RTD or thermocouple?

[Mark Agnello]

> Its sensor is "electronically enhanced" to offer
> faster response time.  To 
> me, that sounds like their datalogger is
> "predicting" what the temp should be 
> based on, maybe by its rate in temp change?  

It is compensated with a circuit that responds to the
rate of change.  This is a common technique with
aircraft EGT sensor signal conditioning.  General
Electric frequently uses a circuit like this on gas
turbine aircraft engines.  The down side is that this
makes the system much more susceptable to noise.  Look
how jagged the line is for the RTD.  Still, depending
on your requirements, this may be acceptable.  

Another key point not mentioned in any of these
discussions is that thermocouple response time in a
gas flow is dependent on the mass flow rate.  The
higher the mass flow rate, the faster the response
time.  I strongly suspect T/C response time at high
EGT conditions is more than good enough. 
Unfortunately, I have never seen published data that
correlates calibrated combustion temperature
measurements with mass flow rate.  Cal ovens all seem
to work with static flow conditions.  

> the sensor does not 
> appear to be more accurate than a thermocouple.  I
> have a hard time believing 
> their black box can actually improve response time
> and maintain accuracy.  
> Any thoughts?

It isn't more accurate.  It just has a different
response characteristic.  Accuracy is relative to what
you are trying to do.  If the intent is to trigger
protective algorithms that ensure you never melt the
turbine in an aircraft engine, then a faster, noisy
response is OK.  Another factor to consider is that
around 1000 deg C, radiation heating of the sensor
from the walls of the manifold is significant.  This
decreases the accuracy of the gas flow temperature
measurement.  You probably can't measure EGT to better
than +/- 50 deg C under these conditions.  The
important thing is to understand the significance of
relative temperature changes.  In other words, note
normal temperature trends in your engine, so you can
recognize bad things happening.  Better yet, try to
combine EGT with O2 measurements for a better
understanding.  That is what I am doing with my
current project.  (1988 Turbo Celica AWD)

Mark Agnello



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