Pyrometer Readings

[T-Ricky]

I've read and searched for about a week and no answer to my question. My pyrometers (both) appear to read low when the sun has raised the temp of the guage housing. One is a Blue Chip FMS and the other ISSPRO. In my previous career as an instumentation engineer (20 years ago) , seems I read something about the difference in temp. of the junction (exh manifold) and the read out guage (ambient). Looking around the parking lot I noticed that most every ones guage was reading less than ambient temp. Engine,s were off for at least 4 hours.



Talk to me.



Rick T.

 

[Diesel Freak]

the Temp of the actual gauge does not matter... . the temp of the TC does... . if you are thinking of a Wheatstone Bridge, that will give you erroneous readings, however a TC just produces a voltage where the Wheatstone Bridge looks for a difference in voltage.

 

[Neal Cassady]

Same Thing

Rick,



Mine behaves the same way. If my truck is sitting out in the sun the temp on the gauge will always be approx. 50 degrees below the ambient temp.



IIRC the Westach catalog lists two EGT setups, one calibrated for cold environments and one calibrated for warm environments. Something to do with the average temp expected at the junction of the thermocouple/gauge wiring. They have an example with a correction factor based on calibration temp and ambient temp.



I may have to go dig out the catalog to see if I remember that correctly.



Here is a link to the web catalog, my memory was close; however, I'm not sure if that explains what is going on.



http://www.frostalarm.com/cat/p52.html



Dave

 

[Michael Hughes]

When I ordered my Westach EGT/Boost gauge, I was told that the thermocouples are part of a differential temperature system, the cold side of the probe (the end of the wires) needs to be in the cab (around 70F) to get the best readings.

 

[LSMITH]

I agree with Diesel Freak on this, my SPA reads the temp of the manifold after the engine has been shut off for several hours regardless of gauge or interior temp. If you are using a thermocouple the amount of voltage it generates is the determining factor in the gauge reading. We are not using differential temperature gauges for our pyrometers.

 

[Ethdee]

Rick, I've got an instrument background as well.



I just finished a nice 6-paragraph response, and deleted it before posting, after seeing the link that Dave D posted.



I'll clarify something on that drawing. Where they show the cold junction, that means the wires coming from the gauge are copper. Wherever the thermocouple wires end becomes the cold junction. I don't particularly care for that design, for reasons you've already discovered.



Some automotive gauges, like VDO, run thermocouple extension wire (it has electrical characteristics similar to thermocouple wire) all the way to the gauge. I'm sure they also use a fixed cold-junction temperature, but the cold junction is now moved to the gauge, where I think the temperatures are less likely to vary as much as underhood temperatures, if that's where the cold junction is.



Some gauges use "transmitters. " If anyone knows whether or not the transmitters use a fixed cold-junction temperature, I'd appreciate the information.



You see, thermocouples don't generate voltage based solely on temperature. They tell the difference in temperatures between the hot and cold junctions. You have to know what the cold junction temperature is in order to calculate the hot junction temperature accurately. I'm sure the automotive gauge manufacturers saved a few bucks by assuming the temperature was 70 rather than actually measuring it and performing the calculation.



Hope this helps.



Jim

 

[T-Ricky]

Thanks guys. Jim you really jogged my memory from 20 years ago. Being an air traffic contoller for all that time I've forgotten more than I knew about engineering. I'll build a thermal block to put over the Blue Chip and guages along with the thing in the windshield when I'm parked.



Thanks' Rick