Looking for a slim liquid level sensor - Page 1
Forum section: electronics / projects

author: peter-h
author_rank: Super Contributor
timestamp: April 12, 2023, 11:58:02 am
content: I need something, 2 terminals, which can be pushed down a 7mm tube, and detects when it is enveloped by liquid. It needs to switch OFF a 12V DC pump when the liquid reaches the sensor.The liquid is a TKS de-icing fluid which is part alcohol part water and has just enough conductivity to present a few megohms across two metal spikes about 2cm long and 5mm apart. I built that and it worked (with a suitable MOSFET-input comparator) but suffered spurious detections simply by the liquid splashing onto the two pins and using the conductive path which is always going to be present via their base.So the pump would cycle on off on off but at least it did turn off and stay off when the liquid reached the sensor. The whole thing was also quite dodgy, due to the microamp currents involved.I reckon a capacitive proximity sensor would do the job but I can't see anything small enough.And I can't hack the whole setup to do it properly...Also I am stuck with the 2 wires.There is no fire hazard; the liquid does not ignite.For background, that tube, about 1m long, feeds a 30 litre tank, and I need to pump the fluid in through that pipe until the tank is full, but there is no way to detect "full" other than the liquid rising up into the pipe.Does anyone have any great ideas?A fairly sophisticated approach would be a coil around a piece of ferrite, and excite it in an oscillator at a frequency which causes a significant mechanical movement in the core (magnetostriction); the frequency would change when immersed in water. I believe this has been used for mass flow coriolis meters.

author: bookaboo
author_rank: Frequent Contributor
timestamp: April 12, 2023, 03:55:22 pm
content: Haven't implemented this one myself but was on a seminar yesterday where a solution for level sensing was to use a small pump to push all the liquid out of just such a pipe, then measure the back air pressure with a BME820 or similar. Apparently very accurate and repeatable, not sure if you need real time atmospheric pressure to calibrate but I would assume so. No idea how they physically implemented it though, was just sketched.

author: Xena E
author_rank: Frequent Contributor
timestamp: April 12, 2023, 04:44:15 pm
content: Been there got the T shirt...If the pipe is plastic, a coil wound on the outside as part of a tuned circuit/resonant circuit, could work. If its just a switching sensor you need, perhaps there is a capacitive one you could use. Synatel Ltd (Canock, Staffordshire, UK), make remote sensor types that can have a probe attached to suit your purpose: I'd say what you need would be feasible to achieve with one of those, may be worth talking to them, rather than trying to reinvent the wheel.

author: Terry Bites
author_rank: Super Contributor
timestamp: April 12, 2023, 09:47:49 pm
content: FDC1004 and some flex pcb.I got the eval kit- FDC1004EVM.Astonishingly good even with a tiny sensor element.

author: pqass
author_rank: Super Contributor
timestamp: April 12, 2023, 10:45:45 pm
content: No wires down the tube. Instead, put in a smaller diameter plastic sensing tube, open-end down the fill-tube. This is how washing machine level sensing works. The end outside the tank goes to a small enclosed cylinder with a diaphragm on one flat side. Use capacitive, optical, or magnetic (hall-effect) means to determine level based on the diaphragm excursion.You may need to weigh down the end down the fill-tube if the fill-tube ends at the top of the tank. You can use a metal plug on the end as long as you provide a hole(s) just above it were the fluid can enter. I don't think it should sit at the very bottom since the sensing tube needs to be emptied so it can equalize with the outside air pressure yet you want enough length such that the air pressure above the rising fluid will move the diaphragm enough to trip at high-water mark.You may even be able to use a commercial washing machine pressure switch. I'm sure they make lighter duty versions of this. You'll have to search for them.Maybe something like this:https://canada.newark.com/omron-electronic-components/2smpp-02/pressure-sensor-37kpa-1-2-6mm/dp/16T8356It has a top-end of 37kPa; about 1/3rd of an atmosphere. By my understanding [, which could be wrong], if the sensing tube is filled 1/3rd with fluid, then that would max-out the sensor. Therefore, depending on how deep your tank is, keep 1/3rd of the sensing tube in the tank (beginning half way down) and remainder determines where up the fill tube the sensing-tube must exit the fill-tube and attach to the pressure sensor.

author: peter-h
author_rank: Super Contributor
timestamp: April 13, 2023, 07:57:49 am
content: That's cunning.Actually one could just use back pressure sensing. When the fluid reaches the bottom of the tube, the pressure will suddenly rise, I think. That is how e.g. MH Oxygen O2D2 regulator detects the start of your inhalation cycle - from 2 metres back, along a thin pipe.

