I'm having a hard time picturing exactly how you mean Brad, but I think I will avoid any material which can absorb moisture.
I have thought that I would use a tube which I will refer to as the wind tunnel/tunnel, and mount the blades with three arms such as computer fans are constructed. I had also thought that maybe I can find blades from a small computer style fan, I have quite a few of those, with a diameter around 4cm.
If a sett of blades such as those could do the job then mounting magnets on the outer edge of the blades would be very easy, and all that is needed is magnets strong enough to clearly register on a magnetic sensor positioned on the outside of the tunnel wall which would be something like maybe 4mm away. The wall being around 2mm thick, once I have determined which blades to use I'll go to a store to look for plastic tubes.
Hoping that a air chamber that is simply a peace of plastic tube is going to display a linear relationship between flow and RPM.
I think the paper idea is a good one aside from the fact that there will be so much moisture in the air, I have begun to plan for buying a 3D-printer but that is still some distance into the future.
But it would be cool to design and print the tunnel with the necessary holes for mounting the laser and sensor as well as the armature for the turbine-blades.
Another problem I have, I am having a hard time using the tips from post #7 by SunnySkyguy.
I started searching for suggested search terms and found my self quickly getting to know that these kinds of sensors are made for a particular wave-length and as such I stopped looking at sensors and started to look for a laser, first I checked out laser diodes at mouser(my favourite is digikey but as I live in Europe it is not viable due to the extra cost it brings) but the cost of laser diodes like that is not an option.
So it's probably Ebay that is the solution, but I know so little about lasers.
Is it acceptable to use a ordinary laser pointer such as this
laser pointer module?
There are also laser modules such as **broken link removed** which has TTL Modulation of 0-20khz which I don't even know what it actually means. I of course know of TTL but what is modulation of a laser between 0 and 20kHz...
The duty cycle of that module is specified as: Duty Cycle 45 seconds ON, 15 seconds OFF
There are also **broken link removed** running at 12V that has TTL, and then there is
kits which contain driver, laser module & heat sink which appears as overkill in this instance.
But the prices of the products linked to are quite a bit higher than what the most basic green laser pointer module costs, and if 0-20kHz modulation is useful... I am probably the worst person to be expressing any thoughts about this as I really don't know anything at all but I can't imagine why modulation would be good for...
In any case, those laser pointer modules could easily be incorporated into my design by solder off that push-button that is used to activate the laser and replace that with a transistor controlled by the microcontroller controlling the apparatus.