Dual Thresholds in Gas Detection Systems
Implementing dual thresholds is a foundational strategy in modern gas detection alarm logic. The first threshold, often labeled as low alarm, triggers at a concentration level that indicates potential risk but not immediate danger—typically set at 20–25% of the Lower Explosive Limit (LEL) for combustible gases. This early warning allows operators to initiate inspection or ventilation before reaching critical levels.
The second threshold, the high alarm, is set at 50–60% LEL, where immediate action is required: process shutdown, evacuation, or activation of emergency ventilation. This dual-tier approach prevents false alarms while ensuring timely response. In systems such as the GDE series infrared gas detectors, these thresholds are fully programmable via on-device menus or remotely through RS485 communication, enabling precise configuration based on site-specific risk assessments.
Threshold logic must also account for sensor type. For instance, infrared sensors used in the GDE820 provide stable, drift-resistant performance over long durations, making them ideal for fixed installations where false alarms are costly. In contrast, electrochemical sensors used in toxic gas detection (e.g., CO, H2S) may require more frequent threshold recalibration due to sensitivity to temperature and humidity—a factor mitigated by the built-in automatic temperature compensation in GDC and GDA series detectors.
Hysteresis and Threshold Stability
To prevent alarm oscillation near threshold boundaries, hysteresis is implemented. For example, if a low alarm triggers at 25% LEL, it will only reset when the concentration drops below 20% LEL. This 5% hysteresis band ensures stable operation and avoids repeated relay toggling during fluctuating gas levels—critical in environments like chemical processing plants or confined spaces.
The GM810/GM820 gas alarm controllers support hysteresis configuration at the system level, allowing centralized management of all connected detectors. This is especially useful when integrating multiple sensor types—catalytic, infrared, and semiconductor—into a single monitoring network.
Time Delays for Alarm Validation and System Protection
Unvalidated alarms can lead to unnecessary process interruptions. A time delay—typically 5 to 30 seconds—is introduced between threshold crossing and alarm activation. During this period, the system verifies that the gas concentration remains above the threshold, filtering out transient spikes caused by sensor noise, temporary leaks, or environmental fluctuations.
For example, the GDC811 detector allows delay settings to be adjusted via its onboard interface. A 10-second delay on the low alarm reduces false trips while still providing early warning. High alarms may use shorter delays (e.g., 2–5 seconds) to ensure rapid response.
Delays also play a role in ventilation control logic. When a low alarm is confirmed after delay, the system can activate auxiliary ventilation fans. If the high alarm threshold is crossed, a separate delay may be applied to allow ventilation to reduce gas levels before escalating to process shutdown—preventing overreaction in well-ventilated areas.
Sequential Delay for Multi-Detector Zones
In large facilities, multiple detectors monitor a shared zone. A sequential delay logic ensures that only the first detector to cross a threshold triggers an alarm, while others in the same zone are suppressed for a defined period (e.g., 60 seconds). This prevents alarm flooding and helps identify the source of the leak. The GM820 controller supports such zone-based logic through its modular programming interface.
Interlocks: Safety-Critical Action Triggers
Interlocks are hardwired or software-based connections between detection systems and process equipment. When a high alarm is confirmed, interlocks can automatically shut down pumps, close valves, or de-energize circuits. These actions are not optional—they are enforced by safety protocols.
For example, in a fuel storage facility, a high methane reading triggers an interlock that cuts power to non-intrinsically safe equipment and activates emergency ventilation. The GTYQ-GDA100V series detectors support dual relay outputs (low and high alarm), each capable of driving interlock circuits rated up to 2A@125VAC. These relays are fail-safe: loss of power or sensor fault results in relay de-energization, ensuring safe state in emergencies.
Interlocks must be tested regularly. The GDE and GDC series detectors include a manual relay test function, accessible via infrared remote or front panel, enabling verification of interlock integrity without gas exposure.
Fail-Safe Interlock Design
All interlock circuits should follow a fail-safe design principle. If communication is lost between a detector and controller (e.g., due to cable damage), the system defaults to alarm state. The GM810/GM820 controllers monitor detector health via heartbeat signals over RS485. A missing signal within a set interval triggers a fault alarm and activates pre-programmed interlock responses.
Ventilation Control Strategies
Ventilation is a primary mitigation measure. Demand-controlled ventilation uses gas concentration to modulate fan speed or duty cycle. A low alarm activates fans at 50% capacity; a high alarm ramps to 100%. This strategy reduces energy use while maintaining safety.
The 4-20mA output from detectors like the GDC810 can directly interface with variable frequency drives (VFDs), enabling analog control of fan motors. Alternatively, relay outputs can switch fan banks in stages. For complex systems, the GM820 controller supports ladder logic programming to implement staged ventilation sequences.
In confined spaces, ventilation may be interlocked with entry permits. The detection system must confirm zero alarm status before allowing access. This logic is implemented in the controller and linked to access control systems via dry contact relays.
Integration with IoT and Cloud Platforms
Modern systems integrate with cloud-based monitoring platforms via 4G or Wi-Fi modules. Alarms, sensor health, and ventilation status are logged and visualized in real time. Historical data enables trend analysis and predictive maintenance. For example, a recurring low alarm pattern may indicate a developing leak or sensor drift—early signs that can be addressed before failure.
The GDE820 with infrared sensor and built-in RS485 interface seamlessly connects to cloud gateways, enabling remote configuration and firmware updates. This reduces on-site maintenance and ensures compliance with evolving safety standards.