Measuring Air Oxygen Levels with a Lambda Sensor? - Page 1
Forum section: electronics / projects

author: StuartA
author_rank: Regular Contributor
timestamp: November 03, 2020, 01:20:09 am
content: The original Lambda sensors couldn't be used to measure O2 in air (which is to say, in a place where O2 levels could be low, e.g. a confined working space).I wonder if the more modern 5-wire "broadband" sensors could form the basis of a "DIY" O2 sensor?Two of the connections go to a platinum heater wire, and the current through that is used to get the sensor element to about 700'C, so they don't need the heat of the exhaust gas.Then there is a ground connection and two other wires but I'm not sufficiently knowledgeable to really understand what they do and how to get a signal out of them on to a DMM.Take a look athttps://www.tiepie-automotive.com/en/articles/oxygen-sensor-broadband. I guess that even then, they probably don't work at anything other than very low O2 levels?Yes, this is me being cheap by not wanting to spend $80 plus shipping to the UK on a real O2 sensor

author: m3vuv
author_rank: Super Contributor
timestamp: November 03, 2020, 06:55:47 am
content: this may give you an idea where to start,with the 4 wire zirconia types,car o2 sensor wires are mostly as follows:heater+2x white wires,ie 12v supply to them,black=signal out,normaly switches at 1hz if i remember,i cant remember the ground wire colour but the body of the sensor,will be at ground anyway,im talking about car sensors here !,you normaly can expect to see a voltage of 0.6/0,8v,think 0,6 was for an afr of 14.7-1 if i recall,best looked at with a scope,regards m3vuv.

author: mck1117
author_rank: Contributor
timestamp: November 03, 2020, 07:51:37 am
content: They do work all the way to free air (~21% oxygen).I'm working on a project to build an open source controller for the Bosch LSU 4.9 and LSU ADV sensors here:https://rusefi.com/forum/viewtopic.php?t=1856They really are quite complicated, and it's essentially impossible to get one working (including the sensor) for sub-$80.

author: ace1903
author_rank: Regular Contributor
timestamp: November 03, 2020, 08:28:06 am
content: In confined space it is CO2 and other chemicals that will make one unconscious I believe(much faster than lack of oxygen).It is far more effective to detect increase of CO2 to more than 1%(or some better chosen value) than to detect drop of oxygen to some low level.In cars lambda sensors are heated to working temperature by flue gases.If you use them in regular air you will need to heat them all the time, so those 80$ will be far more cheaper on long run.Additionally moisture from breath will affect the reading also. Fire risk to have something glowing red next to you in confined space

author: mck1117
author_rank: Contributor
timestamp: November 03, 2020, 10:02:51 am
content: Quote from: ace1903 on November 03, 2020, 08:28:06 amThat's not quite true - wideband sensors have the gain of the pump cell altered by their temperature, so they're designed to run hot enough that they always require heating, even when in the exhaust stream. Since there's no way to cool the sensor, if you want consistent (ie, closed loop) temperature control you have to run hot enough that the heater is required at all times.Narrowband sensors (aka just indicate if lambda is > or < 1) don't require temperature regulation, since the gain doesn't matter so long as the offset is consistent with temperature (which it is).

author: ace1903
author_rank: Regular Contributor
timestamp: November 03, 2020, 10:34:38 am
content: I agree to technical details about heating. My point is that battery that will be used for heating will have quite big size which is drawback when working in confined space.Oxygen sensor for medical use are like battery and are kind of "self powered".

author: richard.cs
author_rank: Super Contributor
timestamp: November 03, 2020, 10:37:19 am
content: Quote from: ace1903 on November 03, 2020, 08:28:06 amPerhaps, but humans can detect high-CO2 and they can't detect low O2. Feeling out of breath is triggered by high CO2 not lack of O2, this is why inert gasses that displace O2 are so dangerous.

author: Circlotron
author_rank: Super Contributor
timestamp: November 03, 2020, 10:44:26 am
content: From the little I know about automotive EGO sensors, one side had to be exposed to the gases under test and the other side to atmosphere. If the whole thing is surrounded by the gases under test I don't know whether it would work.

