Thermocouple problem

Hi guys ,

This is slightly off topic , but there seems to be some very knowledgable people from a wide range of subjects and background
on this site , so i hope you will forgive me

We have a vacuum furnace with a survey frame in it

The survey frame is an oblong wire frame and has four n type thermocouples at the front and four at the rear

The thermocouples are in holders at the top (left and right) the bottom (left and right ) and the same at the rear of the frame

my question is this :

The front bottom right thermocouple reads 4 to 6 degrees lower than the rest , when we do a survey

We have swapped thermocouples and still get a colder recorded temp in this corner at a set temp of 450 deg c

Any thoughts gratefully received

Thanks
Mike

Either the temperature is actually different, there is some unwanted characteristic in the wiring, or possibly, the analog device that reads the thermocouple is out of calibration.

I would try swapping the channels to which the thermocouples are connected to see if the one that is "off" still reads differently when connected to a different channel. That would rule out, or identify the analog input device as the problem.

Paul

what are the thermocouples connected to? ... is there a separate transmitter for each one? ... is there ANY difference between how the transmitters are mounted? ... specifically, is there a chance that the transmitter for the "problem" thermocouple is somehow getting "hotter" than any of the others? ...

basic line of reasoning: it is NOT how hot the thermocouple gets that produces the temperature reading ... instead it's the DIFFERENCE between the thermocouple's "business end" and the "cold-junction" (transmitter, etc.) end ... in other words, if you were to put the thermocouple AND the transmitter BOTH in the oven, then the reading would always be ZERO ... so ... if one of the transmitters is somehow getting hotter than the others, then a lower reading would be expected from that one ...

remembering a life long gone: I used to work on restaurant equipment for a living ... I had a LOT of competitors who never could seem to properly calibrate the controls on Pizza Hut's ovens ... they'd put the thermcouple probe into the oven - and then set the "meter" device ON TOP of the oven ... naturally the device got nice and warm up there - which gave a much lower-than-correct reading ... they'd jack the thermostat way up to compensate - and eventually dial in a "perfect" temperature setting ... but the pizzas would always burn ... finally I'd get called in ... I'd hang my "meter" from the ventilation hood with a piece of chain ... good reading ... good adjustment ... good pay ...

man I miss those days sometimes ...

good luck with your problem ... and I hate to mention the obvious - but is there any chance that the "problem" area in the oven really IS just a slightly different temperature? ... how hard would it be to temporarily relocate a "good reading" thermocouple probe over right next to the problem one? ...

If by 'swapped T/C's' you mean that you substituted another T/C and the same physical point measured too cold with two different T/C's, then either that physical point is too cold or there's an error in
- the thermocouple itself
- the thermocouple and/or lead wire connections
- cold junction compensation
- analog input calibration
- reused thermocouples

It would be rare to find a common mode problem so consistent that you would see a consistent, fixed offset in one channel only, but theoretically possible (pretty remote).

Do all T/C's come from the same calibrated spool?
Are they all constructed the same?
Fixed to the rack fixture the same way?

Has this particular vacuum furnace passed a survey in the past, or is this a "first time" survey?

Are you reusing thermocouples or are these first time use T/C's?

Are the survey T/C's continuous from the hot end to the analog input, in other words, does each T/C 'pass through' a port in the wall of the furnace and then connect to the analog input?

Or are the T/C's terminated on the vacuum side to a terminal block which is then connected on the outside to the analog input with extension wire?

Are plugs and jacks used at any point in the wire run?

Thermocouples that are reused can develop a 'virtual junction' at major heat transition points, typically where the T/C goes from cold to hot, something to do with crystallization of the metals in the T/C wire. As long as that junction stays in the same physical point, the T/C can be reused and the effect isn't noticeable. But if a T/C is moved about (one furnace to another, or used for a survey, removed, then used again for a survey) so that the virtual junction point moves into the hot zone, the T/C then develops a different thermal gradient the virtual junction, and the 'false virtual junction' adds to or subtracts from the thermal gradient.

Can cold junction be calibrated individually per channel, or is it common for all channels? Are the AI terminal connections shielded or exposed so that drafts affect all connections equally?

Is the 'Too Low" channel the outside channel on an 8 AI input card? Is that channel affected by cooling drafts, like from a VFD cooling fan?

Moving the cold T/C to a different analog input channel can indicate whether the problem moves with the AI used or not.

Dan

eebads2391 said:

Hi guys ,

This is slightly off topic , but there seems to be some very knowledgable people from a wide range of subjects and background
on this site , so i hope you will forgive me

We have a vacuum furnace with a survey frame in it

The survey frame is an oblong wire frame and has four n type thermocouples at the front and four at the rear

The thermocouples are in holders at the top (left and right) the bottom (left and right ) and the same at the rear of the frame

my question is this :

The front bottom right thermocouple reads 4 to 6 degrees lower than the rest , when we do a survey

We have swapped thermocouples and still get a colder recorded temp in this corner at a set temp of 450 deg c

Any thoughts gratefully received Thanks Mike

Click to expand...

