Fiber Optic very accurate water detection - Page 1
Forum section: electronics / projects

author: TimB100
author_rank: Contributor
timestamp: October 13, 2016, 09:55:16 am
content: Hi chaps/ladies,Sorry first post on this blog....So I have a need to be able to do measure a fixed volume of pure water (very low conductivity) with very high accuracy. The volume I need to measure is around 30-40ml. It will be a fixed volume.The volume being measured is being filled into a copper tube of about 13.5mm id and around 200ml tall.My first attempt was just 2 probes through an opamp to a ADC input on my micro. This worked great on the bench where I used a self build syringe pump to test repeatability. NOTE the syringe pump was calibrated using a 0.01gram balance.The issue came when I tried to use it in real life and the conductivity dropped. As you would imagine it failed!!!!!!!So now I'm looking at other methods and decided to try a fiber optic method. It will basically involve shining an IR led into the water at the bottom and detecting the level received back up a fiber optic cable that is being dipped into the water. The idea is that there will be an increase in IR falling on the IR Photo transistor as soon as the tip touches the water.Thats the theory any case.My question is do you think it will work or do you know of a better method using some other system? Balance scales are not an option so it has to be a probe of some type.I will later add some sketches to make it clearer.Many thanksTim

author: CJay
author_rank: Super Contributor
timestamp: October 13, 2016, 10:17:04 am
content: Manometer?Load cell?'Squing'?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 13, 2016, 10:25:13 am
content: Ehhh.. you'll have hysteresis, because the fiber will touch the water and form a meniscus that doesn't break until drained some amount.Similarly, you could use dielectric constant. Since there's not much conductivity in that water, but it's still there, it's no less sensible by electronic means -- just use AC!A copper tube seems maybe a little dubious, for super low conductivity water. The metal surface oxidizes, corrodes and dissolves, driven by atmospheric gasses and contaminants. Teflon might be better, if you have the option? In any case, I'm no expert on the matter, but I'm sure there are many excellent sources regarding pure water system design. (Maybe copper is better than I think, in which case, great!)Tim

author: nfmax
author_rank: Super Contributor
timestamp: October 13, 2016, 10:28:52 am
content: How accurate is accurate enough? Is the system at constant temperature, or do you have to worry about thermal expansion of the copper tube? Is this a one-off measurement during an experiment, or does it have to operate continuously for decades? Are you trying to measure an arbitrary volume or do you just want an indication that some set volume has been pumped into the tube? Is the probe to be fixed permanently in place or can it be removed (and cleaned) when not in use?The approach you are describing sounds like what is commonly called a Total Internal Reflection (TIR) sensor, but this is a level-sensing technique, not a volume measurement. The basic principle is that light from a source is coupled into a detector by reflection at a dielectric boundary, the degree of coupling being changed by the medium beyond the boundary (i.e. air or water). They can work well, but like all sensors have their problems - hence my questions!

author: Fungus
author_rank: Super Contributor
timestamp: October 13, 2016, 12:03:26 pm
content: Quote from: nfmax on October 13, 2016, 10:28:52 amThis.There's simply not enough information to answer this question.

author: Kalvin
author_rank: Super Contributor
timestamp: October 13, 2016, 12:09:37 pm
content: A few alternative methods:http://electronics.stackexchange.com/questions/190000/water-level-sensor-circuit

author: TimB100
author_rank: Contributor
timestamp: October 13, 2016, 12:11:43 pm
content: CJay ThanksManometerwill still require some sort of sensor and at 13.5mm dia its is kind of one alreadyLoad cellis really just a balance that I ruled out in my postSquingNot sure what that is, will have to looknfmax ThanksAccuracywith the original probe system repeatable is ~0.01ml I would like to keep thatTempis around 35-40oC and I do have a temp sensor on the copper tube (I may change to SS) so while the maths says that the water/copper expansion will make no discernible difference I can account for it if need be.I'm looking for point detection. The system is the pure water enters from the bottom until it hits the first probe, then I watch it to hit the top probe. I know now that x ml has entered the tube and then empty the tube and repeat the cycle.Probes will not be cleaned but should not get dirty as its clean steam condensate so effectively WFI (water for injection)Usage is either 100% or 4 hours a year.At the level I'm working surface tension is a VERY big issue Even the rate of fill has effect if the water level is concave or convex. My original system had sharpened probes so no water drops could hang, The playing around I have had with the fiber optics is showing I get a tiny drop on the end of my optical fiber around 0.5mm deep. As long as it's on both top and bottom probes it should not be an issue as its the gap that is important and the set up calibration where I calibrate the probe gaps to my fix volume will be done with the drops.The TIR system as far as I can tell has a light source that bounces of the end of the fiber/whatever and you get leakage into the water medium. The think is unless you can feed in the light into the fiber through a y splitter (not found one yet) You have to have a 2 light paths. One providing the light the other taking the reflection back.My thoughts were if the IR Led was in the water there would be a path through the water and when the fiber/probe touches the water it will increase the light transmission up the fiber and can be detected.The fiber option will remove the possibility of DC corrosion of the probes (yes I'm looking at AC) and the very low conductivity issue of the water I have. With a data slicer style buffer detection system any changes in the probes cleanness can be over come.