author: ConKbot
author_rank: Super Contributor
timestamp: November 03, 2020, 03:04:28 pm
content: Quote from: ace1903 on November 03, 2020, 08:28:06 amhttps://officerofthewatch.com/2013/04/26/fatal-accident-during-inspection-of-chain-locker/and this isn't unique either.For confined spaces, O2 levels very much are a concern. For a simple confined space (No other hazardous things) O2 and CO2 levels are thevmain thing to watch. If other hazards exist, obviously they need to be monitored or controlled too. I.e. Monitoring hydrogen sulfide in a chain locker may be a bit silly, but flammable gas monitoring probably wouldn't hurt, as there are flammables on a ship in general.

author: jogri
author_rank: Frequent Contributor
timestamp: November 03, 2020, 04:14:05 pm
content: Quote from: CJay on November 03, 2020, 10:55:39 am"Cheap" and "electrochemical O2 cell" (aka Clark electrode) do not go together... Clark cells need expensive membranes, platin electrodes and constant care to function while zirconium sensors are nearly maintenance free. They don't even require large amounts of current to function, the ones in my lab get powered via PoE. Btw, you don't need large amounts of air and your reference doesn't have to be air as it doesn't get used up->only one gas port needed.

author: StuartA
author_rank: Regular Contributor
timestamp: November 04, 2020, 12:28:40 am
content: Thanks all for the comments. In hindsight, I should have spelt out what my project is. We've all learned by now that anyone badly affected by Covid needs oxygen. I didn't know it until recently, but people with severe respiratory problems canalreadybuy domestic "oxygen concentrators"... but at a price.The underlying technology is very old and simple; pass air through a bed containing particular types of molecular sieves which adsorb nitrogen and you can produce a gas steam enriched up to 90% oxygen. Attached is a 1985 USAF report which is a good place to get started for anyone interested. The internet will findhundredsof papers on this.What I'd like is a cheap O2indicatorthat would work over the range 20-90%; it is only needed for setting up purposes, so if it needs a car battery to run it, that's OK. I could live with 10% accuracy.So, any chance a broadband Lambda will do it, or what else might work? I know from previous experience that the rate at which a candle burns in N2/O2 mixtures will tell you a lot (astonishingly fast in pure O2, remember Apollo?), but even I want something better than that! Thanks.

author: Wolfram
author_rank: Frequent Contributor
timestamp: November 04, 2020, 01:24:06 am
content: There are existing products for evaluating the performance of medical PSA concentrators, check this one out for example:https://highvolumeoxygen.com/product/handheld-ultrasonic-oxygen-purity-tester/According to the website the measurement is ultrasonic, so this could be an option to look into.Edit: There are a range of ultrasonic oxygen sensors available on eBay and aliexpress, ranging from suspiciously cheaphttps://www.aliexpress.com/item/4001240750129.htmlto reasonablehttps://www.aliexpress.com/item/4000846341634.html

author: StuartA
author_rank: Regular Contributor
timestamp: November 04, 2020, 03:11:48 am
content: Quote from: Wolfram on November 04, 2020, 01:24:06 amOh, thanks for that info - those are in my price range. I must try find out they work - just out of curiosity, but ordering one or two seems to be on!

author: StuartA
author_rank: Regular Contributor
timestamp: November 04, 2020, 04:40:04 am
content: It looks as though the really cheap modules have outputs via UART TTL and that does not sound like it's for for me. The more expensive ones ($38) come with a board with an LCD display showing %O2 and flow rate. The work by measuring the speed of sound in the mixture.