author: nachus001
author_rank: Contributor
timestamp: April 14, 2023, 02:40:47 pm
content: HelloTry looking for a TOF sensor (time of flight)these are ranging devices, usually based on laser or some otherlight source that ranges the distance between an objectand a surface by the time it takes to the reflected lightto come back to the sensor. It can be used with liquids byusing the liquid surface as a mirrorYou'd need a microcontroller to read the sensor and eventually activate a relaycheck thishttps://www.sparkfun.com/products/19218regardsNachus

author: peter-h
author_rank: Super Contributor
timestamp: April 14, 2023, 03:58:12 pm
content: Unfortunately this pipe bends around various turns. No way could you get a laser etc down there, although, yeah, if you send light pulses down a fibre, once the far end gets wet, you will have almost nothing coming back. One issue I see is that you might get a droplet adhering to the far end...Clever though!

author: AndyBeez
author_rank: Frequent Contributor
timestamp: April 14, 2023, 04:15:13 pm
content: In a similar vein to@nachus001, another possible concept is to use the refractive index of a glass/perspex rod. Look down a glass rod (say 8mm diameter) and you 'see' the surface at the other end. Place the rod into a fluid and, because the reflective index changes, the end surface vanishes - it goes dark. * This assumes the rod is ground flat at each end * Thus a light source passing down a dry rod will reflect back an amount of light. In fluid, the light shines out the end of the tube into the fluid. This is effect is used to show the 'full level' in budget water filters. So... A laser diode shines down a long (1 meter) glass or perspex rod. At the top laser emitter end a light sensor detects the return scatter. A sudden drop in light and triggers a signal. This concept might work with even an optic fibre?Or you could weigh the tank somehow as it's filling?

author: peter-h
author_rank: Super Contributor
timestamp: April 15, 2023, 07:54:31 am
content: 1) The pipe bends a lot2) No way to weigh the tankWell, it may be possible to weight the tank by attaching strain gauge(s) to its supports. That is quite complicated. I did such a project c. 1980 for weighing lorries (trucks) with the weight displayed in the cab. It worked fairly well. Very hard to calibrate.

author: geggi1
author_rank: Frequent Contributor
timestamp: April 15, 2023, 08:57:02 am
content: If it is possible to have the pipe straight you could use a floater with a small magnet inside and just detect the magnet by a hall sensor.

author: ejeffrey
author_rank: Super Contributor
timestamp: April 16, 2023, 01:33:18 am
content: Have you looked at measuring capacitance between the two wires? That avoids the DC leakage problems you got with your resistive sensor. The capacitance will go way up when the wires are submerged. The main issue would be how large the capacitance of the leads compared to the sense wires.

author: peter-h
author_rank: Super Contributor
timestamp: April 25, 2023, 11:01:34 am
content: Yes I may have to do something like that. The capacitance must vary in roughly the right way.There are capacitive fuel level sensors which are the best available for liquid level; the main challenge with them is to make sure that you can measure the variation at the far end of a long cable. I think these gauges use shielded wires and the shield is driven from a voltage follower so the cable has effectively zero capacitance.

author: tooki
author_rank: Super Contributor
timestamp: April 26, 2023, 02:17:41 pm
content: I built a readout for precisely such a situation: in my case, measuring the level of liquid xenon inside a cryostat, with the capacitive probes a few meters away.In my case, the probes have around 130pF capacitance in gaseous xenon, about 260pF in liquid. So the capacitance tells you the ratio, and thus the level. It’s very linear, but you do need to design the probe to avoid capillary action. (You can calibrate out a small meniscus, but if the whole probe wicks up liquid it’s useless.)The readout electronics are interesting: the core is a specialty IC called the UTI (universal transducer interface) made by a company called Smartec. It can read small resistances and small capacitances (0-30pF and 0-300pF ranges) across very long cables. It uses a charge amplifier to completely ignore cable capacitance. I used the 300pF range of course, and saw no change in accuracy or precision even with 5m of coaxial cable with about 100pF/m. (My test DUT is a 100pF capacitor measured on a big HP LCR meter.)The output of the UTI is a weird sequence of pulse period modulated square waves: a null reference (twice, half length, for synchronization), a reference capacitance built into the readout board, and then the DUT. I used a PIC microcontroller with “signal measurement timer” peripheral to easily measure the periods. From there it’s simple math to calculate the actual values.I created a little RS-422 serial command line as the interface for the control system that runs it.The UTI is an interesting part, just a little bit hard to get, and it’s about €15 each.(I just had to order some because someone forgot to disconnect it before performing 50kV arc-over tests, and they fried 4 of the 6 channels… sigh…)