Why 4 gas became the basic configuration at industrial sites

In portable multi-gas detectors, “4 gas” usually means a configuration that measures oxygen (O2), the percentage of the lower explosive limit of combustible gases (%LEL), carbon monoxide (CO), and hydrogen sulfide (H2S) in one instrument. This combination is widely used because it can check four hazards repeatedly encountered in confined spaces with one relatively small device. It can simultaneously monitor oxygen deficiency or enrichment, fire and explosion potential, CO from combustion processes, and H2S from sewage, manure, and decomposition processes.

But “widely used” and “adequate for every site” do not mean the same thing. OSHA’s U.S. confined-space atmospheric testing procedure requires evaluation with equipment that has sufficient sensitivity and selectivity to identify every hazardous atmosphere that may be present or arise in the space. The testing order is oxygen, combustible gases and vapors, then toxic gases and vapors. This principle is not a list mandating four particular sensors; it is a risk-based approach that says actual hazards must not be missed.

Korean assessment criteria must likewise be read together with site conditions. The Korea Occupational Safety and Health Agency’s 2026 survey on asphyxiation-disaster prevention summarizes “acceptable air” under the Rules on Occupational Safety and Health Standards as oxygen of at least 18% and less than 23.5%, carbon dioxide below 1.5%, CO below 30 ppm, and H2S below 10 ppm. Where there is a fire or explosion risk, combustible gases must also be assessed. Therefore, the four channels of a commonly described 4-gas instrument do not always exactly match the items that must be checked in Korean confined spaces.

The point of “5 gas” is not the sensor count, but the hazard that is missing

A 5-gas detector is not the name of a specification that automatically adds one sensor to an existing 4-gas unit. The fifth channel varies with the work, process, feedstock, by-products, cleaners, refrigerants, fuels, ventilation condition, and applicable regulations. At one site carbon dioxide (CO2) may be sensible; at another, ammonia (NH3), sulfur dioxide (SO2), chlorine (Cl2), volatile organic compounds (VOC), hydrogen cyanide (HCN), or a detection method better suited to a particular combustible gas may be needed.

In particular, CO2 should not be regarded only as an indicator of oxygen displacement. NIOSH describes CO2 as a colorless, odorless, nonflammable gas and gives a recommended exposure limit of 5,000 ppm as an 8-hour time-weighted average, 30,000 ppm as a short-term exposure limit, and 40,000 ppm as an immediately dangerous to life or health concentration (IDLH). Even when oxygen concentration is close to normal, CO2 itself can have physiological effects of concern. Where a CO2 source is evident—such as fermenters, places using dry ice, brewing and food processes, greenhouses, fire-suppression-equipment areas, or manure and wastewater treatment facilities—an O2 sensor alone must not be used to judge the CO2 risk.

Conversely, NH3 may take priority in refrigeration-equipment maintenance, Cl2 in water treatment using chlorine-based chemicals, and SO2 in combustion or smelting processes. In work involving multiple organic solvents, such as painting, tank cleaning, and petrochemical maintenance, screening for VOCs with a photoionization detector (PID) can be useful; however, a PID’s total-volatile signal does not automatically give the precise concentration of each substance. Substance-specific response factors, ionization energies, and the mixture composition must be reviewed.

Accordingly, the shift from 4 gas to 5 gas is more accurately understood not as a “numbers upgrade,” but as bringing hazards outside the standard combination into the measurement system. Before deciding on a fifth sensor, the first question is what could be generated.

Risk assessment for selecting the fifth sensor

Choosing a sensor configuration solely from the industry name on paperwork is insufficient. Even at the same wastewater treatment plant, generated gases can vary with influent characteristics, chemicals, microbial activity, sludge disturbance, season, and work location. Normal operation and maintenance, cleaning, and emergency conditions must also be considered separately. A risk assessment requires at least the following information.

  • Current safety data sheets (SDS) for raw materials, intermediates, products, by-products, waste, and cleaning and disinfecting chemicals

  • Processes that generate gas or consume oxygen, such as nitrogen purging, fermentation, decomposition, combustion, welding, painting, and battery charging

  • Potential ingress through piping, valves, and adjacent equipment, along with past leak, alarm, and near-miss records

  • Gas relative density; temperature; humidity; potential stratification; ventilation dead zones; and workers’ actual breathing locations

  • Applicable laws, permitted conditions in permits, internal exposure limits, emergency-response criteria, and requirements of the authority having jurisdiction

  • Worst-case conditions not only during normal work, but also during opening, discharge, agitation, cleaning, commissioning, power outages, and ventilation failures

Use this information to compare the likelihood of generation, the severity of exposure consequences, and whether the existing 4-gas sensors actually detect the hazard. If a serious gas that cannot be detected is identified, choose the suitable method among a dedicated sensor, a separate single-gas detector, detector tubes, or sampling and laboratory analysis. Forcing every hazard into one portable instrument is not necessarily the right answer. Acute hazards requiring immediate alarms and industrial-hygiene measurements that evaluate long-term average exposure differ in both purpose and method.

IEC 62990-2:2021 also treats the selection, installation, use, and maintenance of toxic-gas and vapor detectors as one life cycle. IEC 60079-29-2:2015 presents the same perspective for combustible-gas and oxygen detectors. In other words, this is not a purchase that ends after deciding the sensor count: the target gas, measuring range, alarm purpose, installed or portable form, testing, and maintenance must be connected. International standards’ scope and Korean legal requirements are not identical, so the applicable laws and current internal procedures of the specific workplace must be checked separately.