author: richard.cs
author_rank: Super Contributor
timestamp: November 04, 2020, 09:57:19 am
content: Quote from: StuartA on November 04, 2020, 12:28:40 amI've got one that I use to supply a small welding/brazing torch. It's a medical one that's no longer certified for medical use.Quote from: StuartA on November 04, 2020, 12:28:40 amSo you need to make a measurement once per setup? Does it need to be electronic at all? How about some secondary school chemistry, trap a sample in a graduated tube over water, remove the oxygen to a solid oxide (burn a metal inside, e.g. electrically heated iron wire, magnesium ribbon, whatever), and measure the reduction in volume to get the fraction of O2 in the gas. The nice thing is that you know the contaminants are just the other atmospheric gasses so simple chemistry works. You might be able to come up with something a bit less clunky, maybe burn and weigh the oxide to get rid of all the faffing with liquids.Note that the concentrators work by selectively removing nitrogen, not by separating the oxygen, so the output is enriched in argon compared to normal air.

author: StuartA
author_rank: Regular Contributor
timestamp: November 04, 2020, 04:46:40 pm
content: Quote from: richard.cs on November 04, 2020, 09:57:19 amYes, I'm aware that N2 is adsorbed on the sieve (and eventually vented as "waste") but argon and oxygen go through the bed.

author: CaptDon
author_rank: Super Contributor
timestamp: November 04, 2020, 06:49:55 pm
content: Just go with electrochemical like everybody uses in scuba diving to test theO2 level of their Nitrox mix. The very same sensor is used on Draeger andOhmeda anesthesia machines in the hospitals. I have years of experiencewith them. They work fine, they don't need temperature compensation acrosssensible room temperatures 60f to 85f. And they last for over a year normally.They work like a battery and the higher the oxygen concentration the fasterthey wear out. At 90% they may last 3 to 4 months, at 21% perhaps 1.5 yearsand that is with a steady gas flow across them. If you seal them up they consumethe oxygen within them and them sort of 'sleep' and will last for years beinginert until you unseal them for use. This is how they are stored for purchase.I think they can be had for under $40 each. Some use an 1/8" mono mini jackand they simply put out a voltage which goes higher with higher O2 levels.They do require a high input impedance buffer like a J-Fet opamp.

author: StuartA
author_rank: Regular Contributor
timestamp: November 04, 2020, 07:04:41 pm
content: The cheapest O2 electrochemical sensor I've seen was over $80 and that would probably have another $20 on top for the shipping to the UK. That's not "out of the park", but if you can recall where the $40 units are, that would be good.Back to using the possibility of Lambda sensors working at ~90% O2??? I am a chemist who dabbles in radio/electronics. I do have a number of oscilloscopes, but in my dreams, I fancied that I could just measure mV (or mA) coming out of the Lambda, possibly via an op-amp?The measurements don't need to go on for long periods; just brief checks to see roughly what O2 is coming out of the concentrator over a period of ~20 seconds, obviously, having let the heater get to temperature before starting.The Aliexpress ones with LCD look interesting, but will take time to get here; I can find a wideband Lambda on ebay quick and cheap.Maybe, measuring the burning rate of a candle is not so bad after all, observe it with my existing light meter

author: LaserSteve
author_rank: Super Contributor
timestamp: November 04, 2020, 07:06:40 pm
content: If their optical O2 sensors work as well as their tough, accurate, CO2 sensors, this is a charm:https://www.co2meter.com/collections/sensorshttps://youtu.be/S65uSKH6YQcI have a few of their CO2 sensors running in the graduate school incubators, a tough, humid, environment, and we love themWe have a Tr250Z thermal sensor as well, and it has also held up in one of the incubators, as I needed to control N2, Co2, H2O, and O2 partial pressures on that project.You might be able to get the sensor direct from LuminiOx, cheaper. But you would still have to build the serial adapter board, which makes Co2Meter's price very economical.Steve

author: StuartA
author_rank: Regular Contributor
timestamp: November 04, 2020, 07:47:34 pm
content: Quote from: LaserSteve on November 04, 2020, 07:06:40 pmInteresting; they have re-invented non-dispersive infra-red spectroscopy, which I thought had died decades ago. But, have you seen thedifferencein price between the O2 sensor which is 0-25% and the one that is 0-100%???I've just bought a used 5 wire lambda sensor on ebay ($12).

