Re: How do you calibrate EGT and CHT ?

Jim Williamson wrote:
> 
> "Bob and Mary Beard" <[EMAIL PROTECTED]> wrote:
> 
> >
> >"Jim Williamson" <[EMAIL PROTECTED]> wrote in message
> >news:[EMAIL PROTECTED]
> >> "ken" <[EMAIL PROTECTED]> wrote:
> >>
> >> >the others in the group may comment on this but I would also caution the
> >> >location of the thermocouple, if placed in the wrong location - distance
> >> >from the chamber you will get an incorrect reading. The instrument may
> >well
> >> >be correct but if too close or too far away then the resulting indication
> >is
> >> >wrong. I would say it is more likely that placement of the thermocouple
> >is a
> >> >problem rather than the instrument itself.
> >>
> >> And with this in mind I offer that calibration for the EGT is somewhat
> >> futile.  The EGT reading should simply be correlated to the plug's color.
> >> The plug is the primary indicator of air/fuel mixture.  It would be nice
> >to
> >> go by EGT readings/numbers across the board (plane to plane, installation
> >> to installation, etc) but too many variables exist - primarily probe
> >> placement.  The pilot of X craft simply needs to know that at X temp the
> >> plug (if the engine were to be shut down right then and the plug color
> >> checked) color would be X color.
> >----------------------------------
> >I believe what the individual asking the original question is after is a way
> >to check his in-flight exhaust gas readings. If this is so it is a valuable
> >tool for saving parts before they become welded together while you are
> >trying to enjoy the view from the air. If it is, as suggested, just a way to
> >check the heat range of the plugs he is using then looking at the plug
> >itself is good enough.
> >    When checking plug color you will need to get the engine to normal
> >operating temp, 3/4 to full throttle, let it run a bit at that setting, and
> >then cut the ignition. The reason being that if you throttle down before
> >turning off the ignition and then check the plug you do run a good chance of
> >getting darker (cooler running) deposits on the plug. This could prompt you
> >to put in a hotter plug when it is not really necessary.
> 
> Yes, IIRC the original post request was how to calibrate the probe/gauge
> set - and several people joined in with suggestions.  It was Ken's topical
> post regarding the issue of probe placement that propted me to put a few
> cents into the mix.
> 
> Probe placement is important because the exhaust stream temperature is
> different depending upon location.  With an EGT probe we're trying to read
> a temperature of the internal combustion champer's temps but are limited to
> using exhaust gasses after they've existed the chamber - they are external
> to the environment we're trying to measure.  They're changing from the
> ideal we're trying to measure.
> 
> On the exhaust setup for my original engine there was a single probe -
> after an asymetric Y-pipe junction (rear cyl was perhaps 12" from piston
> face to probe - front cyl was perhaps 6" from face to probe).  That
> placement would be, temperature wise, showing me LOWER temps than what was
> actually coming from the motor.  From that probe location I might have
> recorded 1100F temps and the said "Heck - I'd better lean it out as
> industry states 1200/1250 should be OK"  then bam - melt down time - but
> why - I was only at 1150 on the gauge????....  The gauge was at the wrong
> place for where the industry expects to record exhaust stream temps.
> 
> If the flyer (or me in the example) would have looked at the plug color
> (and your description of how to check them by shutting down the engine's
> electrical w/o changing throttle position is what I've been told to be the
> preferred method) and noticed the plugs were too light of a tan or were
> whiteish - well it's just too lean - no matter what the gauge is saying.
> 
> The better calibration would be too look at the internal components of the
> motor to see what life is like in there - plug, piston wash / piston top
> condition, ring seize condition.  Once those conditions are known - then
> the gauge numbers are recorded with the knowledge that X number is still
> acceptable.
> 
> Heck - pull the numbers off the EGT gauge.  Run some 3/4 throttle plug
> tests and some 100% throttle plug tests (leaner each time w/o going too
> lean) and simply mark green/yellow/red bars on the EGT face.
> 
> If I'm off the mark in my post - let me know.  My intentions well meant.


I think it goes deeper than what you said, although what you said is
accurate,,,,,

the manufacturer, and the industry as a whole, has taken some pains and
testing to locate the probes properly, such that,

the gauge reading is indicative of the situation inside,,,,,,

and that changing the situation inside with different jetting will give
an anticipated response at the probe,

ie, leaner jetting will in fact, raise the temps,,,,,,,,,,,and hopefully
throughout the anticipated range of temps likely to occur,

locating the probe improperly may actually find a situation where leaner
jetting lowers the readings, and worse, the vice versa may
occur,,,,,,,and you find yourself leaning for what is shown as a rich
condition, and the temps go down,,,,,,,,,,,

this is why I suggest the rotax engines rather than the many other
potential ones,,,,,,,good probe locations with predictable readings,
lots of good info for typical condidtions, etc,,,,,,,,,,

cuyuna has none, no suggetsions, just some jetting for theirs and a
myriad of other muffler systems,,,,,,,

got them in trouble years ago, when QS bought aftermarket crappolla
mufflers for their 430's,

rotax learned from their mistakes and sells the muffler with the
engine,,,,,,,,period,,,,,,,,
-- 


Mark Smith                        
Tri-State Kite Sales           http://www.trikite.com      
1121 N Locust St
Mt Vernon, IN 47620          mailto:[EMAIL PROTECTED]