author: Kalvin
author_rank: Super Contributor
timestamp: October 13, 2016, 12:13:33 pm
content: Weigh it?TDR:http://www.edn.com/design/sensors/4433411/Use-Time-Domain-Reflectometry--TDR--for-low-cost-liquid-level-measurement--Part-III

author: TimB100
author_rank: Contributor
timestamp: October 13, 2016, 12:21:42 pm
content: Posts made while answering so addressing more hereT3sl4co1lhysteresisYes its dealt with as I only measure on the up fill.dielectric constantI am paying someone to look at the probe system. The idea being that you set up an AC signal and look for it to be attenuated by the water. However I see it that to alter a freq of say and RC system involves a reduction in the R value and at the conductivity level of the water 1.7 us/cm your not going to get much of a change. Hence I was looking at non electrical system and this post.Copper pipeYes not the best system hence want to use SS. The thing has to be manufactured and involves joining of materials so SS will be better.

author: Kalvin
author_rank: Super Contributor
timestamp: October 13, 2016, 12:41:57 pm
content: Make a long coil and measure the coil's Q-value as a function of the water level/volume.Make a plate capacitor and measure the capacitance as a function of the water level/volume.Make an LC-oscillator and use the plate capacitor above as the C and measure the frequency as a function of the water level.

author: TimB100
author_rank: Contributor
timestamp: October 13, 2016, 12:49:05 pm
content: KalvinUnfortunately all the capacitance system you propose would have very poor accuracy

author: Kalvin
author_rank: Super Contributor
timestamp: October 13, 2016, 12:53:06 pm
content: Make a "bird flute", the water level will act as the stick. Measure its frequency as a function of the water level:http://dunum.ch/WebRoot/Store2/Shops/172505/5377/A6EE/CFC2/308F/BA9D/D91A/30F9/0B0A/Birdflute_482c304998340_m.jpg

author: nfmax
author_rank: Super Contributor
timestamp: October 13, 2016, 12:57:17 pm
content: How about a thermal probe? The 'classic' way of doing this is with a PTC thermistor, with an applied voltage to heat it just above ambient temperature. The current taken when the thermistor is immersed in the water increases markedly, due to increased convective heat loss. You might have to build your own probes using SMD thermistors: obviously you want to keep the probe size down. The advantage of this technique is it doesn't depend on the electrical properties of the liquid, and isn't too much affected by biofilm. Perhaps scaling might be an issue for you, though?If the basic PTC sensor can't be made to work, you might try an NTC bead thermistor in a potential divider with a fixed resistor. Switching the DC supply voltage across the potentiometer between 2 levels will give you a square-ish wave with rounded-off edges (like a low pass filter) as the thermistor heats & cools. Measuring the time constant should tell you if the thermistor is in air or water.Edit to add: Your volumetric accuracy requirement gives a water level accuracy requirement of 0.07mm - that's going to be tricky! Even variation in surface tension depending on the cleanliness or otherwise of the copper tube could be a significant source of error.

author: Kalvin
author_rank: Super Contributor
timestamp: October 13, 2016, 01:21:28 pm
content: Quote from: nfmax on October 13, 2016, 12:57:17 pmLaser and webcam/image sensor: Shine the laser to the water surface and let it reflect to the image sensor. The image sensors have pixel resolution down to micrometers: Sensor size: 1/4", Pixel size: 5.6 x 5.6 um, Frame size: 640 x 480.Edit: The ambient vibrations can be cancelled out by averaging. (Ambient vibration level was mentioned by nfmax in a message below).

author: nfmax
author_rank: Super Contributor
timestamp: October 13, 2016, 01:27:14 pm
content: What is the ambient vibration level going to be like? Is the flow entering the tube likely to be turbulent? These effects could give you problems!

author: Fungus
author_rank: Super Contributor
timestamp: October 13, 2016, 01:28:19 pm
content: How about something that floats in the water and rises out of the top of the tube to break a beam of light?(or tilt a mercury switch, or whatever...)It might be worth conformal-coating the inside of the pipe as well. Copper will go nasty with time no matter how pure the water is.

author: Fungus
author_rank: Super Contributor
timestamp: October 13, 2016, 01:30:38 pm
content: Quote from: Kalvin on October 13, 2016, 12:53:06 pmHah! That's a good one!Seriously though, it might work. Make something that taps the side of the tube with a solenoid and listen to the frequency.

author: Kalvin
author_rank: Super Contributor
timestamp: October 13, 2016, 01:52:20 pm
content: Ultrasound interferometering should give accurate results with high resolution.

author: nfmax
author_rank: Super Contributor
timestamp: October 13, 2016, 02:02:20 pm
content: I don't think ultrasound in air would work - resolving 0.07mm requires a time accuracy of 200ns, which is likely to be swamped by sound velocity changes due to ambient conditions (pressure, temperature, humidity, airflow). Ultrasound in watermightbe able to do it (since the speed of sound is about 4.5 times faster) but not if the water is flowing.Optical interferometry could easily resolve 0.07mm, but it becomes difficult to keep track of all the fringes as the water level will change over many, many wavelengths. How about closing off the top of the tube with a conductive cap and measuring the (microwave) resonant frequency of the resulting cavity?Edit to add: What @grouchobyte said - I also work professionally in fibre-optic sensing!

