Gas detection, DIY? - Page 1
Forum section: electronics / general

author: tmbinc
author_rank: Frequent Contributor
timestamp: May 22, 2025, 10:52:00 am
content: I assume this is a stupid idea but hear me out(tm):The idea is methane leak detection. Methane has two absorption peaks in the MWIR/LWIR band, one around 3.25um (so clearly not LWIR) and one around 7.2um to 8.2um. This partially overlaps with what e.g. FLIR typically publishes for their uncooled VOx LWIR sensors. The overlap is not great, but existing. My understanding is that this is how e.g. the GF77(a) works, a 320x240 camera in the $20k range.Typical gas cameras work by having a bandpass that covers the absorption spectrum, so you essentially see the gas as "dark clouds". Due to the generally small size of the band pass, there isn't a lot of signal so it needs a low noise sensor for real-time detection. The quoted NETD of 25mK, which I assume is the raw value (that isn't entirely meaningful for gas detection), matches that of the carefully binned Tau2+, so at this point I assume "it's really just a fancy VOx LWIR camera" with an optical filter.The bandpass filters themselves are unfortunately quite expensive, around $600 in the typical places.Now, how can we be cheaper?https://amt.copernicus.org/articles/15/1395/2022/has an excellent idea: Instead of imaging the absorption range directly, image a.) "everything" and b.) "everything else", and then subtract "everything else" from the "everything", leaving the absorption range. A filter for "everything else" is incredibly simple - just fill a gas cell with the target gas at a sufficient concentration. They are doing this for NO2 in the UV range, but can't we do this with CH4 in the ~8um range?In their example, they have two discrete cameras that are well-tuned, but I would imagine this may work better with a single camera and a moveable filter. Take two GaAs (or other material that's transparent at 8um) windows, glue them together to form a cavity, and replace the air with the target gas (CH4 in this case). Then, build a twin (just to match the optical performance as much as possible) and keep it with air. Build a small setup where these filters can be swapped into the front of a camera, and essentially build a "lock-in thermography" setup, where you synchronize with the filters. E.g. target a swap rate of, say, 1 Hz, as a trade-off between unusable time when the assembly is moving, scene movement, and pixel noise that can be averaged. (Combine swapping the filter with an FFC can use the dead time for better NUC.)The energy that will be removed is quite little, but for static scenes, the differences can be integrated (which, after all, is the magic of lock-in cameras). The detected difference in energy, as measured in mK, can be significantly smaller than the NETD of the camera.And finally, different to a regular gas camera setup, we also have the "everything" frames; because the passband (after subtraction) will of course also contain all the blackbody radiation, we can remove (some of) this by extrapolating from the regular image - thereby amplifying the energy difference due to gas absorption.Is it worth trying?

author: DaJMasta
author_rank: Super Contributor
timestamp: May 23, 2025, 02:16:54 pm
content: How badly do you need to detect methane at a distance?I don't think fundamentally it can't work, but how practical would a <1 fps low dynamic range inspection camera be? Would it be worth the cost of two matching cameras, a filter switcher, and the filter itself?These pricey narrowband gas detection filters really do give you the kind of few-nanometer FWHM performance to pretty reliably block out most outside-of-band illumination, so using the gas itself as a filter would be blocking out generally only a tiny portion of the spectrum visible to the sensor. If you've got 14 bits worth of readout data and a minimum of 1 LSB of noise in the readout (and it's likely going to be higher even with well matched cores at stable temperature and with a good NUC), how much change you can expect for the high concentration filter could you expect - maybe 4 or 5 LSB in the scene? If so, that would mean your minimum detection limit for your target could be on the order of 5-10% percent, which is only going to give you a clear detection pretty close to the source and without much wind. There's a little absorption from water in this band too, so under humid conditions it could be worse.The cost also isn't just the gas itself, but a gas cell sufficiently transparent in the band of interest. An FTIR gas cell using probably KBr would work, but unless you're buying used it's easily as much as the narrow band filter even before the second core & lens and the physical switching mechanism.So it could be an interesting experiment, and the advantage would be that you could swap filter gasses once everything was going and get more flexibility, but for site inspection I don't think it's quick (or small) enough to compete with the pricey commercial options, and if you know your gas of interest and there aren't multiple, it's probably a good bit cheaper just to use a single core and get the filter.

author: bap2703
author_rank: Regular Contributor
timestamp: May 31, 2025, 02:26:34 am
content: Quote from: tmbinc on May 22, 2025, 10:52:00 amI have a hard time seeing the whole2 cam - 1 gas cellbeing cheaper than a filter.And the practicality of the setup will be abysmal.You could also build a FTIR while you are at it

author: vinnieboombatz
author_rank: Newbie
timestamp: August 10, 2025, 01:12:30 am
content: I have a project that I would like to try to visually see isoprene, outdoors, at a distance of anywhere between say, 1 foot to 200 feet. But, I don't have a lot of money. I thought of buying something like this attachment for my smartphone and then placing some kind of filter in front of the camera lens to filter just for the wavelength spectrum for isoprene:https://www.flir.com/products/flir-one-gen-3/?vertical=condition+monitoring&segment=solutions&srsltid=AfmBOorm8fOrriZkvNFwGP072NclRj8oplkbALqnmV2NwWoAs4g9tCQOBut, looking at the specs, I don't think(?) it has a wide enough spectrum to cover isoprene.Does anyone have any lower cost ideas on how I can visually see isoprene without spending huge amounts of money? I can't afford something like a FLIR optical gas imaging camera.BTW, I don't have any experience in gas or gas imaging - I just had an idea for an experiment that I would like to try if it does not cost too too much money.Thanks a ton in advance for any ideas you may have!

