Today's Message Index:
----------------------
1. 08:40 AM - Re: Thermocouple compatible bulkhead connector? (rampil)
2. 10:21 AM - Re: Re: Thermocouple compatible bulkhead connector? (Ron Quillin)
3. 02:51 PM - Re: Re: Thermocouple compatible bulkhead connector? (Noel Loveys)
4. 07:27 PM - Re: Re: Thermocouple compatible bulkhead connector? (Richard E. Tasker)
5. 08:32 PM - Re: Re: Thermocouple compatible bulkhead connector? (Doug McNutt)
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| Subject: | Re: Thermocouple compatible bulkhead connector? |
I think someone is confused here.
The purpose for keeping the alloys the same in the path to the voltmeter
for T/C measurement has nothing to do with certification. It has to do
with Accuracy! If there is a point where the metal changes, it creates a
new thermocouple (usually a "cold point") These additional thermocouples
will change the net voltage at the voltmeter and thus the temperature reading.
There is almost always a cold junction at the meter itself to
contend with. If you have several cold junctions in the loop at different
temps and all in the same direction of metal change, your reading will
be very inaccurate. Yes, even if the pins are gold plated!
But then, who among us ever bothered to test the accuracy of their
CHT or EGT probes?
Thus it has always been. Knowing the reasons for certain guidelines lets
you make sensible plans
--------
Ira N224XS
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=285636#285636
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| Subject: | Re: Thermocouple compatible bulkhead connector? |
At 08:39 2/9/2010, you wrote:
>But then, who among us ever bothered to test the accuracy of their
>CHT or EGT probes?
If the craft has been sheltered in a hangar, most every time the
master/avionics is turned ON.
How close are all the probes to ambient and one another?
Absolute accuracy, as good as whatever reference you have.
Relitive accuracy, how do they diverge?
No, that's not a test at operating temperature, but it does verify
the probe is there and connected...
Ron Q.
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| Subject: | Re: Thermocouple compatible bulkhead connector? |
I agree with your comment on the accuracy which is why if there are
dissimilar metals in the circuit of the T/C it will never get certified.
Noel
-----Original Message-----
From: owner-avionics-list-server@matronics.com
[mailto:owner-avionics-list-server@matronics.com] On Behalf Of rampil
Sent: February 9, 2010 1:09 PM
Subject: Avionics-List: Re: Thermocouple compatible bulkhead connector?
I think someone is confused here.
The purpose for keeping the alloys the same in the path to the voltmeter
for T/C measurement has nothing to do with certification. It has to do
with Accuracy! If there is a point where the metal changes, it creates a
new thermocouple (usually a "cold point") These additional thermocouples
will change the net voltage at the voltmeter and thus the temperature
reading. There is almost always a cold junction at the meter itself to
contend with. If you have several cold junctions in the loop at different
temps and all in the same direction of metal change, your reading will
be very inaccurate. Yes, even if the pins are gold plated!
But then, who among us ever bothered to test the accuracy of their
CHT or EGT probes?
Thus it has always been. Knowing the reasons for certain guidelines lets
you make sensible plans
--------
Ira N224XS
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=285636#285636
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| Subject: | Re: Thermocouple compatible bulkhead connector? |
Sorry, but I have to speak up regarding the confusion... While what
seems to be the consensus is literally true, in reality it really
doesn't make any difference.
The way a thermocouple works is by a phenomenon called the Seebeck
effect. This has a complicated physics description, but the net result
is that any metallic material that is exposed to one temperature at one
end and another at the other, will generate a small voltage across its
length. Each material has its own characteristic voltage per degree
difference between ends. A thermocouple takes advantage of the
different voltages by pairing two different materials - one generates
one voltage and the other generates another. Because they are connected
at one end (where the desired temperature measurement is to take place)
one can measure the voltage difference at the other end. A so called
"cold junction" compensation is usually provided in the gauge or meter
that displays the temperature so that the readout shows the actual
temperature at the far end rather than the temperature difference
between the two ends.
Ideally the same material should be used from the temperature
measurement point to the meter that displays the reading.
Practically speaking this is not necessary and is a source of a great
many old wives tales.