author: Kalvin
author_rank: Super Contributor
timestamp: October 13, 2016, 02:13:46 pm
content: Quote from: nfmax on October 13, 2016, 02:02:20 pmThe reflected laser beam from the water and the webcam/image sensor should give accurate results down to few micrometers depending of the image sensor's pixel size. The image sensing will give absolute water level as it doesn't have the problems related counting the fringes or nulls. Should there be any ambient vibration, averaging the image sensor results will cancel out the ambient vibration. Use theOpenCVand some Python for the image processing.

author: leeatljs
author_rank: Contributor
timestamp: October 13, 2016, 02:30:24 pm
content: If you get a bi-colour 3 or 5mm LED, wire one LED to illuminate, wire the other as a photodiode into an op-amp. With no water, internal refraction will illuminate the 'photodiode'. With water, most of the light escapes out. This will only work if the water doesn't leave a residue on the LED surface.The heated thermistor works great, although I have had trouble with 'sludge' growing on the thermistors.I have designed out the thermistor method in favour of a capacitive sensor and this works well for a water level of 10cc (in my application)

author: TimB100
author_rank: Contributor
timestamp: October 13, 2016, 02:35:39 pm
content: Just to save typing. This is not a new project for me so I have explored most of the options being posted before. 95% of that posted here would be lucky to resolve to 1mm accuracy. That is way to coarse for me.My measuring tube is filled from the bottom at a flow rate of around 12ml per min that should be around 1mm every 1.3 seconds I aim to sample at 100hz.For those wanting to see micro level sensing in action there is a videohttps://youtu.be/oA-0UgrdPBUWhat I do not understand from the video is how it work.The way I propose my system to work is that there is a better light path generated by the water coming into contact with the fiber that is detected by the IR opto transistorRef Microwave. If there was a simple microwave TX RX unit that could look down the tube and accurately measure the level to 0.1mm I would go for that. As That would also give me flow rate. Currently I get that using a micro pressure sensor. (Which is not that good)ThanksTim

author: nfmax
author_rank: Super Contributor
timestamp: October 13, 2016, 02:36:52 pm
content: Quote from: Kalvin on October 13, 2016, 02:13:46 pmAveraging wont help though, if there is a meniscus on the water surface - the tube ID is only 13.5mm

author: TimB100
author_rank: Contributor
timestamp: October 13, 2016, 02:38:39 pm
content: leeatljsThe bi colour Led sounds interesting as that means no splitter. I did not know you could get a diode to act as a detector.

author: Kalvin
author_rank: Super Contributor
timestamp: October 13, 2016, 02:47:56 pm
content: Quote from: nfmax on October 13, 2016, 02:36:52 pmWouldn't that affect the contact based measurement like micro level sensing shown above as well? The non-contact optical method doesn't suffer from the surface tension problems or contamination of the measuring probe like the micro probing or any other contact-based method will probably do.

author: nfmax
author_rank: Super Contributor
timestamp: October 13, 2016, 02:48:43 pm
content: That's an interesting problem you have there, @TimB100!

author: ajb
author_rank: Super Contributor
timestamp: October 13, 2016, 02:55:54 pm
content: Quote from: TimB100 on October 13, 2016, 02:35:39 pmThe description for that youtube video says that the fiber has a silver coated tip. So when not in contact with the water, the silver coating reflects most of the light back up the fiber, then when the water comes up the sides of the fiber the light starts escaping out of the sides of the fiber through the water, and less light is reflected back up the fiber. The amount of light lost depends on how far up the fiber the water gets, so there's a narrow range where it's possible to detect a level rather than just contact/noncontact, but measuring that reliably will depend on careful control of the amount of light applied to the fiber. Using a reflector on the tip of the fiber eliminates the problem of droplets/meniscus clinging to the face causing changes in reading.The "Y splitter" you'd be looking for is anoptical circulator.

author: TimB100
author_rank: Contributor
timestamp: October 13, 2016, 04:40:29 pm
content: Thanks ajbThat optical circulator looks good but to expensive for production use.I have a 1mm od fiber cable and trimmed the cover of when doing initial tests. Looking at the fiber in the video it seems to be a Single Mode cable which might be only 0.009mm od WOW!So might try and find a cable with e core of say 0.5mm or less to eliminate the issue of drops clinging to the endI have come away knowing more than I started with so its been a good dayCheersTim

