1. Extension (X) vs. Compensating (C) Grade

Connecting a thermocouple to a transmitter requires wires that match the thermoelectric properties of the sensor. Using copper wire creates unwanted "Cold Junctions" at the connection point.

• Extension Grade (Suffix 'X'): The wire is made of the exact same alloy as the thermocouple itself (e.g., KX uses Chromel/Alumel). It has the highest accuracy and mimics the sensor perfectly up to high temperatures (usually 200°C insulation limit).
• Compensating Grade (Suffix 'C'): Used for noble metals (Platinum R/S/B) where the wire would be too expensive. It uses cheaper copper alloys (e.g., Copper/Copper-Nickel) that mimic the EMF curve of Platinum only over a limited range (typically 0-100°C or 0-200°C). Outside this range, significant errors occur.

Rule of Thumb: Always use Extension grade for base metals (K, J, T, E, N). Only use Compensating grade for Platinum (R, S, B) to save cost.

2. The Color Code Nightmare (IEC vs ANSI)

Mixing up USA (ANSI) and International (IEC) standards is the #1 cause of reverse polarity errors.

• ANSI (USA): The Negative (-) lead is ALWAYS RED. The Jacket color matches the Positive lead color (e.g., Type K = Yellow).
• IEC (Europe/Global): The Negative (-) lead is ALWAYS WHITE. The Jacket color matches the Positive lead color (e.g., Type K = Green).

Dangerous Confusion: A Type J sensor. ANSI Black is Positive. IEC Black is the Jacket. If you see a white wire, is it Positive (ANSI) or Negative (IEC)? You must know the standard used.

3. Insulation Selection

The environment dictates the insulation, not the sensor type.

• PVC: Cheap, moisture resistant, good for general use up to 105°C.
• FEP/Teflon: Excellent chemical resistance, good for up to 200°C. Standard for chemical plants.
• Fiberglass: Required for high heat (up to 480°C). Not moisture resistant (absorbs humidity).
• Kapton: Used in radiation environments or high-vacuum (aerospace).

4. Law of Intermediate Metals

Compensating cables work based on the Law of Intermediate Metals. As long as the two junctions (Sensor Head connection and PLC connection) are at the same temperature, intermediate metals don't affect the reading. However, Compensating cable is only matched to the TC curve for a limited range. If the connection head gets too hot (e.g., >100°C on an SCA cable), the "Compensation" fails, and errors of 5-10°C can appear.