> There is a hardware variable. You need to time a CYCLE which
> needs an accurate level-sensing switch, or count cycles in a
> fixed TIME, which means the switch is less stringent. An ordinary
> Schmitt Trigger is good for 1000:1 or 0.01%. If you count 1000
> cycles you get 1 ppm. That's why they prefer to count cycles
> rather than microseconds (or whatever).

JT> Depends on how the pulse was received I suppose, if it's iffy,
JT> you're likely to get a fair amount of jitter, which would
JT> fluctuate the readings by a good amount (it might average OK,
JT> but the jitter would make it useless for human readability).

 All this talk has forced me to make some calculations. At 6,000
rpm in top, the tailshaft is going at 100Hz and this explains the
problem. I've been talking through my hat, you're wright and I'm
worng.

 Shit! That hurt.

 1 - 100Hz is the crossover where *nothing* works! If you
measure a fixed period (Frequency), you need 1 second to get 100
pulses and that's too slow, and if you measure time between pulses
(Period), you have to fake it at low speed where the pulses are
1-second apart (2kmph).

 The only answer is hardware - either gear the sensor up 5-times, or
use a multi-slot interruptor.

 As for jitter, that's not a problem... just write it into the
software - update every fifth of a second.

 Shit!

Regards,
Bob
 

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