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MLX IR sensors for cloud detector

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#1 tomb18

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Posted 24 September 2021 - 06:49 PM

Hi

I have built a cloud sensor using the MLX IR temperature sensors.  However, I find that the field of view is very large.

There are other MLX sensors that have a much narrower field of view but I am wondering if they are waterproof as the regular one is.  They look like they have only a plastic baffle or something restricting field of view.  Thewy look like they can collect water.

Anyone have any experience?

Thanks

 

 

 



#2 tomb18

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Posted 24 September 2021 - 07:04 PM

Ok, I have now determined that they use a metal housing with a small hole that covers the standard MLX sensor.  How to block the hole which will collect water?  A germanium window?



#3 freestar8n

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Posted 24 September 2021 - 08:43 PM

I have used a variety of melexis sensors for clouds over the years.  I don't think any of them are intended to be exposed to weather directly - so I think they should be shielded somehow.

 

A thin sheet of plastic shouldn't lose much flux - and it's easy to test just by pointing the sensor at something warm or cold and seeing how much the measured value changes when you put the sheet in front of the sensor.  I just used plastic wrap.

 

I happen to be setting one up now and will try different forms of plastic to see if something more durable will work.

 

Frank



#4 tomb18

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Posted 24 September 2021 - 11:40 PM

Well the plastic should absorb the IR.  A germanium window in 1cm size costs around $30.

I've had the base model outdoors now for a couple of years with no ill effects.  The narrower FOV sensors though will fill up with water.  SInce they use the same sensor I guess that doesn't matter much. 

It will be interesting to see your results.

Tom



#5 freestar8n

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Posted 25 September 2021 - 03:11 AM

I think you can't go wrong buying a special window for IR, but keep in mind the window has two characteristics:  the inherent extinction coefficient of the material, and the thickness of the window.  Plastic wrap may have higher extinction coefficient, but at the same time you know it is very thin.  And if you mainly just want to know if it's clear or not - there is less need for an exact measurement of temperature.

 

So in the years I have been using these sensors, I either used them attached to the telescope and completely uncovered - or in dedicated setups, fully exposed to the environment but covered in plastic wrap.

 

As it happens, right now I am setting up a full spectrum measurement with multiple sensors from UV to IR - and I think plastic wrap will work for the whole range.  It doesn't pass UV-C but I hope I don't have to worry about that.

 

And with all these things, if you have the sensors it is very easy to try different materials just by passing them over the sensor and seeing if the signal changes dramatically.

 

Frank



#6 lambermo

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Posted 25 September 2021 - 09:41 AM

I have an Melexis MLX90614-BCC exposed to the weather for several years now and indeed as expected it collects rain and dust in the cavity and the signal is barely usable anymore. I added the BCC as an experiment next to a more regular BAA sensor.

 

skytemp_small_now-1d.png

 

The graph shows the difference of the two sensors. The BAA gets dirty too but can be cleaned at least, somewhat. Because the signal quality deteriorates over the years.

 

I have looked at proper band-pass filters for these sensors but did not find any with a 'reasonable' cost.

I might test the thin plastic foil that was mentioned for the next batch of sensors.

 

-- Hans



#7 tomb18

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Posted 25 September 2021 - 12:37 PM

Hi

That's interesting.  So it appears to be about 5 degrees different.

This is the window I was thinking about:

https://www.knightop...um-windows.html

 

Coating adds a lot to the cost.

Tom


Edited by tomb18, 25 September 2021 - 12:37 PM.


#8 tomb18

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Posted 25 September 2021 - 12:40 PM

Do you guys worry about dew on the sensor? I have a dew heater under it (resistor epoxied in place)  so it makes replacement impossible without destruction of the sensor.

Tom




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