PROBLEM SOLVED - Protective cover for 5.5 to 14um

[Phil1977]

Hi,

does anyone have experience what can be used as a FIR-window to seal the housing of a contactless thermometer?

Following project: A friend asked me if I can build him a thermometer that displays the surface temperature of the tires of his race car. A GY-906 BCC from Melexis seems to be the perfect choice, it´s small enough for the wheel housing, it has an appropriate detection angle and black rubber should usually be very well suited for pyrometry.

The main issue is: Even if the sensor does not need to function if it´s wet, it should survive the conditions next to the wheel. I definitely need some window to protect it.

I had the following ideas:
- Thin PE window: Cheap and easy to replace, but not very transmissive and very prone to dirt.
- ZnSe-window: Highly transmissive, but more expensive and very easy to scratch
- Ge-window: Much to expensive, also easy to scratch
- "Periscope" of two gold coated glasses: No hermetic seal, but easy to replace

Do you know any other promising material or setup?

[Fraser]

I had a thermal camera from an F1 car a while ago. The camera was used to monitor tyre temperatures. I think I still have the tablet PC and software that was connected to it, but not the camera. It was ruggedised and used a Germanium window. In F1 cost is less of an issue, yet the physics of the situation remains. Ducted air flow is used to keep the lens clear of debris during use. In industry it is common to use disposable lens protectors made from Polyolefin film. The film is kept very thin (10 Microns), to reduce transmission losses, but it is fragile as a result. The alternative is a non-disposable lens protector that uses Germanium, ZnSe or thin silicon to provide a more robust protective barrier. Germanium and ZnSe coated windows are not that expensive from China, especially when small diameters are involved. Automotive night vision thermal cameras (Autoliv) also use Germanium windows to protect the lens assembly but those windows are heated and expensive unless found in a scrap yard on a night vision equipped BMW or Audi.

Making a thermal imaging sensor water and ballistic resistant whilst maintaining measurement accuracy can be a challenge and tends to cost money to do well. Note that the F1 thermal camera had a frame update rate of 120 frames per second and the associated downloadable data logger held masses of data points for later analysis. It was a pretty comprehensive tyre monitoring system that actually showed the temperature gradient across the tyres surface.

Fraser

[Fraser]

You might find this short article interesting……

https://www.renvale.com/texense-tyre-temperature-sensor-wins-award

Fraser

[langwadt]

IP65 sufficient? https://www.digikey.dk/en/products/detail/dfrobot/SEN0256/10279716

[Phil1977]

IP65 sufficient? https://www.digikey.dk/en/products/detail/dfrobot/SEN0256/10279716

Sounds promising, but somehow I don't trust the IP-rating. They just write the housing is made of "mental" but don't  specify anything for the IR-window. I suppose it´s a recessed window out of ZnSe and if you get dirt into this zone the accuracy will completely vanish...

In industry it is common to use disposable lens protectors made from Polyolefin film. The film is kept very thin (10 Microns), to reduce transmission losses, but it is fragile as a result. The alternative is a non-disposable lens protector that uses Germanium, ZnSe or thin silicon to provide a more robust protective barrier. Germanium and ZnSe coated windows are not that expensive from China, especially when small diameters are involved. Automotive night vision thermal cameras (Autoliv) also use Germanium windows to protect the lens assembly but those windows are heated and expensive unless found in a scrap yard on a night vision equipped BMW or Audi.

The hint for the polyolefin film is great! I just checked it, even mundane wrapping film from the kitchen is quite transparent. I think I use a Ge or ZnSe window from China with together with a disposable polyolefin film on top. My unit won't ever get near to a formula one car so it seems more than sufficient. Thank you!

[Vipitis]

The lepton based FLIR one and CAT phones have this ~11mm Window that is hard carbon coated. It might be Ge. Maybe you can buy a few of those from broken/parts units on eBay?

[ArsenioDev]

Raw silicon also works nicely if you can get large DSP (Double side polished) wafers.

[Phil1977]

Thanks again for all the ideas. As mentioned, the wrapping foil seems to be the solution to the dirt problem.

Thankfully the inside of the wheel house shows where the aerodynamics blow most and least dirt. We just place the sensor where the least dirt is in the wheelhouse, and then we put a wrapping foil over a ZnSe window. Probably the sensor will not work reliably when it is wet, but that´s okay.

The melexis sensor is around €10, the ZnSe-window around €6 and the wrapping foil nearly €0 - so it´s no problem if once in a time something gets destroyed.

[CRC]

I got a box of like 20 ZnSe windows about 12mm diameter for only a few dollars on ebay. I epoxied one into a 3D printed housing made of ASA for a vehicle mounted camera. It works fine.