The engine is now running the way it should, I have 32 hours of
flight time on it, 8 left to finish my phase 1. Went out for 2
hours today, 75 degrees on the ground, temps never went over 190.
One thing though- my egts are always high- running around 1800 up to
1950. dead cold the egts show about 230 degrees, but that is way
below their designed operating temperature, and I have not changed
the settings that my EM2 came with. Anyone have any opinions?
Brian Trubee
How far from the exhaust port are they.
Sounds like a problem with the cold junction.
http://www.omega.com/prodinfo/thermocouplewire.html
"Wire that is used in a thermocouple from the point of sensing to
the point of cold junction compensation (cjc end) where the signal
is measured. A thermocouple is a sensor for measuring temperature,
that consists of two dissimilar metals that are joined together at
the sensing end. Different thermocouple types (e.g. J, K, T, E, etc)
use different mixtures of metals in the wire. At the cjc end, the millivolt
value provided by the thermocouple represents the difference in
temperature of the sensing end as compared to the cjc end (also
called the reference end)."
"There are many factors that can impact the usable length of a
thermocouple. As a guideline, under 100 feet with 20 AWG or thicker
wire in an area free of electromagnetic interference usually is
fine. Two of the main factors in determining usable thermocouple
length are total loop resistance and preventing electrical noise
getting into the signal. Because different thermocouple wires are
made of different materials, the resistance will vary based on the
type as well as the wire diameter and length. The allowable loop
resistance is affected by the input resistance of the amplifier
circuit to which it is attached. But as a guideline, typically the
objective is to keep the total loop resistance under 100 ohms. Loop
resistance is determined by multiplying the length in feet by the
resistance per double feet
(remember 1 foot length of run includes 1 foot from each of the two
t/c wires) as shown in "Resistance Vs Wire Diameter" table for
thermocouple type and gauge. Remember in your calculations to
include the probe (when used/applicable) in addition to the wire
length. The second major factor in running a thermocouple wire is to
keep it away from any electromagnetic fields. Thermocouple wire
creates a low voltage signal and should not be run near power wires,
motors, etc. To help minimize noise pickup, a metal over braid or
twisted shielded wire is commonly used."
I don't have a EM2 but there should be an adjustment for the cold
junction voltage like a potentiometer. Perhaps the leads are
reversed. Polarity of the leads is important. Try reversing them.
You may not splice or shorten the thermocouple wire. If necessary
folds them up but leave them the length they came with.
Paul Lamar
I take it that by "DEAD COLD" you mean right after a cold start. If that is
the case, I would think that is normal. The temp should rise quickly to a
warmed up stable temp at idle (2000RPM). Then as you put more load on the
engine the temp will rise from there to a new stable point.
Thermocouples are very sensitive to extension wiring. Special couplings for
the Thermocouple type are required. That is because EVERY change in metal
chemistry will introduce another cold junction. You need to use the
thermocouple wire right to the amplifier device (gauge). I am used to working
in plants. The newer ones have transmitters that convert the small voltage
generated by the thermocouple into a 4mA to 20mA signal back to the control
room. Older plants had thermocouple wires run all the way back which may be
500 feet in some instances. When they are 40 years old and have had some
exposure to H2S there can be a lot of trouble finding the problem.
Shorten could be possible depending on the design of the gauge and how much
adjustment there is. The stuff we work on has a lot of adjustment. Exhaust
pyrometers are usually designed for a given length lead. Note: RED is
negative when it comes to thermocouples.
wikipedia.org/wiki/Thermocouple
The distance from the port as Paul mentioned will affect the reading. Also,
which type T/C do you have? Should be K. J may do it but will be off at the
run temps or fail early.
Dale Davies
The thermocouples are installed correctly, with the possible exception of the
polarity of the wiring. I'll check that. I had a small issue with polarity on my
crank angle sensor, if you recall. That was a real hoot to fly around with, let me
tell you.
Brian Trubee
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