Help with thermocouple confusion

[danw]

The allowable error by ANSI standard, is about ±4°F, ±2.2°C, new, out-of-the-box, never subjected to heat. Performance only degrades with use.

Note that the error is plus/minus. If one T/C goes one direction (polarity), and the other goes the other direction - the difference can be quite substantial.

 

[iant]

just a few off the cuff thoughts, appart from the points raised.
1. Is the laser temp sensor and both Temperature probes reading from exactly the same spot?????
2. Is the chart reader taking it's information at the same time?
3. What type of stabilisation does the chartreader's input do?
4. Are both the T/C and Chart recorder syncronised?

Might not mean much - but poses some additive errors in the display process - dosent it.

EDIT
Also - could not help myself - using the watch example.
watch one watch for 1 second
then
look at the other watch does it show exactly the same time as the other watch.
= it can not - that was 1 seccond earlier

 

[Lancie1]

1. Is the laser temp sensor and both Temperature probes reading from exactly the same spot?????

I can answer that one: It is always no, because two objects cannot occupy the same exact physical space (because they do not have 0 volume). Six inches in many applications (furnace temp measurement) can easily have 10 degrees F difference, specially if they are mounted verticaly one above the other.

Erika, it back-to-school time for studying thermocouples!

 

[iant]

@Lancie - hi mate - it was a rhetorical question (I knew it was not possible) worked on many blow Moulders and a few injection ones as well.

I was meaning exactly what you answered - there are many heating elements and heating zones.
Unless the testing is at the same point the temperature reading will naturally be different.

 

[leitmotif]

Ripley

I can only add that everything that has been said is all good stuff and well worth your time to read twice. There is a lot of info here and some is bit hard to grasp.

If I had to start somewhere to get information I would start with Omega - I have found their literature to be accurate and fairly easy to read.

I believe the best way to go is continuous thermocouple wire - no splices cold joints (except at sensing end). With special wire you can splice but for my needs 100 feet or less I have been told it is besst to go with TC wire and no splices. Worked well so far and when all said and done about same cost.

Get rid of the copper wire and get the right stuff
OR if you can - determine what the error may be and add in an error factor.

To get pragmatic here with some rough assumptions
ASSUME 170 C and an allowable error of 1%. So error either way can be 1.7 or your readout can be between 171.7 to 168.3. this is real close to what you are reading and would be acceptible. I think +/- 2C is more than good for most industrial operations. I think your customer is expecting too much of his equipment and for the most part it is just fine. Overcome the errors ie copper wire and you should be in great shape.

IF you are at sea level you can generally rely on melt temp of ice and that of boiling water. That will give you a two point calibration curve which may be just good enough - it may be reaching too far since you are working around 170 C. If at elevation then you will have to correct for barometric pressure - which you really should do at sea level for best accuracy - but generally close enough for most work.

Dan Bentler

 

[DwSoFt]

Im surprised nobody brought this up yet. why not switch the Thermocouple on the chart recorder and the one into the plc. like dont move the thermocouples, just put the chart recorder reading off the plc TC, and vise versa. see if you see the exact opposite problem as before. that would help to prove there is no actual problem with his equipment. i also agree ab out using the thermocouple wire over regular wire

 

[Lancie1]

I suppose Erika solved the thermocouple problem, because now she is working on the VFD problem!

 

[theripley]

I suppose Erika solved the thermocouple problem, because now she is working on the VFD problem!

Yes I hope I did. All your comments were eye opener to me. Thanks a lot y'all