Waterproofing thermopile - Page 1
Forum section: electronics / general

author: tszaboo
author_rank: Super Contributor
timestamp: October 17, 2019, 10:01:32 am
content: I have a sensor project in mind. It is a thermopile sensor, that is monitoring a temperature of an object, placed about 300 mm from each other. I've used thermopiles before, however that was an indoor application, so the sensor was just mounted on the PCB, and it was sticking out of the enclosure. Sensor was like this:The application I have in mind is outdoors. It would be cleaned by a pressure washer sometimes. So how do I make this waterproof? IP67 or IP69K. I guess I have to place a window before it, and the glass in that must be germanium? What materials can I use to not filter out the IR spectrum? The temperature can be anywhere between -20C and 200C. Do you know any suppliers that would carry such glass windows?

author: bap2703
author_rank: Regular Contributor
timestamp: October 17, 2019, 11:42:02 am
content: That sounds more complex than just waterproofing.The temperature range is that of the object you want to measure?If it's the range that the thermopile must withstand then the problem is even more complex.

author: Ultrapurple
author_rank: Super Contributor
timestamp: October 17, 2019, 11:56:49 am
content: There are certainly options for IR-transmissive windows (FLIR make a line of them for things like inspecting high voltage equipment through the cabinet) and I was surprised just how good they were when I picked one up for pocket money the other month. I'm reasonably certain that this, or a variant, would withstand pressure-washing. Search for FLIR IR Window. But be aware they're not cheap.I think we need to know a little more about what you're trying to achieve before we can give any significant advice. You won't want to put a US$500 window on something you want to be a $10 consumer device...

author: wraper
author_rank: Unknown
timestamp: October 17, 2019, 12:05:40 pm
content: IR thermometer (gun type) Fresnel lenses are made out of of plastichttp://www.fresneltech.com/pdf/POLYIR.pdf. You can also use GaAs or ZnSe. You don't need anything fancy like Germanium unless you need high resolution like in IR cameras.

author: Fraser
author_rank: Super Contributor
timestamp: October 17, 2019, 12:06:09 pm
content: As has already been suggested, a Thermography inspection window is a good option provided it is one rated for exterior use. I mention this as it dictates the window material used. Some will deteriorate quickly in outdoors conditions.Another, likely more affordable option is the lens protection Germanium window from a Fire Fighting thermal camera. This would need to be mounted in a watertight fitting but offers the advantage of having A hard carbon coating on its exterior surface that is VERY durable. Such Germanium Windows also have the required AR coating applied for excellent transmission characteristics. Another advantage is that most are around 1” in diameter so easily mounted using common 25mm photographic lens holder ringsI have seen a whole pile of these Germanium Windows for sale on eBay recently. I will have a look for then in a minute. They were something like $70 each, which is very cheap compared to the retail prices on these.You will need to use a Longwave (7-12um) sensor if using such a window as that is what the AR coating passband for the Microbolometer and BST cameras used in fire fighting.Fraser

author: Fraser
author_rank: Super Contributor
timestamp: October 17, 2019, 12:10:39 pm
content: Look for a Germanium Window on eBay.com. Make sure it is AR coated and for the LW band or whatever suits your sensor wavelength.Less than $25 for this one that looks a sensible size for your needs and it is LW AR coated. Whether it is Hard Carbon coated is unknown. Germanium corrodes in outdoor weather so a Hard Carbon coated type is recommended.https://www.ebay.com/itm/ISP-Optics-12-7mm-1-2-AR-Coated-Thermal-Imaging-LWIR-8-12-m-Germanium-Ge-Window/264165780779?hash=item3d8181ed2b:g:n9EAAOSwhzhcTR-bFraser

author: Fraser
author_rank: Super Contributor
timestamp: October 17, 2019, 12:25:59 pm
content: More window auctions.......https://www.ebay.com/itm/Axsys-IR-Systems-Germanium-Infrared-Window-8-13-microns/390041148740?hash=item5ad0439d44:g:W38AAOxy-1lRHp-Ehttps://www.ebay.com/itm/Optical-grade-USED-Chipped-Single-Crystal-Lens-Germanium-metal-disc-99-999-7-Gr/273850457562?_trkparms=aid%3D555018%26algo%3DPL.SIM%26ao%3D1%26asc%3D60257%26meid%3D2a11afe9148f4058b70de01263e7fb1d%26pid%3D100005%26rk%3D2%26rkt%3D12%26mehot%3Dpf%26sd%3D390041148740%26itm%3D273850457562%26pmt%3D1%26noa%3D0%26pg%3D2047675&_trksid=p2047675.c100005.m1851https://www.ebay.com/itm/Balzers-Germanium-Infrared-Window-Filter-8-11-5-microns/390105677272?hash=item5ad41c3dd8:g:vioAAOxyjPNRFWqJI still have not found the chap selling a pile of Germanium Windows but I will keep looking.For your application, some chipping or pitting should not effect the single pixel sensors performance. Calibration of the sensor needs to be carried out with the window in place as it will have a transmission loss, although very small.Fraser

author: tszaboo
author_rank: Super Contributor
timestamp: October 17, 2019, 12:26:37 pm
content: Quote from: wraper on October 17, 2019, 12:05:40 pmPlastic is definitely a better option than expensive germanium. This is definitely the direction I would head.The -20 to 200 is the measured temperature. The device's temperature is typical -20C to 60C worst case.I'm not looking a fancy high resolution thermal measurement.The application is Industry 4.0, smart metering, tens of thousands quantity. The sensor is something from TE Connectivity or others (still have to do the search for it). So cheap solutions are welcome. 70 dollar is out of the question, because in that case I will just wire a PT100 to the stuff measured. This is supposed to reduce the installation and the device cost, by eliminating the wiring.

