RTD Sensors in Industrial Applications

Process Control

• Process refers to the methods of changing or refining raw materials into usable products
• Raw materials may be liquid, gaseous, or mixtures
• These materials are processed by being transferred, measured, mixed, heated/cooled, filtered, stored, or otherwise manipulated to create an end product
• Examples of process industries include chemical, oil and gas, food and beverage, pharmaceutical, water treatment, and power industries

What is Process Control?

• Process control is about controlling the variables during product creation
• Control is implemented for several reasons:
  • Reduced product variability leading to quality improvement
  • Increased efficiency in production saving resources
  • Ensured process safety reducing the risk of catastrophic events like the Chernobyl disaster

Control Point Instrumentation

• Control point instrumentation refers to the instruments and sensors used to control process variables
• Key instrumentation includes:
  • Resistance temperature detectors
  • Thermocouples
  • Pressure transducers
  • Flow meters
  • Oxygen sensors
  • Servo motors

Process Variables

• Process Variables or PV is measured by instruments and acts as input to the controller.
• Common process variables include:
  • Level
  • Flow
  • Temperature
  • Pressure
  • Density
  • pH (acidity or alkalinity)
  • Mass
  • Conductivity
  • And more

Resistance Temperature Detector (RTD)

• RTD is a temperature sensor using the change in electrical resistance of a metal to measure temperature changes
• Common metals used in RTDs include:
  • Platinum
  • Nickel
  • Copper
• Platinum is most commonly used in industrial applications
• RTDs are protected by a metal sheath forming a probe and housed for protection

RTD Construction

• RTDs are constructed in three ways:
  • Wire wound
  • Coiled element
  • Thin film

Wire Wound RTD

• A resistance wire is wound around a non-conducting core (usually ceramic)
• The wire length is precisely trimmed to achieve the specified resistance (RO) at 0°C
• Lead wires are attached and coated for protection

Coiled Element RTD

• Resistance wire is coiled, placed in a ceramic form filled with non-conductive powder
• This allows for expansion and contraction during temperature changes minimizing error
• Protected with metal sheathing

Thin Film RTD

• Mass produced, smaller, and faster response time
• Platinum deposited on a ceramic base
• Resistance adjusted at 0°C using a laser beam

How RTD Works

• RTDs change resistance proportionally with temperature
• This change is well-known and repeatable over time
• External electronic devices are used to measure this resistance (passing a small electrical current through the sensor)
• Lead configurations (2-wire, 3-wire, 4-wire) compensate for lead wire resistance to ensure accurate measurement

Thermocouple

• A thermocouple is an electrical device with two dissimilar conductors forming an electrical junction.
• Heat (temperature) causes a voltage (emf) difference that can be interpreted to measure temperature.
• Housing is similar to RTD
• Change in voltage/emf interprets the change in temperature

Key Differences Between RTDs and Thermocouples

• Thermocouples have a wider operating temperature range than RTDs(-346° C to 2320° C) compared to RTDs which typically operate from -200°C to +600°C
• Thermocouples have a faster response time (.1-10 seconds) than RTDs (1-50 seconds)
• Thermocouples tend to be less expensive and easier to install than RTDs

Pressure Transducer

• A pressure transducer converts pressure into an electrical signal
• The signal increases or decreases proportionally to the pressure change.
• The electrical signal is used by other devices (like controllers, alarms, loops)
• Common output signals are millivolt, voltage, and 4-20mA output

Mass Flow Meters

• Conventional flow meters (orifice plates, venturis, vortex, turbine meters) measure volume
• They require pressure and temperature transmitters to account for density changes -Thermal mass flow meters measure gas mass flow without additional hardware
• Use constant temperature differential technique with matched platinum RTDs.

O2 Sensors

• Burner setups are typically adjusted for about 3% excess oxygen, which is around 15% excess air
• Ambient and atmospheric conditions can affect oxygen supply, necessitating careful adjustments
• Automatic O₂ trim systems continuously monitor flue gases and adjust fuel/air accordingly using PLC control systems

Servo Motors

• Servomotors are rotary or linear actuators for precise positioning, velocity, or acceleration control in applications like air dampers and fuel valves
• Consist of a motor and position-sensing mechanism
• Used in parallel positioning fuel/air ratio control systems (FARC) systems to independently control various components of systems like combustion air dampers, gas valves etc.