author: nfmax
author_rank: Super Contributor
timestamp: October 13, 2016, 05:05:08 pm
content: Those diameters are CORE diameters, not the actual fibre diameter. Single mode fibre usually has a core around 8 or 9 microns across, while multi-mode fibre is generally 50 microns core diameter. In both cases, the standard diameter of the actual glass fibre is 125 microns. To prevent surface damage causing breakage, an additional protective primary cladding is applied, almost always a soft polyacrylate, giving a diameter of 400 microns. If you buy a reel of 'fibre', this is what you get. Fibre patchleads generally have a secondary cladding, strength members, and a soft silicone jacket giving a diameter of 3mm or so (varies).Fibre with just the primary cladding is quite robust and can be handled with care. The care is mainly care not to stick bits of it into yourself.SM fibre is available (at extra cost) in smaller sizes, generally intended for use in very small coils - we have used 80 micron (glass) diameter fibre. Diameter matters because the water 'drop' that is likely to end up hanging on the fibre will be of around this size. Fibre like this will probably have polyimide coating which is much thinner than polyacrylate (just as good as polyacrylate for protecting the glass surface but not as effective at preventing microbend loss)The fibre in the video had been tapered - presumably using a fusion splicer. I think how the sensor is working is that this taper has the effect of coupling light from the primary mode, guided by the core/cladding interface, into cladding modes, guided by the cladding/air interface (the primary cladding has been stripped). In air, these travel to the cleaved end, which is possibly metallised (?), and are reflected back, still as cladding modes. When they hit the taper, some at least of the cladding modes are coupled back into the core and go to the photodetector. When you immerse the thin part of the fibre in water, the cladding mode is more weakly guided and so escapes from the fibre into the water. This reduces the reflection. The more fibre is immersed, the more light is lost. However it looked as if the function was not monotonic with depth, which is probably a function of the particular cladding modes being excited.The fibre can only be tapered if the primary cladding is stripped. The taper and the sensing fibre will bephenomenallyfragile!Edit to add:SMF-28e from Corninghttps://www.corning.com/media/worldwide/coc/documents/PI1463_07-14_English.pdfSmall diameter SM fibrehttp://fibercore.com/product/polyimide-coated-sm-fiberFibercore are local to us, and for many years I worked together with the guy who is now their CTO!

author: TimB100
author_rank: Contributor
timestamp: October 13, 2016, 06:21:03 pm
content: Thanks NfmaxWhat I'm going to test as soon as my work load work gives me 5 mins is the light transmission of IR through water. The surface water layer should cause internal mirroring and reduce transmission. As soon as the Fiber pierces the water the IR should be able to enter it with less losses and be detected by the ir transistor.I need now to find some better cable than the plastic fiber cable I have.Tim

author: SeanB
author_rank: Super Contributor
timestamp: October 13, 2016, 07:00:17 pm
content: How about using a PTFE rod and machining the desired ID through it. That way you will not contaminate the DI water there, which I assume you are making to injectable water quality. Thus no copper or any metal to touch the water, only either borosilicate or Quartz glass and pure PTFE will do, and all has to be either steam sterilisable or withstand aggressive chemical cleaning. Then you can use a bare fibre top and bottom, fed through a microbore through the PTFE walls ( bare fibre without cladding) and bent gently in the tube so there is a curve in the fibre. Then you can use a constant illumination light source ( or a reference fibre of the same length in a cladding) to illuminate the 2 or 3 fibres evenly ( 3 fibres if you use a red LED in a metal housing with the fibres arranged to see the beam reasonably evenly. otherwise you need another photodiode there to measure light output and drive the LED to a constant illumination level) and then a 3 photodiode block ( thermal mass so each has a constant transfer function that will track) to get the drop in light as the water touches each fibre, and total internal reflection is going to change the light level as it changes refractive index on the curve in the fibre.If you can bend the fibre in the tube diameter then simply drop them down from the top, and then adjust so the droplet on the end is constant, though you probably will simply have to have the drilled microbores at a fixed distance apart and use that as a calibration, polishing the bore to a smooth finish before installing the fibres. Fiddly, but you can thin the tube at the fibre points or use a larger hole and PTFE plugs on the fibre ( 2 different diameters so you can thread them into the tube, tapered to lock in the wall and then held in position with epoxy and a outer collar) to get them in there.PTFE and glass is not going to contaminate the water, and the fibre will also do the microsensing you want for the fine adjustment. Just use the small linear range where the water is only covering part of the fibre as sense range before it is either immersed or not.

author: max_torque
author_rank: Super Contributor
timestamp: October 13, 2016, 07:52:06 pm
content: You want to measure a "volume" of water yeah? So, assuming your measurement system is measuring a linear distance or angular one, then you need the thinest "resevoir" possible, so that tiny changes in volume lead to large changes in distance. Using a 1mm dia meter pipe, that is long would allow you to use basic digital sensing techniques to accurately measure volume. Even better, use a very thin ID tube, coil it up, use a IR beam break or TIR sensor when the water in it gets to the necessary length. If you connect a large volume to the end of the pipe, the pressure rise will be small and linear with volume.Alternatively, how about just pumping in too much volume, and just drill a hole in your current tube so "excessive" water flows out again! Lots of complex dosing systems do this, because it's simple and works!!

author: TimB100
author_rank: Contributor
timestamp: October 13, 2016, 08:19:59 pm
content: SeanBIn one of my original posts I said it was clean steam condensate.max_torqueThe trouble of loads of posts is that new commenters cannot read them all. In one I said that the chamber is emptied in a cycling system.Anything less than say 8mm you get to much surface tension and it will not drain. Also as soon as you get into small dia temperature changes makes a big change in volume/length.I have to know when I have ~30ml so at 8mm dia you're talking 600mm tall, 1mm would need a pipe around 40mtrsAs per the title I'm looking for info on fiberoptic level detection.