author: tszaboo
author_rank: Super Contributor
timestamp: October 17, 2019, 12:28:19 pm
content: Quote from: Fraser on October 17, 2019, 12:25:59 pmI really appreciate the help. But I'm looking for something that is new.

author: Fraser
author_rank: Super Contributor
timestamp: October 17, 2019, 12:33:14 pm
content: Regarding plastic.......1. Only some plastics have decent transmission figures at LWIR2. The lower transmission loss plastics need to be relatively thin to achieve their low losses. This makes them fragile.3. Common Polyelofin thin film plastics are used as lens protectors but they are fragile.4. IR thermometers do commonly use plastic fresnel lenses so you could try that path, but they are not normally exterior rated plastic unless you go for the plastic used on common exterior PIR sensors. Some PIR sensors use a clever reflector array and a plain plastic window, as opposed to a fresnel lens array window. The plastic is thin and fragile however. It also does degrade with age and becomes brittle.For a long lived solution, a simple new or used Germanium Window that is hard carbon coated is the way to go. That is why such an approach is used in industryFraser

author: bap2703
author_rank: Regular Contributor
timestamp: October 17, 2019, 12:42:52 pm
content: Quote from: NANDBlog on October 17, 2019, 12:26:37 pmHave a look at motion sensors then. I guess you are just building a thermographic thermopile based motion sensor 😁

author: Fraser
author_rank: Super Contributor
timestamp: October 17, 2019, 12:48:50 pm
content: You could look at products that are already being marketed as thermal sensors for outdoor use. That could give you a steer on what weather protection approaches are already in use. I suspect you will find a lot of fresnel lens plastics and Germanium or Chalcogenide IR glass windows in use. Look at industrial non contact thermometers from the likes of Raytek and Omega. They offer all-weather housings for the sensors and these are very similar to what you are looking to achieve.You have not stated whether this is a ‘one-off’ project or something that you hope to make many of. Scale of production does effect the financially viable options available to you. Have you considered forms of protection beyond just the window ? I am thinking about the pressure washer scenario...... that is a tough one to cope with due to the pressure involved and potential for non trained operators applying the Lance too close to the sensor housing. You could use a sensor protection tube that would need the pressure washer to be deliberately aimed down the tube to contact the window. This has the serious disadvantage of potential visibility to the thermal sensor, insect ‘invasion’ and dirt collection though. I hope you see what I am trying to say though. You may need to consider how best to avoid direct blasting by a high pressure water jet by some means. Also, is there a need for a lens system to correctly illuminate the sensor with just the target that you wish to monitor. As you will know, the target must fill the sensors field of view if being used for measurement rather than just thermal movement detection.Fraser

author: tszaboo
author_rank: Super Contributor
timestamp: October 17, 2019, 01:00:45 pm
content: Quote from: Fraser on October 17, 2019, 12:48:50 pmTens of thousands. The device would be marketed as IP69K, which is pressure washer from 20cm distance. I know, it is a though requirement. At this point I'm just investigating whether it is possible to replace the PT100/PT1000 that we typically mount with an IR measurement.

author: Fraser
author_rank: Super Contributor
timestamp: October 17, 2019, 01:32:56 pm
content: Wow! Replacing a PT100/1000 with a IR non contact system !I hope temperature accuracy is not required ?Thermography, as a technology is relatively inaccurate with common measurement tolerances of +-2C or 2% of full range. The measurements are effected by changes in air quality and the emissivity of the target. Corrosion, dirt, plant growth (Moss) Rain and dew can become an issue !PT100 measurement systems are often specified where measurement accuracy is important as they cost more than Thermocouple or Thermistor solutions so such needs to be justified. You clearly have a plan in mind but please do investigate the number of measurement uncertainties involved with non contact IR temperature measurement.

author: tszaboo
author_rank: Super Contributor
timestamp: October 17, 2019, 02:27:06 pm
content: Quote from: Fraser on October 17, 2019, 01:32:56 pmSure, the measurement accuracy is worse. In fact we would like to use it as a differentiation to the product. This would be the cheaper one, which is easy to install, and not that accurate. The other is more expensive and more accurate. Market segmentation. The question is if we can actually get lenses that would work for a decade, and if it is actually cheaper to make.

author: Vipitis
author_rank: Frequent Contributor
timestamp: October 17, 2019, 02:46:32 pm
content: It may be worth a shot to ask lens suppliers for a custom product. As in a tiny window needed to fit your sensor and specs. Volume can keep the price down as they may have tons of small pieces laying around.For example my IP68 rated phone has a Window to protect the Lepton sensor below it. It's actually conical in shape and just less then 9mm in diameter at the wide end.

author: tszaboo
author_rank: Super Contributor
timestamp: October 18, 2019, 08:36:31 am
content: After some research, PPMA or acrylic is actually a material worth looking into. The other is HDPE.I've realised that germanium and silicon is used because it blocks visible light.I've also found this company worth looking into.http://www.kube.ch/plastics/index.php]http://www.kube.ch/plastics/index.php] http://www.kube.ch/plastics/index.phpIt seems like long term exposure is a problem for these plastics. I think I can overcome that by replacing the lens at the regular maintenance cycles.Thank you for the brainstorming, this was very helpful.