I was flying an RV6 recently with an EFIS and the RPM was displayed
on the EFIS LCD with tiny numbers about 1/8th of an inch high. No other
tach on the instruments panel. To make matters worse there
was a long delay  between the time you adjusted the RPM and the change
took place on the display.

There are two ways to measure RPM. One way is to measure
the time for one revolution and then have the computer divide
that number into a constant. Lets say it took one second to rotate once.
That is 60 RPM of course. If it took .5 second to rotate
once it is 120 RPM. So fourth and so on. Computers are busy
little beast doing a lot of other things so time is of the essence.
It takes a very powerful computer with floating point math
to do the divide quickly.

Another way is take the pulse from the CAS and up count a decade counter
every rev. Then take a look at the count once a minute, display
it and reset the counter to zero and start over again.
Obviously the update rate on the tach is one minute. Not so hot.

However if we look at the 100 teeth on the ring gear we can
get the up date time (rate) down to 1/100th of a minute or
.6 second. This is the tach for me. I am working on it:)

I used this idea about 35 years ago in my real time road test equipment.
All done in hard ware. No math need be done by a computer.

http://www.rotaryeng.net/IEEE-road-test-sys.pdf

A stand alone device so no computer needed either.

Paul Lamar

Paul

Not sure if this, or similar item can work. It has resolution to 0.1 rpm.
http://www.amazon.com/Digital-Tachometer-Contact-revolution-minute/dp/B0045E0GT2

Henry

We need cockpit display and remote sensing. They don't tell you the update
rate or the max RPM. LCD's are sometimes hard to read in bright sunlight.

Paul