Mike
I did testing on ovens at Boeing from fairly low temp (200 F) to high temp 1800 F (heat treat ovens).
We put a TC in each corner of the "box" and one in the center.

We had the same problem you are facing one TC would not be within tolerance.

FIRST
With a 4 degree error at 450 you are looking at less than 1% error. I asked the question at Boeing and am going to ask you
- do you REALLY need this tight a tolerance?
SECOND
IF you swap thermocouples and get same results in same corner.
IF the door hinges are on left hand and latch is on right I would suspect a leaky door gasket
OR less insulation in that area
OR warped door.

Dan Bentler

I have similar problem before where a TC show a few degree lower than actual and it turn out that the analog input module that i use is 4-channels and all 4 channels need to be use.

If 1 channel is free or not used then all channel will have wrong measurement.

That is the purpose of a survey frame, to determine heat distribution in your furnace.

This is a vacuum furnace. That means that there is one method and one method only by which heat energy is transfered in the furnace - radiant heating. In a vacuum that is line of sight. Its possible to have a cold spot right next to a very hot spot. Under vacuum things are not always intuitive, but if you have only four degrees difference then your furnace's heat distribution is excellent.

Hi Sohwel,

For thermocouple and other low level analogue inputs it is
good practice and often necessary to short out any unused channels.

Wow , gentleman, Thankyou ! !

I will look at all this and give you as much needed info as fast as i can

The thermocouples are brought out to female n type sockets, under neath the furnace

They are wired using extension cable ( correct type ) to a survey chessel recorder on a wheeled trolley alongside the furnace

The whole set of thermocouples inside have been swapped ( when one at the front , read lower than the others )

we have also swapped channels

Then , for good measure , swapped the low reading thermocouple again

we do appear to have a reading of what looks like a lower temp in this area , i just cannot find any reason , so am just trying to rule out thermocouple error completely

Leitmotif , the door insulation has a little bit of gap , but we packed it with resistant wool and found no difference

Alaric , thankyou . . had not considered shadows too much

Sohwell/John , good point will check , its a 20 channel paperless chart recorder , and we are using 8 channels

Dan w /Ron / Okiepc , will need longer to try every suggest , the thermocouples on the survey are new each survey , the wires from furnace to recorder are new there is no seperate transmitter per channel

Another question to do with this , is as a last resort , we are allowed to use a permanent offset to allow for a slight temp skew

How would you apply this and where ?

Thanks guys ! !

When you get into vacuum line of sight issues can become a big deal. Without anything to difuse the thermal radiation from the elements and without any convection you can get some counter-intuitive things going on. (Thats what trips up all the moon landing hoax nuts over shadows and reflections) Also your elements are not going to radiate heat evenly, especially graphite elements. Is your hot zone wrapped in elements top, sides, and bottom? Or just the sides? Is the heaing power supply single phase or three phase? If you have three phase elements take a look at how they are arranged, even a slight variation in one element can cause it to run just a tiny bit cooler. We re-survey our furnaces when we change elements as there can be changes to the hot zone map simply due to element differences.

Also, when you have your survey frame in the furnace, do you have a thermocouple reading from the center? If so, is that your control T/C? If you don't have center T/Cs then do you have vacuum feedthrus where you can you add them? The reason is that a center TC receives radiant energy from nearly all sides if you have wrap-around elements, while the corners to not, so depending on your element design you might find that all 8 corners are cooler than the center.


One thing I have found makes a difference in keeping the elements balanced in three phase elements and parallel single phase elemlents is keeping the connection integrity high at any low-voltage high current vacuum feed-thrus. There is a product on the market called Cool-Amp. Its a silver paste that I burnish into the copper at each connection point to lower connection resistance. It seems to help.

Hi Aleric ,

Yes, it has heating zones, on sides top and bottom ,
3 phase delta wired as single zone heating

we removed all elements and found some of the insulated support blocks had very low readings to earth , replaced ceramics and support rods and re ran survey , unfortunately still 5-6 deg c low in the one (same ) corner , got a lot of ideas from all who replied (thankyou) to try but really frustrating so far .

You mention a 'recorder', which is typically used for furnace surveys. Although your single point deviation is very likely a cold spot, you still might want to investigate whether your recorder has a peculiar offset on that one channel.

Most recorders have at least one, somethimes two or even three places in the setup (configuration) where a channel reading can be offset.

There's always a bias or offset factor on each channel or pen.

Sometimes there's separate cold junction compensation on each channel, where each channel has a CJC offset, but it amounts to the same effect: any given channel can have a setting different than the others.

Then some recorders have a "single point sensor compensation", which is yet another spot for a an offset to be used.

Dan

Hi to All ,

thankyou for all your great input

we could not completely discount the heating elements , so stripped them down to check on insulator blocks , looks like two or three have earthing probs when we test with the meter , so just spent the day replacing insulator ceramics and re testing

will be running it tomorrow on survey and hope this was the main problem

John / Dan ( and all ) thanks for the replies

Mike