Take the example of a connector that has the two thermocouple wires on
each end and the connector contacts between them (i.e. thermocouple wire
from the point of interest to the connector and then thermocouple wire
from the connector to the meter). Yes, the different material of the
contacts will generate a small voltage from one end to the other based
on the temperature difference between the two ends of the connector.
What is this temperature difference likely to be? Probably less that
one degree unless it is a very long connector. Further, both sets of
pins are the same, so the net voltage generated by the connector will be
zero. So the only difference on voltage at the meter will be the very
small temperature difference between one end of the connector and the
other. Considering most of us are measuring CHT (several hundred
degrees) or EGT (1200 -1600 degrees), a degree or two is irrelevant.
If one chooses to use a connector to go through the firewall and then
use regular copper wire to connect between the firewall connector and
the meter in the cockpit, then that would cause an error that might be
measurable. If the firewall is 150 degrees and the cockpit is 70
degrees, then there will be an error of 80 degrees in the display, which
is definitely significant for what we are measuring - if we are trying
to tell what the exact temperature is. However, even in this extreme
case, the typical use of the readout is to compare the temperature of
the cylinders to each other or the EGTs to each other. Even though the
displayed temperature will not be correct, if all thermocouples go
through the same connector, they will all be affected the same and can
still be compared properly.
So...
A connector that is added to a set of thermocouple wires run from the
temperature measurement point to the meter has essentially no effect on
accuracy of the measurement. You can even add lengths of copper wire if
the temperature difference between the two ends of the copper wire are
at the same temperature. Your error in this case will be only the
difference in temperature between the two ends of the copper wire.
There are a few caveats:
All the above regarding connectors and copper wire is true if the two
wires from the thermocouple and connector and copper are run over the
same physical path so they are exposed to the same temperature
differences. If you were to use two connectors - one for each of the
thermocouple wires - placed far apart you could get errors that mattered
(why you would do this I have no idea).
The materials in the path of the two thermocouple wires must be
identical (except for the thermocouple wires themselves of course) -
that way all the other materials will generate the same voltages in each
path so the reading will be only what the thermocouple wires generate
over their length - whether they are one unbroken run or a run broken by
a connector.
If you are trying to measure temperatures of a only few degrees above
ambient, then, while the above is still true, the errors may be too much
for your requirements. If you are measuring CHT and EGT adding a
connector is irrelevant to the reading.
Dick Tasker
rampil wrote:
> --> Avionics-List message posted by: "rampil"<ira.rampil@gmail.com>
>
> I think someone is confused here.
> The purpose for keeping the alloys the same in the path to the voltmeter
> for T/C measurement has nothing to do with certification. It has to do
> with Accuracy! If there is a point where the metal changes, it creates a
> new thermocouple (usually a "cold point") These additional thermocouples
> will change the net voltage at the voltmeter and thus the temperature reading.
There is almost always a cold junction at the meter itself to
> contend with. If you have several cold junctions in the loop at different
> temps and all in the same direction of metal change, your reading will
> be very inaccurate. Yes, even if the pins are gold plated!
>
> But then, who among us ever bothered to test the accuracy of their
> CHT or EGT probes?
>
> Thus it has always been. Knowing the reasons for certain guidelines lets
> you make sensible plans
>
> --------
> Ira N224XS
>
>
> Read this topic online here:
>
> http://forums.matronics.com/viewtopic.php?p=285636#285636
>
>
>
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Message 5
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| Subject: | Re: Thermocouple compatible bulkhead connector? |
At 22:12 -0500 2/9/10, Richard E. Tasker wrote, and I snipped.:
A well thought out answer that says it all. Read it,
There was a time when thermocouples in aircraft were hooked up to sensitive D'Arsonval
current meters, the analog kind with a magnet, a coiled spring, and a
pointer. Those meters measured current rather than voltage and that made the
length and resistance of the wires important. Re-calibration would be required
if, for instance, a thermocouple was shortened during installation or repair.
The solid state digital voltmeters of today have no such problems.
When reading FAA rules of engagement remember that most of them were written 50
years ago.
Richard's use of the Seebeck effect in his description is accurate but perhaps
more complicated than necessary, You won't go far wrong by thinking of a junction
of two metals as a tiny temperature sensitive battery with a known voltage
vs temperature function that you can look up in tables. The Omega Company is
really good at providing those tables.
<http://www.omega.com/toc_asp/sectionSC.asp?section=A&book=temperature>
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