author: Fraser
author_rank: Super Contributor
timestamp: August 10, 2025, 01:10:56 pm
content: Sadly inexpensive DIY gas detection using thermal imaging is one of those topics where I would say, “if it could be done cheaply, with simple equipment, it would already be a commercial product”. Gas detection using thermal imaging is a specialist application that requires high performance thermal imaging cameras and very expensive filters. It is not an area that may be easily explored on a hobby budget or even a decent budget as the cooled cameras that are required are built for gas detection at all stages of development and not just a modified “standard” camera. Adding an expensive filter to a decent quality standard cooled camera might have a chance of working but the camera would lack the image processing modes of a gas detection camera that are designed to pull out the detail of the scene to expose the presence of a gas cloud or leak. Remember, these cameras are designed to detect small leaks as well as large leaks so gas cloud concentration/dispersal is part of the imaging challenge. Gas detection using thermography remains a specialist use for which specialist, better than average, thermal imaging equipment is required and that gets expensive.The uncooled LWIR FLIR One G3 would sadly be useless in this application. You could look, for a used, working, cooled thermal camera of decent sensitivity, and invest in a suitable filter for the gas that you are trying to detect, but the filter is a specialist product, so very expensive. That is the only DIY approach that I can see being practical and actually working with some reliability. It would still need development and likely a host computer running image processing to pull out the gas cloud detail from the image.Fraser

author: vinnieboombatz
author_rank: Newbie
timestamp: August 10, 2025, 03:00:26 pm
content: Fraser,Thanks a lot for the detailed and well though out response. Thanks for telling me "like it is" and your explanation about what my realistic possible choices are here. I have looked a little bit online and see that you can rent an optical gas imaging camera, but the prices even for a day rental were not cheap. The few used cameras that I found sold online were not cheap either. Maybe if I ask around, I can find someone local around here who owns one and would be willing to rent it to me a for a few hours one day for not too much money. We'll see what I can come up with...

author: Fraser
author_rank: Super Contributor
timestamp: August 10, 2025, 03:13:21 pm
content: Renting such specialist kit does make sense if it is only needed occasionally. Cooled cameras are expensive and their coolers need maintenance based on hours of use that is also very expensive, hence the high rental prices.Fraser

author: LaserSteve
author_rank: Super Contributor
timestamp: August 22, 2025, 04:13:55 pm
content: 3.39 um Helium Neon lasers have an overlap on a strong adsorption line of methane. So strong that it was once used to frequency lock the laser with an intracavity methane cell and then tuning the cavity length, a process known as Pound-Drever-Hall. That was used as a wavelength standard and almost used for defining the meter.The 3.39 um HENE line has such a strong gain that it is known to lase with one mirror on the tube and the reflection off of a fingernail. That kind of gain is generally unheard of in lasers. The beam is of small diameter and really low divergence.If your hunting Methane, commercial 3.39 um sealed mirror gas lasers exist.I used to have one with dual 3.39 um and red emission at 632.8 nmSynthetic Sapphire and Fused Silica have been used at 3.39 um as Windows on the tubes that have external cavity mirrors. In fact the lasing transitions of 3.39 and 632 sharean upper state transition, and to suppress 3.39 from dominating the lasing process, and thus nulling out 632.8, they had to switch from fused silica or quartz to glass windows on the external mirror tubes to allow 632.8 to have gain by adsorbing 3.39 um.I have some older anesthesia analyzer cells that use uncoated sapphire from 2.5 to 5.5 um.Gold coated front surface mirrors work fine in this region.So, the optics are not all that expensive.I know of four techniques.1. Scan the beam with a pair of galvo scanners and look at the backscatter or view the scanned image on a camera. The gas cloud shows up as darker then the background.2. Aim the beam on a photodetector a long ways away, and measure the adsorption either CW or using a lock in amplifier and beam chopper for the ultimate precision. (You can aim the beam at a mirror and bounce it back for twice the path length)3. Use the laser as a pointer and watch with the camera. You'll see dust scatter that will stop when it hits a methane cloud. (Really, Really, dependent on atmospheric conditions, and finding large dust particles.)4. Herriot Cell, bouncing the beam between curved mirrors for anywhere from 30 to several thousand bounces, mainly used on a bench.Pacific Lasertec and REO/PMS/EG&G, whomever they are today, make 3.39 um compact lasers.So does "Plasma" in Russia.There are/were commercial gas monitors based on this method.Steve

author: SYOM
author_rank: Newbie
timestamp: September 03, 2025, 08:08:27 am
content: I’ve tried a couple of these FPV thermal camera builds too — they’re a great way to experiment without adding much weight. If anyone’s on a budget, the Thermal Master P3 is actually one of the more affordable USB thermal dongles I’ve found that still gives decent resolution (512×384 with the X3 enhancement). It stays under 30 g with a tiny USB host board, so it’s very flyable.

author: daisizhou
author_rank: Super Contributor
timestamp: September 03, 2025, 11:49:04 am
content: I have a Dräger gas chamber sensor for detecting end-tidal CO2 and anesthetic gases. If anyone is interested I will disassemble it and post it here.Its name is ILIA or ILIA2. The principle is to generate infrared rays through a heating wire, pass through an air chamber, and the photoelectric sensor receives the concentration value.