VDO CHT gauge thermocouple and testing

[juki48]

I know I know, VDO CHT gauges are junk, get Dakota Digital or another better one. So now that that's out of the way, I got a VDO unit new on amazon for $40 so I figured I would mess with it. it comes with a 14mm ring so I need to make a 12mm one. when I started looking into it I found the VDO CHT uses a J type thermocouple, not K like I have read on some forums. I verified this by not only the color of the insulation and wires themselves but by putting a mini connector on it and comparing it to new J and K types using a fluke 52 II thermometer. it's a J.

one weird thing I have found is the gauge seems to be sensitive to output current. this is not normally the case with thermocouples but this is a pure analog gauge reading the mV of the TC junction. a small contact of the TC wires = lower reading than large contact of theTC wires. using the Fluke they read exactly the same, but on the analog VDO there is a difference. the difference is about a needle width above or below the gauge reading, probably around 10°F at 350°F. I may try to calibrate the gauge by changing the contact are of the wires when I crimp them in a loop terminal.

By the way, this guage is almost dead on from 150°F to 400°F with the guage, and therefore cold junction, at room temp. I haven't gone over 400 yet, I have to switch to a different thermal chamber.

I'm also not using the copper extension wires, I'm using J type wire so the cold junction is at the gauge where the temps will be a little more consistent. I'm also using J mini connectors so I can unplug and compare the reading to a real thermometer when installed in the car.

[Max Welton]

Keep track of the cold junction temp. It is allegedly the achilles heal of that system.

Max

[juki48]

yes, there will always be the need to understand the effects of the cold junction with the VDO. I think moving it to the gauge by using TC wire instead of the copper wire will be a huge improvement. a digital system is hands down a better option. I just figured I would post what I've found messing with the VDO.

[juki48]

In regards to my previous comments about current, small TC wire less than .020" doesn't work well. even .020" seems to read low unless I strip and twist about 1/2" at the end, then the reading is correct. once it reads correct adding more contact doesn't seem to change the reading. It must need lots of contact to create enough signal strength to run the analog meter. not sure why but that's what I'm seeing. I'll just use the wire that came with it twisted and crimped into a 12mm terminal then use smaller TC wire all the way to the gauge.

[Dale M.]

In the world of ceramic kilns and glass annealing ovens it common practice to use same type wire for extension to gauge or controller as the thermocouple is designed with and not convert extension to "just" copper wire... Most thermal couple manufacturers recommends to keep within same wire type for extension also...

More information then you ever wanted to know....

http://www.omega.com/temperature/Z/zsection.asp

Dale