We use an electronic optical tach designed for the purpose. Costs $300
from an aviation supplier. If I hold it near a fluorescent light, it
will read 3600 if set on two-blade mode. 60 hertz for one minute is
3600 two-pulse cycles, and since the 60 hertz supplied by power
companies is very accurate (some clocks rely on it), this instrument
is pretty good. It measures in tens (have to add a zero).
Now, my 17-year-old son bought a model airplane engine tach for
$50, and it measures in ones. And equally accurate. Go figure.

Dan


Michelle P wrote in message link.net...
Dan,
ok so I had a weak math moment. 5% at 2000 is 100 RPM, at 2500 is 125.
Weather or not is Tach is out of limits depends on how accurate the
reference device.
Michelle

Dan Thomas wrote:

Michelle P wrote in message link.net...


Sami,
it is allowed to be off by 5%. this is 40 rpm at 2000 or 50 at 2500.
While it is outside limits most places will placard the difference and
call it good. getting a mew one it may not be much better.



5% of 2500 is 125 RPM, not 50. His tach is still out of limits,
though. In Canada the limit is 4%, measured at the centre of the
cruise RPM range.
Most tachs in singles are the magnetic-drag type, and the magnet
weakens with age, so the tach underreads. Replace it, but I think you
need the green and red markings to identify safe ranges; does the
electronic tach have such warning devices?

Dan



--

Michelle P ATP-ASEL, CP-AMEL, and AMT-A&P

"Elisabeth" a Maule M-7-235B (no two are alike)

Volunteer Pilot, Angel Flight Mid-Atlantic

Volunteer Builder, Habitat for Humanity