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1. 03:05 PM - Re: Re: Thermocouple compatible bulkhead connector? (John Loram)
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Subject: | Re: Thermocouple compatible bulkhead connector? |
Hi Ira, I agree with you in all but one detail: when I insert a connector
into a leg of a thermocouple wire I've created three junctions. One does not
generate a voltage, and the other two junctions generate equal voltages
(because they are at the same temperature) and opposite polarity (so they
cancel out).
For example: Take a piece of thermocouple wire and cut it. Crimp a brass
connector pin on one end of the cut and a brass connector socket on the
other end of the cut. Now plug the brass pin into the brass socket. The
Brass/Brass junction does not generate a voltage because it is a junction of
similar metals. However you have created two other junctions; one is
Iron/Brass and the other is Brass/Iron. These two junctions create equal
voltages (because they are at the same temperature) but the voltages are of
opposite polarity, and cancel one another out. The connector has no net
effect.
regards, -john-
> -----Original Message-----
> From: owner-avionics-list-server@matronics.com
> [mailto:owner-avionics-list-server@matronics.com] On Behalf Of rampil
> Sent: Tuesday, February 09, 2010 8:39 AM
> To: avionics-list@matronics.com
> 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
>
>
>
>
>
>
>
> Photoshare, and much much more:
>
>
>
>
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