In-Depth Notes on Types of Sensors and Their Principles

Sensor Classification

• Sensors can be classified based on:

  • Physical Principles: Resistive, Capacitive, Radiative, Optical

  • Output Signals: Electric, Mechanical, Thermal, Optical

Types of Sensors by Output

• **Electric Output Sens:

  • Passive Sensors:

  • Require an external power supply for voltage or current output.

  • Examples: Resistive, Capacitive, and Inductive sensors.

  • Active Sensors:

  • Do not require an external power source.

  • Examples: Electromagnetic sensors, Thermoelectric sensors.

Intrusion Types of Sensors

• Intrusive Sensors:

  • Placed inside the pipeline.

  • Can be in-contact or non-contact with the process flow.

• Non-Intrusive Sensors:

  • Placed outside the pipeline.

  • Also can be in-contact or non-contact with the process flow.

• Invasive Sensors:

  • In-contact with the process flow.

• Non-Invasive Sensors:

  • Have no contact with the process flow.

Examples:

• An Infrared Thermometer is a non-invasive and non-intrusive sensor.

• A Pressure Gauge is an intrusive and invasive sensor.

Primary and Secondary Sensors

• Primary Sensors:

  • Directly measure physical parameters (e.g., temperature, pressure, vibration).

• Secondary Sensors:

  • Convert the output of primary sensors into a form usable by measurement systems.

Example:

• Spring acts as a primary sensor where compression corresponds to applied force.

• Potentiometer acts as a secondary sensor by converting displacement into an electrical signal.

Types of Sensors Based on Physical Principles

• Input Measured Variable and Physical Principle:

  • Resistive: Electric (passive)

  • Capacitive: Electric (active)

  • Optical: Various types

  • Inductive: Mechanical, Thermal, Optical, etc.

Common Temperature Sensors

1. Resistance Temperature Detector (RTD)

2. Thermistor

3. Thermocouple

4. Silicon Temperature Sensor

5. Infrared Temperature Sensor

RTD (Resistance Temperature Detector)

• Resistance increases linearly with temperature (typically -100°C to +800°C).

• Most popular: PT100 Sensors, resistance of 100 Ω at 0°C.

• Equation of resistance change:
  

• Components Used: Platinum, Copper, Nickel.

Thermistor

• Thermistor: Thermally sensitive resistor, generally made from ceramic or semiconductor materials.

• Quick response, fragile, and operates in a limited temperature range.

• NTC Thermistor: Resistance decreases with temperature; equation:
  R(T) = R_0 e^{ rac{b}{T}}

• Types of Thermistors:

  • Bead: Small (0.15mm - 1.25mm) and commonly glass-coated.

  • Probe: Used for liquids with a diameter of 2.5mm.

  • Disc: Used for temperature control, 2.5mm-25mm in diameter.

  • Washer: Long cylindrical sensors.

Thermocouple

• Voltage output from dissimilar junctions generates voltage based on temperature.

• Ranges from -200°C to +2000°C based on material.

• Created with various materials following international standards for accuracy and application.

Comparison of Sensor Types

Silicon Temperature Sensors

• Operate based on semiconductor properties:

  • When current passes through base-emitter of a bipolar transistor, temperature affects voltage produced between them.

• Equation:
  

• Example: MCP9700-E/TO sensor with a temperature range of -40°C to 125°C and an output of 10 mV/°C.

Infrared Temperature Sensors

• Utilizes Planck Radiation Law to measure temperature based on collected radiation:

  • Non-intrusive and non-invasive.

• Emits radiation in the visible and infrared range, good for measuring high temperatures without contact.

Considerations

• The suitability of each type depends on application; no one type is best overall. Key advantages and disadvantages must be weighed for specific use cases.