hi all,

if i were to tacho a ducted fan impellor, that has x number of blades, and i use a tacho and set the tacho to read y number of blades, how do i convert the reading properly?

i am confused.

i thought it might be

[reading x (Y/X)]

this appears to be incorrect, but i'm unsure.

the problem is that when the tacho reads a two blade setup, whenever two blades cross the optical sensor, it registers a rev right?, so theoretically a 7 blade impellor registers 3.5 rpm per rev when measuring on the 2 blade setting....? right?...

so 30k revs in this instance would read as 3000 x 3.5 = 10500.

is this correct?.. or is it [reading x (Y/(X-1))] is it only the 6 blades inbetween the 7th?, or does it include the 7th, so effectively being 4 rpm per rev.

it appears to be picking up all the blades on my fan, as the numbers are repeatable.

Tim.