Oil temp vs. cylinder head temp question, one low and other high Options

[phantom914]

QUOTE (lapuwali @ Apr 13 2005, 02:05 PM)

QUOTE (Jake Raby @ Apr 13 2005, 12:19 PM) Yep, thats the way it was explained to me by a tech at VDO. Because of this I run westach instruments and their thermocouples. Are Westach gauges temperature compensated? The VDO "problem" is a function of how thermocouples work. Unless there's some active compensation for ambient temperature and how it varies from the calibration standard, any thermocouple setup will have the same problem. Note that if the temperature is BELOW the calibrated value, the gauge will read HOT. So, if it's 20dF outside, a cylinder at 300dF will show 350 on the gauge. Thermocouples work by showing the difference in temp between two ends of the thermocouple pair. One spot under the plug, and the other spot at the other end of the thermocouple pair. On the VDO senders, the thermocouple pair is only 12-18" long, so the "cold" end of the pair is still in the engine bay. Since the VDO gauge is calibrated under the assumption that the cold end of the pair is at 70-76dF (I've heard 70, Jake has heard 76), the gauge will always read cold in a 914, as the ambient temp at the cold end of the pair will usually be around 120-130dF. There are some that contend that it's the temp of the gauge, not the temp of the cold end of the pair, that determines the "cold" calibration temp. I don't know for certain either way. If the Westach gauges have a second temp sensor to correct for ambient temp errors, that's a wonderful thing, given that they're no more expensive than the VDO units.

The temperature of the "cold" end of the thermocouple junction is what is important as far as the compensation for ambient temperature. But, depending on design, the gauge itself can also introduce error if it is not compensated for temperature well.

So in other words, the ambient temperature of both the gauge and the gauge end of the thermocouple wire are important. The effect of the ambient temperature on the gauge end of the thermocouple wire (junction) is predictable and can be compensated for, but the effect of ambient temperature on the gauge itself would be different for each design.

In my work with thermocouples, an IC temperature sensor was in the measurement device near the junction to measure its temperature in order to compensate in software. Any meter you buy will need to do this to be accurate and the gauge electronics itself will also have to be consistent under varying temperatures.


Andrew

[tat2dphreak]

QUOTE (Tom Perso @ Apr 14 2005, 03:35 PM)

Yup, that 2056 came right here from Kalamazoo... (IMG:http://www.914world.com/bbs2/html/emoticons/smile.gif) Got that thing "DRIVING" yet? I know it run!!! (IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif) Later, Tom

I need to put the seats in and it will drive... I am going to paint the floor this weekend I think... I got sidetracked with rust (IMG:http://www.914world.com/bbs2/html/emoticons/ar15.gif)

other than seats, all I need is the normal legal hurdles.. insurance,inspection, and registration...