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thermocouple wires

 
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glcasey(at)adelphia.net
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PostPosted: Fri Mar 31, 2006 6:15 am    Post subject: thermocouple wires Reply with quote

Cold junction compensation: I have a system from Grand Rapids
Technology that recommends using ThermoCouple(TC) wire all the way
from the box to the TC connectors on the engine. The CHT uses type J
and the EGT type K. The people that did my panel used copper to a
bulkhead connector (expensive Mil-spec with gold-plated terminals)
and then I, without too much thinking, kept going with copper wire to
each TC connector. Now I'm worried about the errors will occur
because of all the wire material mismatch. The only good thing going
is that the TC wires are equal length so the + and - connectors are
right next to each other. Big errors? Small errors? I'm almost
tempted to pull out the whole thing, bulkhead connector and all, and
run TC wire in one length from the box to each TC. I looked at the
JPI system in my certified aircraft and that's the way it was done.
And then while I'm at it I could buy real TC connectors that will
keep all the materials correct. Should I?

Gary Casey


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nuckollsr(at)cox.net
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PostPosted: Fri Mar 31, 2006 6:33 am    Post subject: thermocouple wires Reply with quote

At 06:12 AM 3/31/2006 -0800, you wrote:

Quote:


Cold junction compensation: I have a system from Grand Rapids
Technology that recommends using ThermoCouple(TC) wire all the way
from the box to the TC connectors on the engine. The CHT uses type J
and the EGT type K. The people that did my panel used copper to a
bulkhead connector (expensive Mil-spec with gold-plated terminals)
and then I, without too much thinking, kept going with copper wire to
each TC connector. Now I'm worried about the errors will occur
because of all the wire material mismatch. The only good thing going
is that the TC wires are equal length so the + and - connectors are
right next to each other. Big errors? Small errors? I'm almost
tempted to pull out the whole thing, bulkhead connector and all, and
run TC wire in one length from the box to each TC. I looked at the
JPI system in my certified aircraft and that's the way it was done.
And then while I'm at it I could buy real TC connectors that will
keep all the materials correct. Should I?

One is never wrong to follow the manufacturer's instructions.
Your concerns for degraded calibration of the instruments
due to mis-wiring are well founded. See chapter 14 in the 'Connection.
Thermocouples are neat things. They work well but only if
one follows the design rules. As soon as you stick a foreign
material (copper) in the loop, one must DESIGN for a specific
set of circumstances that should honored if you want to
maintain system integrity. If the manufacturer calls for
100% t/c wire from measurement of interest to the instrument,
then those are the rules.

Bob . . .

< What is so wonderful about scientific truth...is that >
< the authority which determines whether there can be >
< debate or not does not reside in some fraternity of >
< scientists; nor is it divine. The authority rests >
< with experiment. >
< --Lawrence M. Krauss >


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william.p.dube(at)noaa.go
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PostPosted: Fri Mar 31, 2006 8:25 am    Post subject: thermocouple wires Reply with quote

In a thermocouple circuit, the voltage is generated along the wire as it
passes through regions of different temperature. If part of the circuit
is the wrong alloy (like the connectors, contacts, joints, wire,
corrosion,) and there is a temperature difference across these regions
of incorrect alloy, you will get the wrong temperature reading.

The error will depend on how big the temperature gradient is, and
the Seebeck coefficient of the improper alloy in that region. Copper
oxide, for example, has HUGE Seebeck coefficient, so the errors
generated at slightly corroded copper joints can be enormous.

Having said all that, keep in mind that if there is no temperature
gradient, no voltage is generated.

Also, if both legs of the circuit are the same alloy, and are
subjected to the same temperature gradient, no voltage will be
generated. This can be good, or bad. If you have copper wires running
from the gage to the engine block, then TC wire running from the block
to the exhaust probe, the gage will read the difference in temperature
between the block and the probe (approximately.) If the block was hot,
and the probe slightly colder, you would read a negative temperature on
the gage.

Complicating this is the reference (cold) junction compensation in
the gage. It assumes that you have the correct alloy wire everywhere. It
will add or subtract the temperature of the gage from the voltage it
reads from the TC circuit. If the transition from copper wire to TC wire
is not at the gage terminals, but on the engine block, the reading will
be off by the difference in temperature between the block and the gage.

The firewall bulkhead connector and the connector near the engine
are the ones to be concerned about. They are almost certain to have a
significant temperature difference imposed on them. If they are not the
correct alloy, they will introduce an error.

Connectors with the correct alloy are not terribly expensive.
Sometimes you can buy pins for existing connectors that are the proper
TC alloys. Teflon TC wire is also not very expensive. If you want the
reading on the gage to be correct, take the time to do the wiring correctly.

Bill Dube'

Gary Casey wrote:

Quote:


Cold junction compensation: I have a system from Grand Rapids
Technology that recommends using ThermoCouple(TC) wire all the way
from the box to the TC connectors on the engine. The CHT uses type J
and the EGT type K. The people that did my panel used copper to a
bulkhead connector (expensive Mil-spec with gold-plated terminals)
and then I, without too much thinking, kept going with copper wire to
each TC connector. Now I'm worried about the errors will occur
because of all the wire material mismatch. The only good thing going
is that the TC wires are equal length so the + and - connectors are
right next to each other. Big errors? Small errors? I'm almost
tempted to pull out the whole thing, bulkhead connector and all, and
run TC wire in one length from the box to each TC. I looked at the
JPI system in my certified aircraft and that's the way it was done.
And then while I'm at it I could buy real TC connectors that will
keep all the materials correct. Should I?

Gary Casey









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ainut(at)hiwaay.net
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PostPosted: Fri Mar 31, 2006 1:20 pm    Post subject: thermocouple wires Reply with quote

I would.

David M.
Gary Casey wrote:

Quote:


Cold junction compensation: I have a system from Grand Rapids
Technology that recommends using ThermoCouple(TC) wire all the way
from the box to the TC connectors on the engine. The CHT uses type J
and the EGT type K. The people that did my panel used copper to a
bulkhead connector (expensive Mil-spec with gold-plated terminals)
and then I, without too much thinking, kept going with copper wire to
each TC connector. Now I'm worried about the errors will occur
because of all the wire material mismatch. The only good thing going
is that the TC wires are equal length so the + and - connectors are
right next to each other. Big errors? Small errors? I'm almost
tempted to pull out the whole thing, bulkhead connector and all, and
run TC wire in one length from the box to each TC. I looked at the
JPI system in my certified aircraft and that's the way it was done.
And then while I'm at it I could buy real TC connectors that will
keep all the materials correct. Should I?

Gary Casey

















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