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Date:         Sun, 28 Dec 2003 23:02:09 -0800
Reply-To:     David Marshall <vanagon@VOLKSWAGEN.ORG>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Marshall <vanagon@VOLKSWAGEN.ORG>
Subject:      Re: What makes a pyrometer's cable so special? PT-II
Comments: To: Keith Hughes <keithahughes@QWEST.NET>
In-Reply-To:  <3FEFBED5.9090905@qwest.net>
Content-Type: text/plain; charset="iso-8859-1"

Wow! I lot of response to my question. This is great!

The VDO gauge that I am using uses a thermocouple, so says the installation guide. The resistance of it is under zero ohms. The length of the cable that came with the kit is 12' and the additional cable that I bought is 14' Measuring resistance on the red wire, it is 1.2ohm and the yellow wire is 3.2ohm.

The thermocouple end of the 12' that came with the VDO gauge is simply some inexpensive looking 18-22 ga 'eye' connectors crimped on the end of the wires. Each wire is offset by about 1" so they can not touch each other. The VDO gauge end of this cable is just the two wires terminating into the 'open air'. As in you have to crimp on your own female spade connectors to this end. To attach the cable to the thermocouple they supply some small (M2ish) screws with nylock nuts to fasten the coupler to the cable. The cable looks a lot like the cable you see used between a household furnace and the thermostat - i.e. nothing fancy other than a tough outer skin.

David Marshall

Fast Forward Automotive Inc. 4356 Quesnel-Hixon Road Quesnel BC Canada V2J 6Z3

http://www.fastforward.ca mailto:sales@fastforward.ca Phone: (250) 992 7775 FAX: (250) 992 1160

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-----Original Message----- From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com]On Behalf Of Keith Hughes Sent: December 28, 2003 9:43 PM To: vanagon@GERRY.VANAGON.COM Subject: What makes a pyrometer's cable so special?

Well David, it depends;

If it's a thermocouple, then you need to use thermocouple extension wire (of the same type, "T", "J", "K", "R", "S", etc.) and of sufficient gauge to minimize the resistance. The affect of a few feet of, say, 16-18ga extension wire is negligible on the calibration. However, if you use regular copper wire, then you're creating a secondary thermoelectric junction. The thermoelectric potential generated would then result from the temperature differential between the TC tip and the connection point to the extension wire - *not* between the "meter" (where the zero junction compensation is measured) and the tip. The resulting error is the difference between the ambient temperature at the meter, and that at the connection to the extension cable (plus some minor thermoelectric offset caused by the additional junction at the meter terminals).


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