Temperature and Humidity Sensing in Intelligent Building Systems

Introduction to Temperature Sensors

  • Definition: Temperature sensors are devices that measure the amount of heat energy generated by an object or system. They allow for the sensing or detection of physical changes in temperature through either an analogue or digital output.
  • Categorization: Temperature sensors are generally divided into three primary groups based on their working principles:
    • Electro-mechanical sensors.
    • Electronic sensors.
    • Resistive sensors.
  • Common Industrial Types:
    • Thermostats.
    • RTDs (Resistance Temperature Detectors).
    • Thermistors.
    • Thermocouples.
  • Primary Focus Areas: The lesson focuses specifically on the Thermocouple, RTD, and Thermistor.

RTD - Resistance Temperature Detectors

  • Working Principle: RTDs are resistive temperature devices that capitalize on the physical property that the electrical resistance of a material changes in response to temperature variations.
  • Classification of Resistive Devices:
    • Metallic Devices (RTDs): These rely on the resistance change in a metal. The resistance typically rises linearly with temperature.
    • Thermistors: These are based on resistance changes in a ceramic semiconductor. The resistance rises or drops nonlinearly with a rise in temperature.
  • Resistance-Temperature Variables:
    • RR represents the absolute resistance at any given temperature.
    • R0R_0 represents the resistance at 0∘C0^{\circ}C.
  • Key Characteristics and Performance:
    • Accuracy: Known for excellent accuracy over broad temperature ranges. High-precision RTDs can have an accuracy of 0.01 Ω0.01\,\Omega at 0∘C0^{\circ}C.
    • Stability: RTDs are extremely stable. Common devices drift less than 0.1∘C/year0.1^{\circ}C/\text{year}, with high-stability models reaching 0.0025∘C/year0.0025^{\circ}C/\text{year}.
    • Sensitivity: RTDs have low resistance (commonly 100 Ω100\,\Omega) and their resistance changes only slightly with temperature (<0.4 Ω/∘C< 0.4\,\Omega/^{\circ}C).
    • Measurement Challenges: Because resistance changes are small, special configurations are required to minimize errors caused by lead wire resistance.
  • Applications of RTDs:
    • RTDs are ideal for applications requiring small package sizes, high accuracy, and linear output.
    • Interchangeability: They can be replaced without the need for recalibration.
    • Specific Usage Areas:
      • Overload protection for electric motors.
      • Semiconductor protection in electronic circuits.
      • Thermal management and temperature compensation.
      • Temperature regulation in industrial process control.
      • Automotive applications (air and oil temperature sensing).
      • Appliance sensing (heating and cooling).
      • HVAC industry (Chilled Water/Condenser water temperature sensing).
      • Specific requirements for "Green Mark" certification.

RTD Measurement Principles

  • Passive Nature: RTDs are passive devices. To obtain a reading, an electric current must be passed through the device to produce a measurable voltage.
  • Self-Heating (I2RI^2R):
    • The excitation current causes internal heat generation, known as self-heating, which can introduce measurement errors.
    • Specification: Self-heating is specified as the power required to raise the RTD temperature by 1∘C1^{\circ}C (e.g., 1 mW/∘C1\,mW/^{\circ}C).
    • Minimization: Self-heating is minimized by using the smallest possible excitation current, typically between 1 mA1\,mA and 5 mA5\,mA.
  • Medium Dependency: The degree of self-heating depends on the surrounding medium. An RTD can experience self-heating up to 100 times higher in still air compared to running water.
  • Linearity and Temperature Coefficient (Alpha):
    • The output of an RTD is relatively linear compared to other devices like thermocouples.
    • Alpha ($\alpha$): The temperature coefficient differs by RTD type. It is defined as the change in resistance from 0∘C0^{\circ}C to 100∘C100^{\circ}C, divided by the resistance at 0∘C0^{\circ}C, and further divided by 100∘C100^{\circ}C.
    • Equation for Alpha:         α=R100−R0R0×100∘C\alpha = \frac{R_{100} - R_0}{R_0 \times 100^{\circ}C}
    • Example: A 100 Ω100\,\Omega platinum RTD (R0=100 ΩR_0 = 100\,\Omega) with α=0.003926\alpha = 0.003926 will have a resistance of 139.26 Ω139.26\,\Omega at 100∘C100^{\circ}C.