author: Kalvin
author_rank: Super Contributor
timestamp: October 13, 2016, 08:51:34 pm
content: Although you are looking for a fiber optic solution, here is a quick test with a cheap 640x480 webcam bought from eBay measuring the water volume in a translucent plastic container. The container's diameter is approx. 45mm, the initial water volume was approx. 10ml, each additional volume was approx. 0.2ml. The water level change is approx. 37 pixels per 0.2ml, thus one vertical pixel in this setup corresponds to 0.2ml/37 = 0.005ml. With better optics and camera/image sensor, and proper lighting conditions, possibly using polarized light the image quality should be much better. With a thinner 13mm tube vs. the 45mm container used in the experiment, the resolution will be much improved**. However, this was just a proof of concept.** The change of water level height in 13mm tube vs 45mm tube is almost 12:1. As the height of one pixel row represented 0.005ml change in water volume in 45mm tube, there will be 12 pixel change in the 13mm tube for the same 0.005ml ie. one vertical pixel corresponds to 0.005ml / 12 = 0.0004ml in 13mm tube.

author: filssavi
author_rank: Frequent Contributor
timestamp: October 14, 2016, 06:43:37 am
content: The simplest thing you can do (for me anyway) is use the capacitive sending, so put two electrodes in the water, apply an AC excitation signal of suitable frequency/amplitude, and measure the capacitance between the 2 electrodes, as water's relative permittivity is much higher than air's One (58/80 to 1) you should see the capacitance increasing as level goes up

author: C
author_rank: Super Contributor
timestamp: October 14, 2016, 11:13:38 am
content: optical circulatorThey use power splitters in cheap fiber optic systems. Think in addition to the power split, it is also somewhat of a directional coupler. I am thinking of the fiber trunk where the power tap is just wrapped around trunk some distance and heat fused. Thinking from lower cost angle.Going a step, Time of Flight.Think of using fiber length for a fixed TOF sensor. A split and combine of equal length, no amplitude change. Unequal length of one after split should give a step change for an input amplitude change. Could need a reference to compare measured tof against, like a three way split.Your TOF change could be to small for cheap.If you have a frequency that changes some based on level, today you might think of a x___ phase locked increase. Using harmonics is often forgotten. For a 1 Hz change the 1000x harmonic changes 1000 Hz.You could use two of the harmonics in to a mixer and use same circuit to repeat this.

author: Kalvin
author_rank: Super Contributor
timestamp: October 14, 2016, 12:10:40 pm
content: The OP wanted the measurement repeatability to be 0.01ml which corresponds to water level change of 75 micrometers (0.075 mm) when the tube diameter is 13 millimeters. The TOF measurement will require measurement resolution to be less than 0.5 picosecond (assuming that the light travels in a fiber 20cm/ns: 2cm/100ps, 2mm/10ps, 0.2mm/ps, 0.1mm/0.5ps).

author: TimB100
author_rank: Contributor
timestamp: October 14, 2016, 12:26:24 pm
content: Hi,Just to say I'm going to write up after work a very detailed description of all the issues involved in this project.I really appreciate all the suggestions but 95% of them have already been thought of and dismissed.CheersTim

author: C
author_rank: Super Contributor
timestamp: October 14, 2016, 01:17:28 pm
content: Quote from: Kalvin on October 14, 2016, 12:10:40 pmThink you are only thinking of TOF of Fiber only. One side is fiber, side(s) is fiber, water & or air).So it the ratio of water & air TOF.With adjustment would result in level change or no level change.Probably still to small.Might also be possible to circulate signal to make change larger.Edit oscillatorSome here have mentioned using water to change value of a cap creating and oscillator that changes frequency by water level.Using a time delay you can also create an oscillator. Ten feet of cable vs 100 feet in feedback path.So you have an oscillator with radio, light or sound creating a time delay with the water vs air TOF change creating the frequency change in oscillator. Sound for example should bounce back when it strikes the water and container bottom. These standing waves should add at certain distances.

author: LaserSteve
author_rank: Super Contributor
timestamp: October 14, 2016, 01:59:04 pm
content: TI makes some wonderful noise rejecting capacitance sensing chips. FDC 1004 is a favorite around here at the University, especially with its driven shield and low cost EVM with decent logging software. You can get a hydrophobic or super hydrophobic material for the outside of the probe. Flatteningthe "Contact Angle" with the right material on the outside of said probe reduces or modifies meniscus to tolerable levels.Steve

author: max_torque
author_rank: Super Contributor
timestamp: October 14, 2016, 02:59:23 pm
content: Quote from: TimB100 on October 13, 2016, 08:19:59 pmIt would seem much easier to me to simply use a cheap, easy to drive pump (say a perastalitic one etc) to push the water out of a narrow tube than spend thousands of man hrs on complex fibre optic detection of infinitesimally small distances. KISS is a prinicipal that seems to have been forgotten rather these days.....And of course, only the bit with the sensor in needs to be narrow (to give you the high resolution), the rest of the pipe can be any diameter you want, so no need to have a "Long" or "large" system. Use physics to your advantage is my suggestion!

