---------------------------------------------------------- Avionics-List Digest Archive --- Total Messages Posted Thu 02/11/10: 1 ---------------------------------------------------------- Today's Message Index: ---------------------- 1. 03:05 PM - Re: Re: Thermocouple compatible bulkhead connector? (John Loram) ________________________________ Message 1 _____________________________________ Time: 03:05:39 PM PST US From: "John Loram" Subject: RE: Avionics-List: 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: > > > > ------------------------------------------------------------------------------------- Other Matronics Email List Services ------------------------------------------------------------------------------------- Post A New Message avionics-list@matronics.com UN/SUBSCRIBE http://www.matronics.com/subscription List FAQ http://www.matronics.com/FAQ/Avionics-List.htm Web Forum Interface To Lists http://forums.matronics.com Matronics List Wiki http://wiki.matronics.com Full Archive Search Engine http://www.matronics.com/search 7-Day List Browse http://www.matronics.com/browse/avionics-list Browse Digests http://www.matronics.com/digest/avionics-list Browse Other Lists http://www.matronics.com/browse Live Online Chat! http://www.matronics.com/chat Archive Downloading http://www.matronics.com/archives Photo Share http://www.matronics.com/photoshare Other Email Lists http://www.matronics.com/emaillists Contributions http://www.matronics.com/contribution ------------------------------------------------------------------------------------- These Email List Services are sponsored solely by Matronics and through the generous Contributions of its members.