No go for a hall sensor or an encoder for the brake RPM. 

Or is it easier to use the engines rpm? 

Thanks.  I'm gonna read more about the sensors but I'm quite sure it will work on aluminum. I have seen a distance chart for another sensor with different kind of materials. and the maximum distance was way lower with aluminum than for steel. So it should work .

I have disassembled the brake so I'm going to check how many vanes it is before I'm put it back again. 

I'm going to try and see it it will work before I'm making a trigger wheel.

For that sensor I think the npn is the correct type. It was a bit hard to judge from the picture ;-). 

For the original sensor there must be magnets embedded in the aluminium, no sensor senses alu that I know of. But double check that it really is 12 of them. You should calculate the number of pulses per second, it should be minimum 200. For 12 triggers per revolution it means that it will work fine over 1000RPM. It works below too, but then you should turn off inertia compensation (normally not super important on an engine dyno). 

Alternatively you can do what we did on our water brake, we just added a trigger wheel and the standard RPM sensor. You can see it in the water brake videos in our youtube channel. 

Ok. I'm gonna use it inside the water brake as they do originally! It's a old dynamite 9" straight vane absorber.

If l remember correct it will be 12 teeth per revolution and the material is aluminum!  That should work? 

I think this one Will be perfect But do you know if l should use use no, nc or nc+no for the sensor? 

https://www.google.com/amp/s/m.aliexpress.com/item/2030055517.html

That may work. You need one that triggers on ferrous materials (iron), since you need a trigger wheel, see wiki. Also the supply voltage is 5V, so make sure it works with 5V +/- a little bit.

Okay thanks. Like this one?

https://www.ebay.com/itm/Hall-Effect-Sensor-Proximity-Switch-NPN-3-wires-with-magnet-Normally-Open-N-X6M8/223137797373?epid=25012086485&_trkparms=ispr%3D1&hash=item33f40c94fd:g:FQ4AAOSwf2pblSQ0

Why I'm asking you about this is because l need a sensor in range of m4-m10 so it will fit the brake. so l can't use your sensor. 

Do you know where l can buy a good sensor ? 

Hi,

No, that does not work. You need a digital sensor, such as a hall effect sensor or an encoder. They should be of type open collector (sometimes called npn), since YourDyno has an internal pullup resistor.