author: Kleinstein
author_rank: Super Contributor
timestamp: October 14, 2016, 03:34:05 pm
content: The surface tension / surface wetting will be a real problem. The surface tension per se is not the problem, the difficult thing is that wetting may not be fully reproducible with some more or less sticky points at the surface. Copper is not that perfectly corrosion resistant and thus might have a surface that changes in wetting and might develop nasty spots.I personally would choose a thinner diameter (which would ease the measurement) and use a kind of glass tube so that an optical measurement from the outside is possible.A capacitive measurement can get tricky, as at low frequencies the conductivity gets important as the water will be more like a poor conductor, reducing the spacing and not really sensing the dielectric function of water (which is less sensitive to impurities). The cross over can be rather high in frequency (e.g. 100 MHz range). Also isolators might change there properties with humidity / water contend. So while I like capacitive sensing, i would use it here.

author: TimB100
author_rank: Contributor
timestamp: October 14, 2016, 05:20:08 pm
content: Here is are the notes I made. As they contain pictures I have it as a PDF

author: LaserSteve
author_rank: Super Contributor
timestamp: October 14, 2016, 05:59:11 pm
content: What is the minimum amount of water to be dispensed ? What is the dead volume of water, the part that can remain in the cylinder as a minimum load?That is a big question as well.At work we use precision pumps for this, off the shelf and controlled by USB or a serial port.Steve

author: TimB100
author_rank: Contributor
timestamp: October 14, 2016, 07:06:33 pm
content: Hello LaserSteveThere is a constant flow of condense, I'm sampling it constantly to get the volume/timeThe dead volume is whatever I want it to be as the design is not finalised. But it will probably be around 10mlAs per the first answer I'm not dispensing I'm measuring.

author: max_torque
author_rank: Super Contributor
timestamp: October 14, 2016, 08:35:10 pm
content: Quote from: TimB100 on October 14, 2016, 07:06:33 pmCould you do a simple sketch of the system? Might help us get an idea for how it looks and sizes etc.Is the flow of condensate constant? at what rate? Is it being condenses else where and being piped to the measurement cell? Average measurenments or real time?Perhaps some sort of resonant sensor? Measure a drop or two at a time, through changes in the fundamental frequency or amplitude of a tuning fork with a (small) bucket on the end ? Once full, a servo quickly tips the condensate out. The tips per minute are your flow rate??

author: max_torque
author_rank: Super Contributor
timestamp: October 14, 2016, 08:38:29 pm
content: Sorry, just seen the .pdf you stuck up there^^^^ which answers those questions!

author: max_torque
author_rank: Super Contributor
timestamp: October 14, 2016, 08:44:49 pm
content: One technique that worked on a larger scale (it was a 220l oil drum!), but could possibly be scaled down is to use a diferential pressure sensor and a pump to drive water OUT of a vertical tube that runs inside a tall thin measurement cylinder. The pump runs continuously, i used a tiny vane type pump, and takes air from the top of the main cylinder, and pumps it into the measurement tube. When water collects in the main tube, the pressure differential between the top of the main cyl and the messurement cyl is the height of water above the bottom of the measurement tube, because the pump keeps any water from entering the measurement tube. You need an accurate diff pressure sensor, but these days that isn't THAT hard, and need to remove the pressure pulses from the pump with software filtering of the dP signal (again not too hard). Using a diffPressure sensor means it isn't skewed too much by baro changes, but you may want to correct for changes in air density? (measure temp and baro too)

author: max_torque
author_rank: Super Contributor
timestamp: October 14, 2016, 08:52:43 pm
content: Something like this (excuse crappy sketch!)

author: DaJMasta
author_rank: Super Contributor
timestamp: October 14, 2016, 10:01:59 pm
content: Could you coat the inside of the reservoir with a hydrophobic coating to narrow it and get more precise measurements of distance to the top? Do you have control over the shape of the reservoir? A capacitive sensor could be just fine if you had a flattened reservoir that was sufficiently thin, though I think that's going to be a challenge if you're really trying to get such minute changes noticed. If you have to stick with the tube, you could string a wire through the middle (or better yet, a narrower tube) and if everything was covered in a non-conductive, hydrophobic coating, you could make a very nice little capacitive level sensor. Have you considered floating something on top of the reservoir and using a sensor to measure the height of the float? Could be easier to get accurate than trying to measure the water level itself.My real question would be why you need such precise resolution - hundredths of a mL will be busy condensing on the side if you're just condensing from steam whenever it's in operation. Would waiting for longer and measuring a larger change (tenths or individual mL) be sufficient if you interpolated and found the rate of change between viable samples, or do you need such precision in real time? If you can take lots of samples and average them together, then some noise in your readings is fine.If you are condensing into a vessel, then just going by weight may be entirely viable because you can physically decouple the reservoir on a load cell - it wouldn't necessarily have to be fixed in place.

