A toxic gas detector is one of the most fundamental pieces of safety instrumentation in modern industry. Unlike combustible gas monitoring, where the concern is fire and explosion, toxic gas detection protects people directly: it warns workers before airborne concentrations of hazardous gases reach levels that can impair judgment, cause injury or become life threatening. Three gases account for a large share of industrial toxic gas incidents worldwide – hydrogen sulfide (H2S), carbon monoxide (CO) and ammonia (NH3). This overview explains how detectors for these gases work, summarizes the key properties and widely recognized exposure limits for each gas, and outlines the practical points that procurement and EHS teams typically weigh when specifying a detection system.
How Toxic Gas Detectors Work
Most fixed toxic gas detectors for H2S, CO and NH3 use electrochemical sensor cells. Inside the cell, the target gas diffuses through a membrane and reacts at an electrode, generating a small electrical current that is proportional to the gas concentration in air. The transmitter electronics convert this signal into a reading, usually expressed in parts per million (ppm), and pass it to a local display, relay outputs or a central controller.
Electrochemical technology is well suited to these three gases because it can resolve low ppm-level concentrations, draws little power and is available in gas-specific versions. It does, however, come with characteristics that buyers should understand. Sensor cells are consumable items with a finite service life, and readings can drift over time, which is why periodic bump testing and calibration are standard practice across the industry. Cross-sensitivity is another consideration: some cells respond partially to gases other than their target, so the expected background gases at a site are worth discussing during selection. For a broader look at fixed-point instruments and mounting formats, see our gas alarm detector product center.
Hydrogen Sulfide, Carbon Monoxide and Ammonia: Key Properties
Although all three gases are commonly monitored with similar sensor technology, their behavior in air and their toxicity profiles differ in ways that directly affect detector selection and placement.
Hydrogen sulfide (H2S) is a colorless gas with a characteristic rotten-egg odor at low concentrations. Relying on smell is dangerous, because H2S rapidly fatigues the sense of smell, so rising concentrations can go unnoticed without instrumentation. H2S is denser than air and tends to accumulate in pits, sumps, tank bottoms and other low-lying or confined spaces. It is encountered in oil and gas production, refining, wastewater treatment, pulp and paper, and biogas operations. Occupational limits are strict: the ACGIH TLV is 1 ppm as an 8-hour TWA with a 5 ppm STEL, and the EU indicative occupational exposure limit is 5 ppm (8-hour TWA) with a 10 ppm short-term limit.
Carbon monoxide (CO) is colorless and odorless, which makes instrumented detection the only reliable warning. It is produced by incomplete combustion and is a concern around furnaces, boilers, engines, parking structures, steelmaking and any process involving burners or exhaust gases. CO has nearly the same density as air, so it mixes readily throughout a space rather than settling or rising sharply. Commonly referenced limits include the ACGIH TLV of 25 ppm (8-hour TWA) and the EU indicative limit of 20 ppm (8-hour TWA) with a 100 ppm short-term value.
Ammonia (NH3) is a colorless gas with a sharp, pungent odor. It is significantly lighter than air and tends to rise and collect near ceilings and roof spaces. Large industrial inventories are found in refrigeration systems, fertilizer production and chemical processing. Ammonia is both toxic and, at high concentrations, flammable, with a flammable range of roughly 15 to 28 percent by volume in air; in practice, toxic alarm thresholds in the ppm range are reached long before flammability becomes relevant. The ACGIH TLV is 25 ppm (8-hour TWA) with a 35 ppm STEL, and the EU indicative limit is 20 ppm (8-hour TWA) with a 50 ppm short-term value.
Placement: Let the Gas Density Guide You
Detector location is as important as detector choice, and gas density is the starting point. H2S detectors are generally mounted low, typically within roughly half a meter of the floor or grade, and close to likely release points such as pump seals, drains and sample stations. NH3 detectors are generally mounted high, near the ceiling or above refrigeration equipment, because the gas rises. CO detectors are usually installed at breathing-zone height, on the order of 1.5 meters above the floor, since the gas disperses evenly and the goal is to measure what people actually inhale.
Airflow patterns, ventilation inlets, dead air pockets and the location of personnel all modify these general rules, so a site walk-through or drawing review is a normal part of system design. Confined spaces, valve stations and areas with intermittent occupancy may be evaluated for additional coverage. Typical layouts for refineries, chemical plants, cold storage and wastewater facilities are outlined in our applications overview.
Alarm Levels and System Integration
Fixed toxic gas detectors are normally configured with at least two alarm setpoints: a low alarm that prompts investigation and a high alarm that triggers evacuation, ventilation or process actions. Setpoints are commonly aligned with the occupational exposure limits that apply at the site, and local regulations or company standards take precedence in every case.
Integration is the other half of the specification. Detectors with 4-20 mA analog outputs remain the workhorse for connection to PLCs and DCS systems, while RS485 digital communication can be considered where many points share a single cable run. A dedicated gas alarm controller ties multiple detectors together, drives sounders, beacons and relays, and gives operators a single panel for status and alarm management – see the gas alarm controller product center for single-zone and multi-channel options.
Why ASA
ASA supplies a coordinated range of fixed gas detectors, dust monitoring instruments and gas alarm controllers, which allows toxic gas points for H2S, CO and NH3 to be specified alongside combustible gas coverage within one system architecture. Our engineering team supports customers through sensor selection, cross-sensitivity questions, placement planning and controller configuration, and we are accustomed to working with industrial buyers across Europe and the Middle East who need clear documentation and responsive communication. Whether you are outfitting a single plant room or planning site-wide coverage, we respond quickly with practical proposals. To discuss your monitoring requirements or request a quotation, contact the ASA team.
Disclaimer: Product availability, specifications, measuring ranges, alarm configurations and certification coverage vary by model and region. The information in this article is provided for general reference only and does not constitute a performance guarantee, safety assurance or compliance statement. Always refer to the official product documentation and confirm suitability, certification status and local regulatory requirements with our team before selection, installation or use.