author: mck1117
author_rank: Contributor
timestamp: November 06, 2020, 10:58:40 am
content: Quote from: StuartA on November 04, 2020, 07:04:41 pmA Bosch LSU sensor will not be able to measure 90% O2. Bosch only specifies the sensor up to free air - 21% or whatever. Extending the characteristic curve out all the way to 100% O2 would require more pump current than the maximum allowed by the datasheet.Quote from: StuartA on November 04, 2020, 07:04:41 pmYou can't just measure an output of the sensor - it requires a control loop to modulate the pump current such that the output voltage is half scale (~0.45v). Bosch then provides a lookup table to convert from pump current (current provided by the controller, not the sensor) to lambda or oxygen %.

author: ace1903
author_rank: Regular Contributor
timestamp: November 06, 2020, 12:12:38 pm
content: First post was edited several times. From detecting low concentration now talking about measuring up to 90%.Now we are entering equally dangerous zone. 90% oxygen is serious fire hazard. I believe that that requires serious system engineering.Starting from valve types ending with tubing. Bad tubing can produce static electricity, sparks and fire.Check in history what happened with space missions with purely oxygen environment.For medical purposes gas mixture is tightly controlled and I believe far from 90% oxygen.Second thing is that with molecular sieves probably it is impossible to reach 90% purity.Third thing is that one will use oily lubricated compressor because it is cheaper. Oil and enriched oxygen recipe for disaster.This starts to look like initial DIY respirator hype. Trying to save 80$ and human life at same time does not go hand in hand.

author: Wolfram
author_rank: Frequent Contributor
timestamp: November 06, 2020, 01:05:03 pm
content: PSA based medical oxygen concentrators usually reach 95 % or higher O2 concentration at 60 % of the rated oxygen flow rate, if the sieve beds are in good condition.I agree that a 700 degree sensor near pure oxygen can be a real hazard, I've personally managed to set fire to the inside of a hose with flowing oxygen and it turned into a jet engine almost immediately.Measuring the oxygen concentration from PSA concentrators is a solved problem, and most solutions are based on ultrasonic sensing. Getting off-the-shelf sensors to work even if you have no experience with UART communication, is significantly simpler than trying to shoehorn an automotive lambda sonde into an application far removed from its designed purpose. Using a simple Arduino and some widely available serial communication example code, this is a quick problem to solve. And once you have the readings from the unit, you can convert them to an analog voltage, transmit them or display them as required.

author: StuartA
author_rank: Regular Contributor
timestamp: November 06, 2020, 03:58:31 pm
content: I don't refute any of your comments about the issues, but I see this as thinking about building a "Kart" before moving on to building a "Formula One car".As for oxidisers, I've worked for many years with pure O2, dinitrogen tetroxide (really very toxic and corrosive) and 98% hydrogen peroxide (really very unstable in the vapour phase), which are of course, rocket propellants, together with their associated fuels. A concentrator configuration I which might try in my garage (or back yard) with suitable PPE is not necessarily a configuration I would lend to my grandmother. She might get the Mk X version.I'd never heard of UART until a few days ago and I've never been within a hundred miles of an Arduino. The last time I did any programming was back in 1986 (a Commodore PET with 4K of memory). The ultrasonic modules on Aliexpress,with LCD display, look very interesting, but are 20-40 days delivery. Also, some of the feedback about "budget" oxygen sensors suggest that people have often been disappointed with them. To the extent that I'd like to use a sensor, maybe hiring a good a one would be a good way to go.So, the solenoid valves arrived yesterday, I already have some sieve (we used to use it for taking water out of N2O4) and a stainless steel column and lots of other bits and pieces, so it's getting closer...

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 06, 2020, 04:49:37 pm
content: Quote from: ace1903 on November 03, 2020, 08:28:06 amUnless the OP was addressing VERY specific environments, then I do agree with this. More effective to detect the amount of CO2 and possibly other gas like CO.