In the management of urban infrastructure across Europe and America, the safety monitoring of sewer systems remains a core concern in municipal engineering. When discussing confined space operation safety, a critical question frequently arises: can combustible gas detectors effectively detect hazardous gases within sewers? The answer to this question concerns not only technical feasibility but also directly impacts the construction of urban public safety systems.

　　I. The Complex Composition of Sewer Gases

　　Urban sewers are not merely conduits for wastewater; they harbour a unique microbial ecosystem. Under anaerobic conditions, the decomposition of organic matter produces methane-dominant biogas. This colourless, odourless gas has an explosive limit of 5.0%-15.4%, classifying it as a typical combustible gas. However, the greater peril lies in hydrogen sulphide (H₂S) – a highly toxic gas with the characteristic odour of rotten eggs. At concentrations of 150ppm, it can cause olfactory paralysis; 250ppm induces pulmonary oedema; and levels exceeding 500ppm pose an immediate threat to life.

　　Additionally, sewer systems may harbour mixed gases including carbon monoxide (CO), carbon dioxide (CO₂), and ammonia (NH₃). These gases are not only combustible but can also combine to create compound hazardous environments through concentration. For instance, when methane reaches 10% by volume and oxygen falls below 12%, asphyxiation may occur due to oxygen deprivation even before the lower explosive limit is reached.

　　II. Technical Principles and Detection Capabilities of Detectors

　　Modern combustible gas detectors primarily employ two core technologies:

　　Catalytic combustion sensors: Detect gas concentrations through changes in platinum wire resistance, exhibiting high sensitivity to hydrocarbons such as methane and propane. These sensors are widely used in natural gas pipeline monitoring, typically covering detection ranges from 0-100% LEL (lower explosive limit) with accuracy up to ±2% FS.

　　Infrared optical sensors: These utilise infrared spectral absorption principles to identify gas molecular structures, proving particularly suitable for complex gas environments. For instance, certain high-end models can simultaneously detect six combustible gases including methane, ethane, and propane, with significantly superior cross-interference resistance compared to traditional sensors.

　　For the specific environment of sewer systems, professional equipment must incorporate triple protection:

　　Explosion-proof design: Incorporates Ex ia IIC T4 Ga-rated explosion-proof construction, ensuring stable operation even when methane concentrations exceed limits.

　　Multi-gas detection: Integrates electrochemical sensor arrays to simultaneously monitor critical parameters including H₂S, CO, and O₂.

　　Environmental adaptability: Features IP67-rated waterproof and dustproof construction, with an operating temperature range spanning -40°C to 70°C.

　　III. Monitoring Solutions in Practical Applications

　　In major international cities such as New York and London, municipal authorities have established tiered monitoring systems:

　　Fixed Monitoring Stations: Laser methane detectors deployed at critical points like valve chambers and pressure regulation boxes enable 24-hour continuous monitoring. For instance, certain models employing TDLAS (Tunable Diode Laser Absorption Spectroscopy) technology achieve detection accuracy of 0.1ppm with response times <3 seconds.

　　Portable detectors: Enforcement personnel are equipped with pump-aspirated quadruple detectors displaying simultaneous readings for CH₄, H₂S, CO, and O₂ concentrations, with data storage capacity for 100,000 entries. During London sewer maintenance operations, such devices enable workers to complete gas concentration checks within 10 seconds, significantly reducing emergency response times.

　　Intelligent Early Warning System: Monitoring data is transmitted in real-time to the control centre via IoT technology. When gas concentrations exceed thresholds, a three-tier response mechanism is automatically triggered:

　　Tier 1 Alert (50% LEL): Initiate localised ventilation

　　Tier 2 Alert (80% LEL): Close relevant valves

　　Tier 3 Alert (100% LEL): Evacuate personnel and activate alarms

　　IV. International Standards and Certification Systems

　　European and American markets impose stringent certification requirements for gas detection equipment:

　　EU CE Certification: Must comply with EN 50271 standards, encompassing 12 tests including electromagnetic compatibility and explosion-proof performance

　　US UL Certification: Equipment must pass UL 2075 standard testing, focusing on sensor lifespan (typically >5 years) and resistance to poisoning

　　ATEX Certification: Equipment for explosive atmospheres must obtain Ex d/Ex i classification certification

　　Taking a German detector brand as an example, its products hold ATEX Zone 0 certification for use in environments with persistent methane concentrations, featuring an 8-year sensor lifespan that significantly exceeds industry averages.

　　From New York to Tokyo, Paris to Sydney, combustible gas detectors have become the ‘electronic sentinels’ of urban safety systems . These devices must not only meet stringent technical standards but also adapt to complex and dynamic operational conditions. With the integration of technologies like the Internet of Things and artificial intelligence, future gas monitoring systems will transition from ‘passive alarms’ to ‘proactive prevention’, establishing more robust safety barriers for urban lifeline infrastructure. For municipal administrators, selecting internationally certified professional equipment and establishing a scientific monitoring system represents the optimal solution for addressing sewer gas risks.