Steve Arndt wrote:The TC wire generates a voltage potential based on the temperature difference between the hot junction and cold junction. The length doesn't matter within reason.
You can run one TC lead 10 feet, and another one 100 ft. Same reading on the gauge, meter, etc. This is the job of the TC amplifier circuit and a function of the physics. These wires are run from 5000 ft spools in industrial applications. Do you think they measure the lead length and calibrate it based on that?
The confusion comes where VDO cheaps out and sells plain copper wires as TC extensions. This isn't how it is supposed to be done.
This is what I had always thought, from what I knew about thermocouples from high school, but then someone launches into a discussion on how the length matters.
Now, knowing that TC junctions create tiny little voltages and that every wire has an inherent resistance, I wanted to see if that was where he was going with that.
Steve Arndt wrote:Thanks Pyle.
Maybe my 8 years of ME school are paying off. :p
I always say that "management" is a 4 letter word...
Which is also why I'm not an EE, as I suck at math.
I just get to fix really interesting stuff for a living
Addendum to Newtons first law:
zero vehicles on jackstands, square gets a fresh 090 and 1911, cabby gets a blower.
EZ3.6 Vanagon after that.(mounted, needs everything finished) then Creamsicle.
With all due respect Pile; and I mean that, I do respect your opinion, here's what Westach says:
"We use standard copper wire to extend the leads up to 20 feet.
The cold junction happens as soon as you terminate the thermocouple leads with anything other then more thermocouple wire of the same type. So the cold junction "error" is still at the end of the thermocouple leads where they meet the copper wire. Be this at the back of the gauge or 20 feet away.
The added resistance of the copper wire to the whole system is negligible but after 20 feet it does start to make more of a calibration problem. That is why we hold it to 20 feet, after that it is too severe.
Thermocouple wire has a higher resistance value then standard copper so using that in a passive thermocouple system just doesn't work well.
Now if you were to have the cold junction exactly at 75 deg f and the hot end was at a known temp, say 400 deg f. the gauge will read 400 deg. Now throw in 20 feet of copper and yes the gauge might read 395. But due to the fact that we don't always know exactly what that cold junction temp is on an automotive application with changing temps and driving conditions etc. that has more of an effect on the readout then anything else.
Remember that the thermocouple wire cannot run all the way through the whole system, it has to change to copper somewhere. The connections on the back of the gauge, the wire from the back of the gauge to the circuit board, the wires from the circuit board to the meter movement, the meter movement itself is full of copper wire.
Yes it is true at an engineering standpoint to use thermocouple wire for the whole length but in real world applications it is the point of just where that cold junction is and what it's temperature is.
In the stuff I work on that +/- 5-10 degrees C of variation is not acceptable, so passive readout is out of the question.
If you don't mind a 40 degree "calibration" swing during the course of some days...
For me, it's cold point referenced and at least comparable to a traceable std or don't bother, and passive CHT gauges are dash filler.
I just like to know what my motor is really doing.
Addendum to Newtons first law:
zero vehicles on jackstands, square gets a fresh 090 and 1911, cabby gets a blower.
EZ3.6 Vanagon after that.(mounted, needs everything finished) then Creamsicle.
Actually, based on the location of the cold junctions in my ACVW's, even a 25* variation from the cold junction calibration temp of 75* is very unusual.
If it is 100* outside, the gauge would read 25* lower than actual.
Whether I'm driving my Thing with the top down in Summer, or my Beetle in Winter, I am aware of the cold junction temperature.
So my CHT gauges are not "dash bling". Nice try though. "Really"
As far as I know, only Dakota Digital CHT gauges claim to compensate for cold junction temperature, and they are way too ugly to consider.
Mondshine
My Dakota Digital (DD) gauge mirrors my Fluke TC meter. They are within two degrees of each other from 60 up to 270 F. So the DD CJC works as well as the certified/calibrated Fluke.
Gary, it's great to know the Westach gauges are that accurate, as I'm sure you have personally verified yours.
I know the VDO setups can easily read 50-75F off true even when it's 75 F out.
Considering that the cruise CHT delta between "perfect" and "park it" is 375 vs. 400F, to ME, that's unacceptable.
I don't have a dash gauge anymore, but even my old Actron DVMs TC setup is cold referenced and checks out vs. a calibrated Fluke TC within 2C from 25-300C.
Addendum to Newtons first law:
zero vehicles on jackstands, square gets a fresh 090 and 1911, cabby gets a blower.
EZ3.6 Vanagon after that.(mounted, needs everything finished) then Creamsicle.
Pile-
I have checked both the thermocouple (cht) gauges as well as the thermistor (ot) gauges using a small electric deep fryer and peanut oil which I am able to heat to above 400*.
The gauges have always been in agreement (within a couple of degrees) of several thermometers. Yes, I am still married despite the occasional use of kitchen equipment in the garage.
I do have confidence in Westach products after over 10 years of use. They are made in the US, and I believe them to be much higher quality than the VDO products currently made "wherever".
In my experience, a Beetle has a much better setup for allowing cooling air to reach the fan inlet. My Beetle will rarely exceed 325 cht at 70 cruise.
The Thing, on the other hand has problems in this area. I have solved it with a duct leading directly to the fan inlet which dropped cht considerably.
This month, on my return trip from the Thing meet in Hiawassee, GA to Detroit, oil temp never went over 220, CHT never over 325. I use Brad Penn 10W30 in both cars.
Total time 11:41, Moving time 11:14 690 miles total. I only stopped to gas up and P.