Professional Explanation of Four-in-One Gas Detector Measurement Range

In industrial production and safety management, gas leaks, oxygen deficiency, or toxic gas overexposure are among the most hidden and sudden risks. Four-in-one gas detectors, with their compact size, concentrated functionality, and fast response, have become standard equipment in petrochemical plants, municipal pipelines, coal mines, wastewater treatment, and confined space operations.

Based on Nexisense's long-term engineering experience in gas sensing technology, this article systematically and professionally explains the measurement range of four-in-one gas detectors by addressing three core questions: “What gases does the detector measure, to what extent, and how should alarm values be interpreted?”

What is a Four-in-One Gas Detector?

A four-in-one gas detector integrates four different types of gas sensors in a single device, continuously monitoring the most representative hazardous gases in the environment:

• Combustible gases (expressed as LEL)

• Oxygen (O₂)

• Carbon monoxide (CO)

• Hydrogen sulfide (H₂S)

These four gases cover explosion risk, oxygen deficiency, and acute toxicity—the three key safety dimensions—making it the “minimum safety combination” validated in industrial safety monitoring.

Standard Detection Range Analysis of Four-in-One Gas Detectors

Combustible Gases (LEL) Detection Range

Combustible gases are typically measured based on the Lower Explosive Limit (LEL), rather than direct volumetric concentration.

• Common targets: methane, propane, butane, etc.

• Standard detection range: 0–100% LEL

• Typical alarm settings:

• Low alarm: 10%–25% LEL

• High alarm: 50% LEL

When the combustible gas concentration reaches a certain proportion of the LEL, the risk of explosion is extremely high. The LEL scale directly reflects explosion risk, enabling quick decision-making on site.

Oxygen (O₂) Detection Range

Oxygen is not toxic but its concentration directly affects human physiological safety.

• Standard detection range: 0–30% VOL

• Normal air concentration: 20.9% VOL

• Typical alarm settings:

• Low alarm: 19.5% VOL (hypoxia warning)

• High alarm: 23.5% VOL (oxygen enrichment risk)

Oxygen levels below 19.5% reduce attention and slow reactions; above 23.5%, they significantly increase combustion and explosion risk.

Carbon Monoxide (CO) Detection Range

CO is a colorless, odorless, highly toxic gas and a common source of acute poisoning in industrial and urban environments.

• Standard detection range: 0–1000 ppm (extendable)

• Typical alarm settings:

• Low alarm: 35 ppm

• High alarm: 200 ppm

CO binds with hemoglobin, reducing oxygen transport. Short-term high exposure can have severe consequences; alarm thresholds are therefore conservative.

Hydrogen Sulfide (H₂S) Detection Range

H₂S has a strong “rotten egg” odor but quickly paralyzes olfactory senses at high concentrations.

• Standard detection range: 0–100 ppm

• Typical alarm settings:

• Low alarm: 10 ppm

• High alarm: 20 ppm

H₂S is common in wastewater treatment, oil extraction, and chemical production, and is the most toxic gas among the four measured.

Logic Between Detection Range and Alarm Values

Detection range is not equivalent to safety range. Detection range defines the sensor's measurement limit, while alarm values are safety thresholds set according to regulations, occupational health, and engineering experience.

Reasonable alarm settings should:

• Provide early warning before personnel experience discomfort

• Allow sufficient emergency response time

• Prevent frequent false alarms from being ignored or disabled

Technical Features of Nexisense Four-in-One Gas Detection Solutions

• Highly consistent sensors ensuring comparable data across devices

• Cross-gas interference suppression algorithms reducing false alarms

• Temperature compensation and automatic zero correction for complex environments

• Explosion-proof design meeting Ex ia IIB T4 Ga safety standards

• Modules such as TZHCO-01B can serve as core units for portable or fixed systems

  
  

Key Selection and Usage Considerations

Select Range According to Environment

• Standard industrial environments: standard range is sufficient

• Confined or high-risk areas: consider extended range versions

Calibration and Maintenance

• Perform calibration with standard gas every 3–6 months

• Regularly check sensor response speed and zero drift

• Monitor sensor life cycle to avoid expired use

Proper Wearing and Use

• Wear detector at breathing zone height

• Avoid direct contact of sensor inlet with liquids or solvents

• Do not spray chemicals or cleaning agents near sensors

FAQ

• Is the four-in-one detector suitable for all industrial scenarios? Not entirely. Specialized gases or high concentrations require dedicated gas detectors.

• Can alarm values be modified by users? Some devices allow it, but changes must follow safety standards and regulations.

• Can expired sensors still be used? Not recommended. Accuracy and reliability cannot be guaranteed beyond sensor life.

Conclusion

By simultaneously monitoring LEL, O₂, CO, and H₂S, four-in-one gas detectors establish the fundamental and most effective safety line in industrial sites. Understanding detection ranges and alarm logic is essential for proper selection and use. In gas safety, the value of measurement is not how high it can detect but whether it provides timely, reliable warnings before risks occur.

Nexisense continues to advance four-in-one gas detection technology toward higher reliability and ease of use, guided by engineering practice.