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I am looking for a "cheap" => Less than $60 way to measure the amount of fog. It doesn't have to be an exact calculation.

I basically need to be able to differentiate between different amounts of fog.

Now I was wondering if a particle / dust sensor would do the job in my case: https://publiclab.org/notes/donblair/05-03-2014/how-do-we-measure-particulate-stuff-in-the-air or https://www.sgbotic.com/products/datasheets/sensors/datasheet-SM-PWM-01C.pdf

It seems like the perfect solution, but I'm not sure whether it filters out fog particles because they're not big enough.

The second datasheet states a minimum particle size of 1 um.

Now by googling "Fog particle size" it says "Most typical fog machines create a fog particle size of 1-5 micron."

That should be detected by the sensor module, but are real fog particles of the same size? And I assume these particles are only that big when the fog is really thick?

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  • \$\begingroup\$ The dust sensors you're referring to are optical sensors which detect reflected (by the particles) infrared light. Now it happens that very cheap smoke alarms also use these sensors. I doubt that a smoke alarm will detect fog and likewise I also doubt that these dust sensors will detect it either. Fog is much lower in particle density than smoke or dust. But you could just get a cheap optical smoke alarm and test it. \$\endgroup\$ – Bimpelrekkie Oct 17 '17 at 12:35
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    \$\begingroup\$ According to the info available here, the size of fog particles is usually 5-50 µm. I hope this helps. \$\endgroup\$ – Marius Oct 22 '18 at 10:54
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A simple, reliable method measuring fog is to measure the stray light created by it. Use a blue LED with focusing optics, tubing to filter out stray light from other sources, a roughly 1m distance between openings and an array of photodiodes at various distances from the center as the receiver.

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As @Bimpelrekkie says, smoke alarms typically tend to deal with reflected IR light. It seems like what you want it to detect how much the fog is obscuring normal light. For this, it's fairly easy to just build your own sensor. Simply have an LDR facing a visible-spectrum LED in a box with holes in it, and measure the output of the LDR. As more fog blocks the sensor, the resistance will go up. Converting this resistance into something you can use (whether that's just an analog voltage, or a full display of some sort of percentage) is also pretty easy, but will require a microcontroller. This page may help you. The total cost will be under $40 (under $10 if you use a non-official arduino).

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