Klaus wrote:
>During those test flights I have found out that my RPM indicator reads 300
>rpm too low!
>Any ideas how I could calibrate it?
There are a number of cheap digital pulse counters about - e.g. for cars,
some in kit form. In principle they are very accurate as they count for long
enough for the jitter in the pulse train to relative to the sampling window
to be neglible.
There is a trap though. The input waveform from the 912 ignition system is
not very clean and amplitude varies with speed. Although most of the digital
devices have squaring circuits and trigger off a nice clean waveform to the
counter, they can introduce counts from spikes/noise between the intended
pulses. I have one which says exactly 3000 if I put 50 hz sine waves or
square waves into it, produces entirely sensible readings up to about 2000
rpm when on the engine, but then starts counting extras until at 5000 or so
its reading over 6000 ! No doubt peak input stabilising and filtering can
cure this but that's not for everybody.
So rather than compare "on engine" it would be better to inject a synthetic
audio signal generator signal into the rpm indicator, if you can find someone
with the necessary hardware.
Graham C G-EMIN
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