How to Wire and Calibrate a Thermocouple?

 

Wiring a thermocouple

Temperature is provide by a thermocouple sensor connected to a ACE PLC ACE 1450, 3090, 7096,.. have thermocouples inputs. Our Type K thermocouple is wired as follows: Red wire > Input + White wire > Input – Reverse the wire if the temperature is not correct. Sometimes, with other thermocouples, the + and – can have different colours. See the thermocouple manual.

Precautions for wiring a thermocouple probe to a ACE PLC input

• Use compensation wires of the same type as the thermocouple to avoid measurement errors
  • Cable extensions, if unavoidable, should be as short as possible.
• Observe the polarity of the thermocouple when connecting to the PLC input.
• Avoid unnecessary junctions and loose connections that create parasitic voltages and unstable temperatures.
• Separate the thermocouple wiring from the power cables to limit electromagnetic interference.
• Ground the shielding on one side only if the cable is shielded. Ensure that the GND quality is adequate.
  • or connect the cable braid to 0 VDC
• Check that the controller input is configured for the correct type of thermocouple and measurement range.

 

Calibrate the thermocouples

After selecting the correct thermocouple (Type K) in the configuration wizard…

Choose “Calibrate Thermocouple” from the vBuilder menu.

 

Check therocouple you’d like to calibrate Inputs F1, F2, F3 or F4 forACE-3090 as example Set the current temperature, then, press calibrate You can select a second point to obtain a more accurate measurement. For example, 21°C for the first point and 0°C for the second point.

 

If you copy the thermocouple input (InFloatF4 in our case), into any other tag, just to see the value in debug mode, you will see the temperature appear.

 

Abnormal temperature spikes on thermocouple measurements

The most common problem is poor connections in ACE connectors. You should use crimp ferrules, especially if the wire is very thin.

• Electrical noise
  • EMI from motors, VFDs, relays,..
  • Poor grounding or shield connection
• Intermittent wiring fault
  • Loose terminals
  • Damaged thermocouple or extension wire
  • Vibration induced micro cuts
• Wrong cable type
  • Incorrect compensation cable
  • Mixed thermocouple types in the loop
• Parasitic junctions
  • Dissimilar metal connections at varying temperatures
  • Non isothermal terminal blocks
• Cold junction compensation issue
  • Faulty CJC on the input module
  • Rapid temperature changes inside the cabinet
• Aging or damaged probe
  • Oxidation or contamination of the hot junction causing instability