author: DTJ
author_rank: Super Contributor
timestamp: October 15, 2016, 03:24:04 am
content: Maybe look at gravimetric flow measurement products made by a Jap companywww.onosokki.co.jp/English/english.htm. They were very sensitive, accurate and the concept was straight forward.I guess you've looked at the flow sensors offered by sensirion.com

author: TimB100
author_rank: Contributor
timestamp: October 15, 2016, 08:31:39 am
content: max_torqueWater/Condensate is flowing all the time. The flow rate may vary so I am looking to ascertain the flow rate but I need high accuracy. Its a VERY simple system to fill a tube and time how long it takes to fill the tube between 2 probes. It works, I have proven it. I see no reason to make it more complicated.But the issue is that it's clean water with very low conductivity.It is not impossible to detect very low conductivity water and I'm paying someone to do the R/D. He though is busy as well and not done anywork on it. And as I think its going to have issues I am looking at other systems where I can use the same proven probe system but without the conductivity issues. Hence fiber optic probes.DaJMastaI will probably have the inside of the tube coated in teflon but that is more to prevent drops clinging to the sided and messing with the volume.Restricting the probes section is an obvious option and mentioned in the PDF. But I also would like some idea of real time flow rate and changes in volume at various sections will make using the pressure sensor difficult (but not impossible)The Probe system works I need to be able to make it work with very low conductivity water. See above"why you need such precise resolution" Without it the product does not work. I'm automating a manual system and that requires accuracy to 0.3ml which is relatively easy. but I have to have machine tolerances so aiming for <0.1ml. I can get 0.03 with probes.Its not a condensor it is measuring condense. The condenser is separate.DTJsensirion are £1000+ and would represent 200% of the aimed for BOM costs.

author: Kalvin
author_rank: Super Contributor
timestamp: October 15, 2016, 10:40:37 am
content: Although this capacitive method is not fiber optical method, this might be useful.- The valve "out" is closed and the valve "in" is opened and the fluid will be pumped into the tube.- The capacitive sensor will be used to measure the water level in the tube.- When the predetermined fluid level is reached, the valve "in" will be closed and the valve "out" will be opened and the predetermined fluid volume will flow out.The capacitive sensor is based on a precision capacitance sensor (for example, capacitance sensors from TI as LaserSteve suggested). The capacitance sensor is used as follows:- When the tube is empty, the sensor's capacitance is very low.- When the fluid level reaches the bottom of the sensor, the sensor's capacitance starts increasing. When the fluid level has reached this point, the pump rate can be decreased if needed. This can be use also to monitor the pump health as the fluid should reach this level in a certain time window.- When the fluid level reaches the top of the capacitance sensor, its capacitance will stop changing: This is the detection level that the tube is full.**Now, you just need to adjust the tube's diameter and the capacitive sensor so that the water level tolerance is sufficient for you application. You could form the tube so that it is fatter at the bottom, and it will much narrower at the top in order to get high measurement accuracy and resolution.** This is quite different from determining the fluid level as a function of capacitance: This is not dependent of the actual sensor capacitance or absolute accuracy of the sensor capacitance, but only the change of the measured capacitance. It is very easy to measure the change of the sensor capacitance even if the fluid has different properties or the properties of the capacitance sensor changes in time.

author: TimB100
author_rank: Contributor
timestamp: October 15, 2016, 12:54:26 pm
content: KalvinI have not used capacitive sensors in practice but these are all the issues I see.1 The bore is ~13.6mm dia and I will probably have make it 2 bores in parallel at 11mm dia. It would be very difficult to place plates inside2 There needs to be a top and bottom probe, Its hard enough with a single probe.3 Probes that take up volume make my pressure based instantaneous flow very difficult due to varying volumes.4 Capacitive probes only solve the detection issue but fail on the accuracy.5 2 plates means that one may come in contact with the other. Unless its tilted then it will not and screw up any calibration.Pumps? There are no pumps and introducing one would make the product not work.My need is really really simple1 A way to use a needle tipped probe to electrically detect pure water as soon as it touches the water (doubt it would work but paying someone to look into it)2 A way to use a optical fiber based probe that would work in the same way as the probe above.The later is what I need to test.ThanksTim

author: _Wim_
author_rank: Super Contributor
timestamp: October 22, 2016, 08:53:14 am
content: Would it be possible to use a very hydrofobic air filter at the top of your tube (0.22µm pharma air filter). These filters let air pass though without any problems, but block the liquid. When the liquid hits the filter surface you get an instant pressure incease in your system that you can detect easily?Edit: sorry, just realised water is probably only coming from the bottom in your calibration setup.

author: TimB100
author_rank: Contributor
timestamp: October 22, 2016, 11:26:55 am
content: Well that is a very novel and interesting approach. It could be made to work if you had a vacuum pump or sum such device and a pressure sensor.The issue as I see it through is how you would managed the filter. The bore would be small and very difficult to attach a filter to. Also the water drop would be sucked against the tip so probably not notice the level drop.Very neat idea though.

author: _Wim_
author_rank: Super Contributor
timestamp: October 22, 2016, 12:09:32 pm
content: My original idea was a little different, but as I said, I was mislead by the picture in your pdf were I assumed the liquid was pumped in from below. If that was the case , then you could at a hydrofobic filter at the top of your tube, and when the tube is full, pressure would rise very quickly to the supply pressure. This would be the trigger to open your drain valve. The pressure sensor can be a simple low cost pressure swith, does not need to be accurate. But to work like this, you will need a sort of pump at the supply side to generate the pressure, which you would like to avoid...

