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  1. #1
    Co-Pilot Antoni's Avatar
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    Westach CHT EGT gauge - calibration currents?

    Anyone know the correct full scale currents for the typical gauges used with Rotax 2-strokes? Please form an orderly queue.

    While replacing the senders I checked the FSD current with a resistance box, 2 Volt cell and a multimeter. The milliamps for my dual gauges are:

    EGT 2.67 2.54

    CHT 1.72 1.85

    Those are small percentage differences, except that the EGT pointers (correctly) don’t move at all until a certain current flows, so you’re only really seeing the top end of a much larger scale – the bottom part of the scale is ‘hidden’ if you see what I mean.

    For the same gauge current there’s a significant difference in the reading with the EGT but much less difference with the CHT gauges because they seem to have linear scales.

    Sure enough the lower current EGT side reads higher in flight - and uncomfortably close to 1200 F at times - hence this post.

    Makes a bit of a mockery of Rotax’s specification of not more than 43 F difference between the cylinders.
    "The single biggest problem in communication is the illusion that it has taken place" - G.B.S.


  2. #2
    Co-Pilot Aerial's Avatar
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  3. The Following 4 Users Say Thank You to Aerial For This Useful Post:

    Antoni (08-03-20), Arielarts (10-03-20), Gentreau (08-03-20), MadamBreakneck (08-03-20)


  4. #3
    Co-Pilot Arielarts's Avatar
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    Whilst the EGT and CHT meters are nominally ammeters, I believe they actually operate as (milli) voltmeters to measure what is actually a low impedence source thermo-couple voltage. Just measured a Westach CHT and its coil resistance is ~7.5ohm and the 1mA test current reads 200C - about half scale.


  5. #4
    Co-Pilot Arielarts's Avatar
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    Interestingly, that equates to ~7.5mV. I note that the Westach table linked by Aerial indicates 9.81mV at 400F (=200C). Perhaps they calibrate for a defined resistance drop in the interconnection.

  6. The Following User Says Thank You to Arielarts For This Useful Post:

    Antoni (10-03-20)


  7. #5
    Co-Pilot Antoni's Avatar
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    Begin Can of Worms:

    For what it's worth, Westach intend that the thermocouple senders have their proper supplied 'plug' connectors when used. This is because the crimp connection of the special metal wires used by the senders which go to those individual shiny plugs *form the other thermocouple* of the pair of thermocouples intended to be present in the Westach sender design. The crimped connector's temperature (cold junction temperature) directly affects the overall reading of the gauge (a bit) (forget that for EGT but it is worth noting for CHT).

    The system is 'calibrated' when the temperature sender connector's crimps are at 75 Farenheit.

    So... you shouldn't chop the original plugs off and crimp your own spade connectors onto those sender conductors.

    Even tho that's exactly what a previous owner had done, and for a few years that CHT sender worked 'credibly'.

    Paraphrasing from Westach: Due to the unchanging nature of thermocouple characteristics just check for probe resistance: ~1.5 Ohms CHT or ~2.5 Ohms EGT.

    /Can of worms.

    Edit -
    The Westach stuff seems to work well enough. I'll just follow the cal procedure in Aerial's Westach link, and if necessary bring the EGT gauge home, remove the rivets and alter the torsion springs' tensions so that at least they are accurate at 1200F, the other four calibration test points suggested are not so important.
    Last edited by Antoni; 10-03-20 at 19:32 PM. Reason: Reality
    "The single biggest problem in communication is the illusion that it has taken place" - G.B.S.


  8. #6
    Co-Pilot Arielarts's Avatar
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    There you go - that's where the missing 2mV goes! Westach rely on around 2ohm of loop+thermocouple resistance.
    Changing connectors is no issue (always being careful not to add resistance). So long as each change of metal and back again is at the same temperature, all 'false' thermocouple voltages cancel out. Even minor temperature variations along the harness are going to be small compared with the main sensor's temperature (~200-250C for a CHT). All connections external to the sensor average out to be the 'cold' junction at '75F'. On cold days, there will be an over-reading, and when over 75F, an under-reading.


  9. #7
    Co-Pilot Antoni's Avatar
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    Thanks all,

    I checked the dual EGT as per the Westach procedure and found the readings very slightly high. No point saying how much as it varies (non-linearly) across the scale.

    Yes, the the EGT side with the lower FSD currrent above did read very slightly more slightly high. Just shows that you can't check them by using a variable current. The gauge is at least 18 years old.
    "The single biggest problem in communication is the illusion that it has taken place" - G.B.S.


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