Thermocouples

Overview

Engineering reference data for Thermocouples in process control.

Key Formulas

PID Controller

$u(t) = K_p e(t) + K_i \int e(t)dt + K_d \frac{de}{dt}$

Proportional-Integral-Derivative control.

Transfer Function

$G(s) = \frac{K}{\tau s + 1}$

First-order system.

Variables

Thermocouple Types and Properties

Thermocouple Instrument Ranges and Accuracy

Advantages and Disadvantages

Advantages

• Wide Temperature Range: Capable of directly measuring temperatures up to 2600°C.
• Direct Contact: The thermocouple junction can be grounded and brought into direct contact with the material being measured.

Disadvantages

• Requires Cold Junction Compensation: Measurement requires knowing the temperature at both the hot junction and the cold junction (where wires meet instrument leads). This cold junction temperature is typically compensated electronically.
• Complex Operation & Error Sources: The thermoelectric voltage can be influenced by factors like corrosion along the wire.
• Non-Linear Signal: The relationship between process temperature and the millivolt output is not linear.
• Calibration Challenges: Calibration requires comparison to another thermocouple; removing the thermocouple for a bath calibration may not reproduce the integrated signal exactly.

Thermocouple Classification

Thermocouples are categorized into four classes based on their materials:

• Base Metal: Types E, J, K, N, and T (the most common industrial types).
• Rare/Precious Metal: Types B, S, and R, made with platinum alloys.
• Refractory Metals: High-temperature alloys.
• Exotic/Standards: Includes tungsten-alloy types (e.g., Type W-Re).

Temperature Conversion Formulas

The relationships between temperature scales are:

$$^\circ F = (1.8 \times ^\circ C) + 32$$ $$^\circ C = (^\circ F - 32) \times 0.555$$ $$K = ^\circ C + 273.15$$ $$^\circ R = ^\circ F + 459.67$$

Interactive Charts

References

• Original Source