodium it will leach out the copper into the liquid (turns green). I used titanium rods crimped outside of the water to copper wires to my sensor circuit.Hope this helps...

author: james_s
author_rank: Super Contributor
timestamp: June 08, 2018, 11:44:35 pm
content: Quote from: jmelson on June 08, 2018, 07:51:39 pmThat's clever, I like it. I'm almost ashamed to say the idea never occurred to me.

author: jbb
author_rank: Super Contributor
timestamp: June 09, 2018, 06:34:29 am
content: Another common method is an ultrasonic distance sensor at th top of the tank.The advantages: it doen’t touch the water so it’s clean and it produces a continuous readout (ie many levels)Disadvantages: calibration and temperature compensation required, unreliable in the face of foam or froth.Follow up questions: do you have heigene requirements (eg dinking water) or cleanliness requirements (eg process chemicals)? What kind of tank is it? Metal wall, polymer wall (how thick), concreate?

author: nardev
author_rank: Unknown
timestamp: June 11, 2018, 04:23:13 am
content: Well, this was first of all question for thinking.I have some friends cultivating raspberry and they have some huge water tanks, often spending a lot of time watering it and checking the water levels. There is nice beaglebone sprinkler project so i was thinking in that direction, what are the challenges and solutions.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 11, 2018, 05:05:32 am
content: Pressure sensors don't have to be super expensive. You may be looking at ones intended for an "industrial" application with inflated prices. Here is a $20 5 psi sensor:https://www.ebay.com/itm/Pressure-Transducer-or-Sender-5psi-0-5-4-5V-for-Water-Oil-Gas-Diesel-Fuel-Air-/181099877753So it is more expensive than a pair of wires, but not crazy.Another possibility to consider is to weigh the whole tank (or just one corner of it) with a strain gauge. Then the sensor doesn't even have to go inside the tank.

author: Rerouter
author_rank: Super Contributor
timestamp: June 11, 2018, 05:23:33 am
content: Automotive pressure sensors generally are pretty cheap.Water conductivity is not stable. Even with only rain water.When its raining it will stir up any particulate in the tank. Then over the next few days more will precipitate outThe other fun one is the conductivity shifts in non linear ways with temperature, depending on the contaminates present.

author: floobydust
author_rank: Super Contributor
timestamp: June 11, 2018, 06:01:02 am
content: Plastic tank or metal tank?I've seen touch-capacitance IC's on flex PCB's stuck to the outside of a plastic tank.

author: tpowell1830
author_rank: Frequent Contributor
timestamp: June 11, 2018, 11:14:40 pm
content: Back in the '80s I worked maintenance in an aluminum plant. We had a large water tank that we needed to control the level on and the tank had a glass tube outside the tank so that you could clearly see the level. However, we needed to have the level controlled automatically, so we purchased a capacitive proximity switch to close a valve when the desired level was reached. The switch was mounted where the glass tube was so that it could detect the level through that tube, as the rest of the tank was metal. This capacitive proximity switch worked very well. If you have a plastic tank, you could do the same at different desired levels with separate proximity switches, again depending on the type of liquid that you are measuring, which you still haven't revealed what type of tank and what the liquid is.Hope this helps...

author: Marco
author_rank: Super Contributor
timestamp: June 12, 2018, 01:17:01 am
content: Quote from: EmmanuelFaure on June 08, 2018, 08:48:32 pmFor a presence/absence of water I can see why it wouldn't care ... but if you were to try to determine how much of the two wires is covered by water I'd say you would care. The water basically brings the wires "together", massively increasing the capacitance between them. But the exact capacitance depends on the thickness of the insulation and the height of the water, the plaque increases the thickness of the insulation throwing off any guess of the height.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: June 12, 2018, 01:29:55 am
content: Determine the resonant frequency of the head space (frequency sweep similar to determining a room's eq needs) to get the volume of the head space and, hence, the liquid. Same concept as blowing over the top of a bottle of water. The resonance decreases in frequency as the liquid level drops.

author: todd_fuller
author_rank: Contributor
timestamp: June 12, 2018, 02:01:15 pm
content: What is the end goal? Sounds like you want to reinvent the wheel. There are lots of ways to measure tank level but only a few are actually used in industry.Fluid pressure doesn't require modification to the tank or internal access. If you don't care much about level accuracy then it doesn't need a high precision ADC either.If you want to measure the level itself, Maxbotix sells ultrasonic rangefinder specifically for this purpose, they're $100-150 USD. They claim millimeter level accuracy and output TTL or RS-232.If it's a very large tank, I believe industry uses floats on a reel and measures the line length to the float to determine tank level.