Measurement blind spots that remain even with five sensors

A displayed value on a multi-gas detector does not mean the atmosphere is safe. Sensors respond by particular physical and chemical principles, and each has limits. Catalytic-combustion LEL sensors can generally become less reliable when oxygen is insufficient, and substances such as silicone and sulfur compounds can reduce their performance or “poison” them. OSHA explains that oxygen should be checked first because many combustible-gas sensors depend on oxygen. If a high concentration exceeds the upper explosive limit (USL or UEL), some instruments may show a low value or one that appears nonflammable; it is therefore dangerous to interpret an unexpectedly low LEL reading as safe.

Infrared combustible-gas sensors can measure without oxygen and resist catalytic poisoning, but they may not suit gases such as hydrogen that do not absorb infrared radiation sufficiently. Electrochemical toxic-gas sensors can cross-react with substances other than the target gas and are affected by temperature, humidity, pressure, and sensor life. Non-dispersive infrared (NDIR) sensors commonly used for CO2 also require review of measuring range, response time, moisture effects, and calibration conditions. With different sensor technologies, actual detection capability differs even when the display says “5 gas.”

Sampling method also changes the result. When a pump and hose are used to pre-test a manhole or tank, the transfer time caused by hose length must be added to the sensor response time. OSHA advises measuring each item for at least the manufacturer’s stated minimum response time and, where a stratified atmosphere is expected, testing separately in the direction of travel and the surrounding area. Rather than assuming light gases are only at the top and heavy gases only at the bottom, actually check the upper, middle, and lower levels and the work path. A single pre-entry measurement cannot guarantee against changes from agitation, welding, chemical addition, or interrupted ventilation during the work. If hazards can change, continuous monitoring or periodic measurement that reflects the rate of hazard change is required.

Operating procedures that must change along with a 5-gas transition

Adding one sensor makes instrument management correspondingly more complex. First, the basis for alarm setpoints must be documented. Workers need to understand which of legal limits, occupational exposure limits, process-safety limits, or manufacturer recommendations was used, as well as the meanings of high and low alarms, time-weighted averages, and short-term exposure alarms. Displays and training must also be aligned so that distinct units—%vol, ppm, and %LEL—are not confused.

A pre-use functional check must confirm that test gas actually reaches all five sensors and that alarms operate. OSHA guidance on portable direct-reading gas monitors introduces industry recommendations to perform a bump test or calibration check before each day’s use, following manufacturer instructions. A bump test is a qualitative test of sensor and alarm operation, not an accuracy calibration. If a calibration check falls outside the acceptable range, conduct a full calibration with certified standard gas of a known concentration; if full calibration also fails, remove the instrument from service.

If a fifth sensor is added but only the existing 4-component calibration gas continues to be used, the new channel is not meaningfully verified. Review the component stability of mixed standard gas, cylinder expiry, flow rate, regulator material, and the possibility of reactive-gas adsorption. Reactive gases such as Cl2 and NH3 can also be sensitive to hose and regulator selection, so manufacturer-approved procedures must be followed. Records should retain the instrument number; sensor type and serial number; test-gas concentration and expiry; test result; calibration date and time; person performing it; and fault and corrective-action history.

Site procedures must also be updated. The work permit’s measurement items, measurement points, waiting time, evacuation criteria upon an alarm, attendant’s role, ventilation-restart conditions, and rescue plan must reflect the new sensor. A detector does not replace ventilation or isolation. Training and supervision to prevent people from reapproaching without protective equipment to find the cause of an alarm, or turning off the sensor and continuing the work, are important.

Questions to answer before purchase specifications

When introducing a 5-gas instrument, it must be possible to answer the following questions before focusing on the channel count.

  1. In which process and scenario is the fifth target gas generated?

  2. What measuring range, resolution, response time, and alarm setpoints are required?

  3. Do the sensor’s cross-sensitivity, oxygen dependence, catalytic poisoning, and temperature-humidity limits fit the site conditions?

  4. Is diffusion sampling sufficient, or is a pumped unit needed for remote pre-entry testing?

  5. Does it meet the certifications and protection rating required for its location of use, such as explosion protection and dust/water resistance?

  6. Are the standard gas, docking station, spare sensors, and responsible personnel needed for daily function testing and regular calibration ready?

  7. Who reviews alarm data and inspection records, and what improvement actions will repeated alarms lead to?

Assuming “more sensors means safer” without answering these questions can instead create false reassurance. Conversely, if a particular additional gas is identified through risk assessment and suitable sensors and an operating system are ready, the fifth channel can reduce an important blind spot.

Conclusion: 5 gas is not a new default, but a site-specific decision

Industrial-safety measurement is moving from the conventional baseline configuration of 4 gas toward a more precise reflection of site-specific hazards. But this must not be taken as a declaration that every workplace is now legally required to use 5 gas. Required gases, allowable limits, and measurement methods vary according to work hazards, processes, national and jurisdictional regulations, and permit conditions.

A sound transition does not begin with equipment purchasing. It proceeds by updating the hazard list, identifying gases missed by the existing 4-gas configuration, selecting the needed fifth sensor or a separate measurement method, and then changing testing, calibration, training, work permits, and emergency response together. Ultimately, what matters is not how many numbers appear on the screen, but whether those numbers show the real hazards completely and reliably.

Sources

Cover photo

Original photo on Pexels