non contact 'wet or dry' surface detection

Other Hardware Sensors
1

For a project I am working on I need to measure surfaces if they are wet or dry without to contact them.

At this moment the detection goes by measuring the resistance by contacting the surface connected to an analoge input.
The problem is the surface gets polluted (dry gets wet), carved and the sensor is wearing out by constant sliding the (sometimes ruff) surface.
The arduino only need to know witch section of the surfaces are wet or dry, no percentage.

The surfaces have different colours and reflections and are made of brik, stone, wood and dry wall.
The measuring distance to the surface stays approximately the same with a minimum of 3 to 5 mm.

There are a lot of sensor options but i don't know which one is the best/cheap, where i can find them (or how to make them myself), how to connect and read them with the arduino.

I was thinking electrical impedance is the best for the job.

but also:

-ir
-microwave
-...

Thanks for thinking.

2

Please give more information on what the usage of this task is. How is it setup? Is it inside or outside? How does it get wet? Why do you need to know if it is wet? Some general background often gets the brain cells working.

Weedpharma

3

Fire a laser at the surface then use an infrared spectrophotometer to detect the water vapour.

4

Maybe NIR? Control Engineering January 2016-CE

5

weedpharma:
Please give more information on what the usage of this task is. How is it setup? Is it inside or outside? How does it get wet? Why do you need to know if it is wet? Some general background often gets the brain cells working.

Weedpharma

the usage will be to detect/map witch part of a fresh painted surface (water based paint) is already dry or not yet been painted. At this moment its only for inside use in normal light conditions. The sensor moves fixed at 3 to 50mm above the surface with a variable speed from 0 to 40cm/ sec.

  • i think laser is probably to slow and will damage the surfaces ? (waiting for vapor, burns )
  • NIR seems to be ok but where can i find a separate small cheap sensor?
  • What about electrical impedance?

thanks for thinking,

6

Oh, you need it to be cheap....

If the conditions are very specific (only one type of substrate, only one kind of paint, one color, etc.) you might find something cheap. But for broad ranges of sensing distance, color, substrate, etc.; I doubt you will find something cheap.

You're already measuring impedance with your current analog set up.