nuckolls.bob(at)cox.net
Guest
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 Posted: Thu Dec 13, 2007 8:07 am Post subject: Thermocouple quesions. |
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At 01:18 PM 12/13/2007 +0000, you wrote:
| Quote: |
Good morning,I purchase yor electric book about a year ago,I was
disappointed at the time,because how technical was write ,I was expect a
lot of pictures not electric symbols,I want to build an airplane for my
self ,not became a electronic guru,but ,reading and reading ,now I;m start
to understand and appreciate the amount of information was in that little
package,(and a low cost for the info released),(still thinking need to be
more pictorial,(I'm still don;t know how a diode look like,and how need to
be conected to the circuit,solder ,crimp ,or?
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Thank you for sharing your experience with us.
As you've discovered, the 'Connection is not
a cook-book. There are only rudimentary suggestions
of recipes for success in the form of the Z-figures.
My goals for this effort was not to be a competitor
of Tony Bingelis and others who have done a fine
job explaining the practice of assembling airplanes.
The 'Connection is intended for a small fraction of
the OBAM aircraft community with curiosity and
a desire to understand the simple-ideas that support
the practice.
| Quote: | any way ,I have a dilema ,and until now no body satisfied me with a
logical answer,if its not to much a problem I like to ask you,reading yor
book I fully understand the work of a thermocouple,but,,, I need to know
if the leads from the instrument to the sensor can be cut to fit the rigth
length,I mean, if the number one cylinder lead can be longer then the
number six,my dilema ,and I can 't found in your explanation ,its will be
different or dealy time in the reading ,if the wires are different
lengths?hope I was able to explain my question,wo persons from the same
company(EFIS) give me different answers.its this an opinion or its a
physical law apply?
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About 60 years ago, it was not uncommon for
a thermocouple driven instrument to get all of
it's operating energy directly from the
thermocouple. A good example is this instrument
found on Ebay a few years ago:
http://www.aeroelectric.com/Pictures/Instruments/CHT%20Gauge.jpg
Note that in the lower right corner of the face
we see the notation "2 OHM I/C COUPLE". Further,
on the rear . . .
http://www.aeroelectric.com/Pictures/Instruments/CHT%20Gauge1.jpg
. . . there are only two terminals. I.e, no connections
to power or other electronics. THIS instrument's
calibration is accurate IF and ONLY IF the Iron-Constantan
thermocouple has a total loop resistance of exactly 2.0
ohms. These instruments are generally paired with a
specific thermocouple assembly having the necessary
characteristics and it cannot be altered either for
length or gage of wire.
The modern thermocouple instrument is assisted by
external power and the loop resistance of the thermocouple
is no longer critical. These devices are insensitive
to changes in length, wire gage or both. However,
they ARE still sensitive to poor joining techniques
that introduce parasitic thermocouples operating at
other temperatures causing the instrument to read
in error.
Short answer to your question is that you may alter
the length or gage of wire used as long as you
use good practice for splicing the segments.
Bob . . .
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