LED indicators like alphanumeric digits, bar graphs, and an alarm LED need specialized drive circuitry to manage multiplexing, brightness, and timing. The Display Board / Display Assembly serves this role, taking data from the Control Processor Board and turning it into the signals that light each segment or bar at the right moment. It handles the current limiting and multiplexing required to drive multiple types of displays, so the alphanumeric characters can be scanned, the bar graph segments can be lit proportionally, and the alarm indicator can flash or stay on as needed. The Control Processor Board provides the content and the timing logic, but it relies on the display board to physically illuminate the LEDs. The Power Board simply supplies power to the system, and the Alarm Interface Board handles alarm signaling routes rather than driving the displays themselves. Therefore, the Display Board / Display Assembly is the correct choice.

The control unit serves as the central brain of the system, continuously monitoring sensor inputs and deciding what actions to take. It reads data from detectors and other sensors, checks it against preset limits, and when something out of range is detected, it sends commands to operate equipment—adjusting fans, valves, or pumps, sounding alarms, or isolating a section to keep everything safe. It may also log events and support safe shutdown procedures if needed. This makes it distinct from simply providing power (that’s the power supply), enabling network access (that’s handled by separate communication modules), or housing the detectors (the sensors themselves and their enclosures).

The detector head is built to both sense the gas and report that information. The two main parts that do these jobs are the sensor assembly, which is the actual sensing element (whether electrochemical, infrared, or another sensing technology), and the transmitter assembly, which conditions the sensor signal and sends a readable output to the monitoring system. The other options cover support or broader system components, not the core head itself: power supply/housing are general hardware, a detector array and readout module imply a different type of imaging or multi-sensor setup, and a calibration unit with a control module are separate maintenance and control subsystems. So the sensor plus transmitter pair is the essential combination inside the detector head.

Indication and control panels run on a DC power source because it reliably powers meters, lights, alarms, sensors, and relay coils. A 24 VDC battery is used because it provides enough voltage to drive these loads without requiring excessive current or bulky wiring, while staying within a safe, standard level for industrial control components. 24 VDC is a common, readily available standard for such panels, making it easy to source relays, PLC inputs, and indicators that all expect that voltage. Using 12 VDC would mean higher current for the same power, increasing wiring and heat; 5 VDC is typically reserved for microelectronics rather than powering the whole panel; and 120 VAC would be mains power rather than a battery voltage, which changes safety and isolation requirements.