---------------------------------------------------------- Avionics-List Digest Archive --- Total Messages Posted Fri 02/12/10: 4 ---------------------------------------------------------- Today's Message Index: ---------------------- 1. 06:57 AM - Re: Thermocouple compatible bulkhead connector? (rampil) 2. 07:04 AM - Re: Re: Thermocouple compatible bulkhead connector? (Noel Loveys) 3. 10:01 AM - Re: Re: Thermocouple compatible bulkhead connector? (Richard Tasker) 4. 10:21 AM - Re: Re: Thermocouple compatible bulkhead connector? (John Loram) ________________________________ Message 1 _____________________________________ Time: 06:57:12 AM PST US Subject: Avionics-List: Re: Thermocouple compatible bulkhead connector? From: "rampil" Hi All, I agree with all the comments that follow my snip. The real problem is not the absolute precision or accuracy of these systems which may each in usual practice with the cheap design gauges (though most pricey to purchase since they carry an aviation sticker), be 10% or worse. The issue is the basic lack of understanding in many builders (this applies to boats and cars too!) about things electric. Simple principles of operation are not that hard or foreign. Understanding the most basic principles help designers from big errors, and would as a side effect, eliminate 90% of the traffic on Bob's mediated group. -------- Ira N224XS Read this topic online here: http://forums.matronics.com/viewtopic.php?p=286122#286122 ________________________________ Message 2 _____________________________________ Time: 07:04:27 AM PST US From: "Noel Loveys" Subject: RE: Avionics-List: Re: Thermocouple compatible bulkhead connector? Is it reasonable to believe if the connection is going through a bulkhead that the temperatures will be different on each side of the bulkhead? Noel -----Original Message----- From: owner-avionics-list-server@matronics.com [mailto:owner-avionics-list-server@matronics.com] On Behalf Of John Loram Sent: February 11, 2010 7:31 PM 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: > > > > ________________________________ Message 3 _____________________________________ Time: 10:01:40 AM PST US From: Richard Tasker Subject: Re: Avionics-List: Re: Thermocouple compatible bulkhead connector? Probably different, but by how much? The thermocouple wire is connected on each side to a connector pin, the connector pins connected to each other. All metallic and relatively good thermal conductors. How accurately do you want to measure the CHT/EGT anyway? As Ira noted, the meter is probably the "weakest link" in this accuracy equation anyway. Dick Tasker Noel Loveys wrote: > --> Avionics-List message posted by: "Noel Loveys" > > Is it reasonable to believe if the connection is going through a bulkhead > that the temperatures will be different on each side of the bulkhead? > > Noel > > -----Original Message----- > From: owner-avionics-list-server@matronics.com > [mailto:owner-avionics-list-server@matronics.com] On Behalf Of John Loram > Sent: February 11, 2010 7:31 PM > To: avionics-list@matronics.com > Subject: RE: Avionics-List: Re: Thermocouple compatible bulkhead connector? > > --> Avionics-List message posted by: "John Loram" > > 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? >> >> --> Avionics-List message posted by: "rampil" >> >> 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: >> >> >> >> >> > > > ________________________________ Message 4 _____________________________________ Time: 10:21:00 AM PST US From: "John Loram" Subject: RE: Avionics-List: Re: Thermocouple compatible bulkhead connector? It is reasonable to believe that there is a temperature difference of a few tenths of a degree. Not enough to be perceptible by any of the instrumentation we're using. -john- > -----Original Message----- > From: owner-avionics-list-server@matronics.com > [mailto:owner-avionics-list-server@matronics.com] On Behalf > Of Noel Loveys > Sent: Friday, February 12, 2010 6:54 AM > To: avionics-list@matronics.com > Subject: RE: Avionics-List: Re: Thermocouple compatible > bulkhead connector? > > > > Is it reasonable to believe if the connection is going > through a bulkhead that the temperatures will be different on > each side of the bulkhead? > > Noel > > -----Original Message----- > From: owner-avionics-list-server@matronics.com > [mailto:owner-avionics-list-server@matronics.com] On Behalf > Of John Loram > Sent: February 11, 2010 7:31 PM > To: avionics-list@matronics.com > 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: > > > > > > > > > > > > > > > > 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.