Advanced RTD Equations and Standards

  • Callendar-Van Dusen Equation: Used for high-accuracy conversion between resistance and temperature to account for slight non-linearities.
    • Standard Equation (Simplified): Rt=R0[1+At+Bt2+C(t−100)t3]R_t = R_0 [1 + At + Bt^2 + C(t-100)t^3].
    • Variables:
      • RtR_t: Resistance of the RTD at temperature tt.
      • R0R_0: Resistance at 0∘C0^{\circ}C.
      • tt: Temperature in ∘C^{\circ}C.
      • A,B,CA, B, C: Specific Callendar-Van Dusen coefficients.
    • Note: For temperatures above 0∘C0^{\circ}C, the coefficient C=0C = 0, reducing the equation to a quadratic expression.
  • Industry Standards:
    • DIN 43760: European Standard (α=0.00385\alpha = 0.00385).
    • US Industrial/American Standard: (α=0.003911\alpha = 0.003911).
    • International Temperature Scale (ITS-90): Issued by the National Institute of Standards and Technology (α=0.003926\alpha = 0.003926). This is commonly used for wire-wound RTDs.
    • IEC 60751: Issued by the International Electrotechnical Commission.
    • B.S. 1904: British Standards Institution.
  • Calculation Example (Pt-100):
    • Goal: Find resistance at 400∘F400^{\circ}F using ITS-90 standard.
    • Conversion: ∘C=(∘F−32)×59^{\circ}C = (^{\circ}F - 32) \times \frac{5}{9}. Therefore, 400∘F=204.4∘C400^{\circ}F = 204.4^{\circ}C.
    • Polynomial for t>0∘Ct > 0^{\circ}C: R=R0[1+3.9848×10−3(t)−5.870×10−7(t2)]R = R_0 [1 + 3.9848 \times 10^{-3}(t) - 5.870 \times 10^{-7}(t^2)]
    • Result: At 204.4∘C204.4^{\circ}C, the resistance is approximately 179 Ω179\,\Omega.

Selection Criteria for RTDs

  • Advantages:
    • Required for high accuracy and stability requirements.
    • Consistency over wide temperature ranges.
    • Useful for area sensing (vs. point sensing) to improve control.
    • High level of standardization.
    • Repeatability and ease of interchangeability.
  • Factors to Consider Before Selection:
    • Temperature rating/range.
    • Tolerance and Accuracy.
    • Interchangeability (Maintenance considerations).
    • Response time.
    • Distance from the control or measuring equipment.

TMP 36 Temperature Sensor

  • Operating Mechanism: TMP 36 sensors use solid-state technology. They utilize the phenomenon where the voltage drop between the base and emitter (VbeV_{be}) of a transistor increases at a known rate as temperature increases.
  • Output: The chip precisely amplifies this voltage change to generate an analog signal directly proportional to the temperature. The chip performs internal complex calculations to output a direct temperature reading.
  • Advantages of TMP36:
    • No moving parts.
    • Precise and robust.
    • Requires no calibration.
    • Functional under diverse environmental conditions.
    • Consistent and inexpensive.
    • Simple to use.
  • Technical Characteristics:
    • Operating Temperature Range: −40∘C-40^{\circ}C to 125∘C125^{\circ}C.
    • Operating Voltage Range: 2.7 V2.7\,V to 5.5 V5.5\,V.
    • Linear Scale Factor: 10 mV/∘C10\,mV/^{\circ}C.
    • Output Voltage Range: 0.1 V0.1\,V to 2 V2\,V.
    • Offset Voltage: 0.5 V0.5\,V.
    • Arduino Supply Voltage: 5 V5\,V.
    • analogRead Integer Range: 00 to 10231023.

Arduino Nano 33 IoT

  • Description: A low-cost, low-power microcontroller board based on a system-on-a-chip architecture.
  • Microcontroller Type: Based on the low-power Arm® Cortex®-M0 32-bit SAMD21.
  • Connectivity: Features the u-blox NINA-W10 low-power chipset for integrated Wi-Fi and dual-mode Bluetooth connectivity.
  • Pinout Configuration Details:
    • Digital Pins: D2 through D13. (Note: D13 is connected to the built-in LED).
    • Analog Pins: A0 (D14) through A7 (D21).
    • Communication Pins:
      • SDA (PB08) and SCL (PB09) for I2C (SC4).
      • CIPO (PA19), COPI (PA16), and SCK (PA17) for SPI (SC1).
      • RX (PB23) and TX (PB22).
    • Power and Support Pins:
      • VIN: Input voltage.
      • +5V and +3V3: Power outputs.
      • GND: Ground pins (multiple).
      • RESET: Reset pins (multiple).
      • AREF: Analog reference (PA03).