Message text written by Fred Fillinger
>In noting discussion re temps, I am wondering if anyone is testing
their temp gauges for accuracy, before drawing conclusion and doing
mods.From experience in designing digital readouts from the type sensors on
the Rotax, the issues aren't trivial.<
With the uMonitor you calibrate the senders as part of the setting up
procedure. I agree most senders (except thermocouples) have a non linear
resistance/parameter characteristic. RMI supply their own senders with the
unit rather than use the Rotax ones which overcomes the design problem.
Obviously the calibration needs to be optimised at the critical points in
the scale. For instance fuel gauges are usually calibrated at full and
empty and the non linearity in between is not so critical. For CHT the high
end of the range is more critical and therefore the upper setting can be
calibrated using boiling water. For the low end melting ice in water gives
approx zero deg C. Although thermocouples are ideal for temperature it is
important that their cold junctions (ends of the leads) are in a stable
cool environment. The thermocouple is measuring the difference between the
hot and cold junctions. With that in mind my thermocouple connectors are
aft of the firewall where the temperatures are more stable giving greater
accuracy. Calibration with digital instrumentation is much easier than
conventional analogue gauges as the calibration points are set using
digital software. In use, although analogue is easier to see at a glance
accurate readings and slower trends are better observed using digital. An
rpm gauge is the only engine gauge which I feel needs analogue display.
Ironically these are often the ones which need the most setting up. Mine
overeads by 200rpm at 5800rpm. Until I had checked the accuracy of my
digital rpm I was only using 5600rpm for take-off losing significant
performance.
Nigel Charles
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