Thermocouple type-K

  • #26
jim hardy
Science Advisor
Gold Member
Dearly Missed
9,847
4,888
Yes i was the one who first mentioned gas . I thought that's what it was.
Being from Florida i haven't seen a lot of 'Furnaces' - to me that word means an appendage to central air conditioning unit, usually gas fired .

Definition of furnace

  1. : an enclosed structure in which heat is produced (as for heating a house or for reducing ore)

Sorry for the derailment. But i do hope it helps NTL with his thermocouple .

Presumably your thermocouple connects to some sort of electronic temperature controller ?
 
  • #27
38
0
Yes i was the one who first mentioned gas . I thought that's what it was.
Being from Florida i haven't seen a lot of 'Furnaces' - to me that word means an appendage to central air conditioning unit, usually gas fired .



Sorry for the derailment. But i do hope it helps NTL with his thermocouple .

Presumably your thermocouple connects to some sort of electronic temperature controller ?
Yes, I think it was my bad that from the beginning I did not mentioned the details. but it is an experience for me any question must be in details.
yes it will be connected to a temperature controller. The problem is here. The out put of thermocouple to a temperature control unit must be two. But as you can see in the photos there are 3. I still do not know what is the reason for that.
 
  • #28
779
527
The two wires going to the brass terminals are (+) and (-) thermocouple output. The third wire is unexplained, but may have been intended to ground the thermocouple.
 
  • #29
jim hardy
Science Advisor
Gold Member
Dearly Missed
9,847
4,888
I agree with @Asymp

it would make sense to ground a thermocouple that's in proximity to high powered electric heaters.
Especially if it's not in a thermowell .

But machinery is full of surprises....
If we knew the nature of the device that reads the millivolts we could venture better guesses.

old jim
 
  • #30
Averagesupernova
Science Advisor
Gold Member
3,916
818
My guess the third wire is to help with noise imunity while preventing the whole thermocouple from floating. I would think there would be better noise immunity when this third wire is grounded back at the amplifier/transmitter than just grounding it at the weld. I would think of it as a shielded twisted pair microphone cable.
 
  • #31
779
527
My guess the third wire is to help with noise imunity while preventing the whole thermocouple from floating. I would think there would be better noise immunity when this third wire is grounded back at the amplifier/transmitter than just grounding it at the weld. I would think of it as a shielded twisted pair microphone cable.

A propensity for noise issues depends on controller instrumentation amp design as well as TC construction and wiring. I ran into this when replacing an ancient Barber-Coleman MACO V control system with a (then, only recently introduced) Allen-Bradley SLC500 PLC equipped with a half dozen 1747-NT4, 4 channel thermocouple input modules. Temperature measurements drifted by a degree or so, some more than others, and apparently at random, but closer investigation revealed it depended on how many zones were simultaneously calling for heat. Turned out the -NT4 module was more sensitive to common mode noise than the MACO V (several years later A-B introduced an -INT4 module with better isolation), and a quick fix was to swap out the original grounded thermocouple probes for ungrounded ones. Another option was to replace all the TC extension wire runs with shielded TC extension cable, ground the shield at the TC plug negative connection (in the process introducing an ongoing maintenance issue), and bond the negative TC extension wire at the module's analog common.

1747-NT4_Input.jpg
TC input wiring

It was a long walk to get here, but most other manufacturers also recommend bonding a grounded thermocouple's cable shield to the source (TC) rather than at the instrument side. The cable shield for an ungrounded TC is grounded at the input module.

Ungrounded thermocouples aren't a panacea, and don't necessarily help with noise. Many TC instrumentation amps connect the inverting input to ground through a high resistance. It they don't, any charge that builds up on the wires through op amp reverse bias leakage, static, etc. has nowhere to go, and affects measurements.

AD_TC_Figure 6_Circuit.jpg


Figure 6 was borrowed from an excellent Analog Devices technical article, "Two Ways to Measure Temperature Using Thermocouples Feature Simplicity, Accuracy, and Flexibility" regarding their TC amplifiers.

In the OP's situation I'd bet there is a screw within reach of the TC ground wire with witness marks indicating prior connection.
 

Related Threads on Thermocouple type-K

Replies
3
Views
2K
  • Last Post
Replies
6
Views
9K
Replies
3
Views
2K
  • Last Post
Replies
1
Views
2K
  • Last Post
Replies
10
Views
1K
Replies
1
Views
2K
  • Last Post
2
Replies
27
Views
33K
S
  • Last Post
Replies
5
Views
2K
Replies
1
Views
4K
Top