author: DTJ
author_rank: Super Contributor
timestamp: October 22, 2016, 01:14:40 pm
content: Good idea by Wim.Check out Goretex filters. They do some circular disk types. Might be quite simple to install.Vacuum pump==>=<== press sensor or switch|||--------------| Goretex disk| || || known || volume || || || |==X===X==|| |||| \===out valve||\ === Inlet valveopen inletapply vacuumpressure will drop when cavity fullclose inlet valveopen out valvepump air in vac port to expel volume of waterYou could stick a thread in from the side to calibrate or change the set volume.

author: TimB100
author_rank: Contributor
timestamp: October 22, 2016, 01:25:14 pm
content: So its a neat idea but for my implementation there would be a number of issuesHere are some of themI need to time the fill so a bottom sensor is mandatoryGortex of any filter that would be reasonably non water transmittance would not have enough airflow to allow gravity draining. I have to go to special measures to get it to empty as it is. (due to surface tension)I have a 0-400mm h20 pressure sensor any pressure in the vessel above 400mm h20 would kill the sensor in a second.WimIt does fill from the bottom

author: _Wim_
author_rank: Super Contributor
timestamp: October 22, 2016, 01:40:41 pm
content: "I need to time the fill so a bottom sensor is mandatory" => I am not sure what you mean by this? The pressure swith, or some additional sensor you use ?"Gortex of any filter that would be reasonably non water transmittance would not have enough airflow to allow gravity draining => I have to go to special measures to get it to empty as it is. (due to surface tension)"=> This is for sure true. But if you use a reversable pump for the vacuum, you could use pressure to empty the container again."I have a 0-400mm h20 pressure sensor any pressure in the vessel above 400mm h20 would kill the sensor in a second." => Is this sensor also on the tube? Depending on the sensor type, 10 times the max range for overload is not very uncommen. To avoid very sharp pressure spikes, you could install the hydropohic filter between a soft gasket (or allow for some other type of deformation in the reservoir). This creates a little buffer/flex, making the rise time of the pressure peak slower and allowing you to detect the pressure rise, before it is to high to damage the second sensor. The flex could even be the pressure sensor itself (types with the stainless steel membrane).

author: TimB100
author_rank: Contributor
timestamp: October 22, 2016, 01:50:38 pm
content: My main aim is to accurately get a flow value but the flow needs to be so accurate that conventional flow meters will not work. Hence I measure the time it takes to fill a fixed volume, a bit of maths and have a flow rate.Going back to my main question on the use of fiber optics some issues I have since found out.IR is attenuated by water but in my case the distance the IR beam has to travel is such a small distance not sure if it will be an issueThe angle of reflection to bounce IR is very low so in my tube example I doubt I would get much.Some day soon I will get some time off to get in the shop to do some tests.CheersTim

author: meeder
author_rank: Regular Contributor
timestamp: October 22, 2016, 02:53:30 pm
content: What is the accuracy needed? Do you need temperature compensation as well since the volume is dependent on the temperature as well.Edit: what is the estimated flow rate?

author: meeder
author_rank: Regular Contributor
timestamp: October 22, 2016, 03:04:17 pm
content: Tim, is 0,05% accurate enough? If so than you can use a flowmeter.

author: TimB100
author_rank: Contributor
timestamp: October 22, 2016, 04:36:39 pm
content: meederFlow is around 8-16ml per minI am aware of flow meters that work on doppler effect using ultrasonics but they start at £800 for the low end. I have gone up to £400 units but they failed to be reliable and bubbles in the flow cause issues.I need ~0.1% accuracy.

author: meeder
author_rank: Regular Contributor
timestamp: October 22, 2016, 04:49:28 pm
content: Air inclusion is indeed a major problem for a flowmeter. The accuracy isn't an issue since Coriolis type massflow meters can reach 0,05% but they come at a cost. The air however will be a problem for any flowmeter no matter what the technology is.

author: EmmanuelFaure
author_rank: Regular Contributor
timestamp: October 22, 2016, 06:17:04 pm
content: Tim, by any chance is this an academic project?

author: TimB100
author_rank: Contributor
timestamp: October 22, 2016, 07:22:58 pm
content: EmmanuelFaureNo its for a future product I want to develop. I have my own company and it will earn me money.Hence why I pay people to do work for me.Cheers

author: bitslice
author_rank: Frequent Contributor
timestamp: October 22, 2016, 08:06:08 pm
content: Quote from: TimB100 on October 13, 2016, 12:11:43 pmWould adding a tiny amount of surfactant aid in achieving a more accurate measurement?A concave surface sounds a bit more repeatable than a convex one, which I suspect is dependant slightly on the surface of the tube?Just throwing it out there as you've looked at everything else.

author: TimB100
author_rank: Contributor
timestamp: October 22, 2016, 08:35:45 pm
content: I cannot inject surfactants continually into the feed.If I could I would find a way to contaminate the feed to make it more conductive and solve my issues using my existing probes.Surface tension is an issue but not insurmountable. Just have to remember its there.