A thermistor is a type of resistor whose resistance changes significantly with temperature. There are two main types: NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient). NTC thermistors are more common in temperature sensing. Their resistance decreases as temperature increases. This characteristic is exploited in temperature measurement. In a microcontroller-based system, a thermistor is typically connected in a voltage divider circuit. The microcontroller measures the voltage across the thermistor and, using a known resistance value (typically a fixed resistor in the circuit), calculates the temperature using the Steinhart-Hart equation or a simplified approximation.

Advantages of thermistors:

• High sensitivity: Significant resistance change per degree Celsius.
• Relatively inexpensive.
• Small size.

Disadvantages of thermistors:

• Non-linear response: Requires calibration or approximation formulas.
• Limited temperature range compared to thermocouples.
• Can be affected by self-heating due to current flow.

Comparison with thermocouples: Thermocouples generate a voltage proportional to the temperature difference between two dissimilar metals. They offer a wider temperature range than thermistors and are less susceptible to self-heating. However, they require more complex signal conditioning (amplification and cold junction compensation) and are generally less sensitive.

Gas sensors are devices designed to detect and measure the concentration of specific gases in the environment. Several technologies are employed for gas detection:

• Electrochemical Gas Sensors: These sensors rely on an electrochemical reaction between the target gas and an electrode. The reaction produces a current proportional to the gas concentration.
  Strengths: High sensitivity and selectivity for specific gases (e.g., CO, O2, H2S).
  Weaknesses: Limited lifespan, affected by temperature and humidity, require regular calibration.
  Application: Carbon monoxide detectors in homes and industrial safety systems.
• Semiconductor Gas Sensors (Metal Oxide Sensors): These sensors utilize a metal oxide semiconductor material whose electrical conductivity changes upon exposure to certain gases. The change in conductivity is measured as a change in resistance.
  Strengths: Relatively inexpensive, wide range of gases can be detected (e.g., CO, VOCs, methane).
  Weaknesses: Lower selectivity compared to electrochemical sensors, susceptible to humidity and temperature changes, can be affected by poisoning.
  Application: Air quality monitoring, detection of flammable gases in industrial environments.
• Infrared (NDIR) Gas Sensors: These sensors measure the absorption of infrared light by the target gas. The amount of light absorbed is directly proportional to the gas concentration.
  Strengths: High accuracy and stability, suitable for measuring a wide range of gases (e.g., CO2, methane, propane).
  Weaknesses: More expensive than other types, require a stable infrared source.
  Application: CO2 monitoring in HVAC systems, methane leak detection in natural gas pipelines.

A closed-loop control system for a chemical reactor temperature control would typically involve the following components:

Block Diagram:

+-----------------+      +-----------------+      +-----------------+
| Temperature     |----->| Controller      |----->| Actuator        |
| Sensor          |      | (PID Algorithm)  |      | (Heater/Valve)  |
+-----------------+      +-----------------+      +-----------------+
^                                              |
|                                              |
+----------------------------------------------+
|
|  Setpoint (Desired Temperature)
|

Function of each component:

• Temperature Sensor: Measures the reactor temperature and provides a feedback signal to the controller.
• Controller: Compares the measured temperature to the setpoint and calculates the control signal needed to adjust the actuator. A PID (Proportional-Integral-Derivative) controller is commonly used.
• Actuator: Adjusts the reactor temperature by adding heat (e.g., by opening a heater) or removing heat (e.g., by opening a coolant valve).

Potential Sources of Error and Mitigation:

• Sensor Error: Inaccurate temperature readings can lead to incorrect control actions. Mitigation: Use a high-quality, calibrated temperature sensor. Implement sensor redundancy (multiple sensors).
• Actuator Lag: The actuator may take time to respond to the control signal. Mitigation: Use a controller with a suitable tuning to compensate for lag. Consider using a faster actuator.
• Disturbances: External factors (e.g., changes in ambient temperature, changes in reactant flow rate) can affect the reactor temperature. Mitigation: Design the controller to be robust to disturbances. Implement feedforward control to anticipate disturbances. Use a temperature jacket to maintain a more stable environment.
• Controller Tuning: Incorrectly tuned controller parameters can lead to instability or poor performance. Mitigation: Use appropriate controller tuning techniques (e.g., Ziegler-Nichols method). Implement adaptive control to adjust controller parameters online.