Four-Gas Monitor

A four-gas monitor is a portable, direct-reading gas detector that measures four common atmospheric hazards at once: oxygen (O2), flammable gases as a percentage of the lower explosive limit (LEL), hydrogen sulfide (H2S) and carbon monoxide (CO). It sounds audible, visual and vibrating alarms when readings cross preset levels.

Four-gas monitors matter because the most dangerous atmospheres often give no warning. Oxygen deficiency has no smell, carbon monoxide is odorless, and hydrogen sulfide can deaden the sense of smell at high concentrations. In confined spaces, sewers, tanks, pits and refineries, a working, properly set monitor is often the only thing standing between a worker and a fatal exposure.

What Does a Four-Gas Monitor Measure?

• Oxygen (O2): measured as percent by volume. Normal air is about 20.9 percent. Low readings can mean displacement by other gases or consumption by rusting or combustion; high readings increase fire risk.
• Combustible gases (LEL): measured as a percentage of the lower explosive limit, usually with a catalytic bead or infrared sensor calibrated to a reference gas such as methane or pentane.
• Hydrogen sulfide (H2S): measured in parts per million (ppm). Common in sewers, oil and gas, pulp and paper, and wastewater.
• Carbon monoxide (CO): measured in ppm. Produced by engines, heaters, welding and incomplete combustion.

The monitor shows real-time readings and usually tracks peak, short-term (STEL) and time-weighted average (TWA) values for the toxic sensors.

Alarm Levels and Reference Values

Alarm setpoints should be based on the regulations and exposure limits that apply to your work, and on the manufacturer's instructions. Useful reference points include:

• Oxygen: OSHA's permit-required confined spaces standard defines oxygen-deficient air as below 19.5 percent and oxygen-enriched air as above 23.5 percent. Monitors are commonly set to alarm at these levels.
• LEL: OSHA considers an atmosphere hazardous when flammable gas or vapor exceeds 10 percent of its LEL. Many employers set the low alarm at 10 percent LEL or lower, with a higher second alarm.
• H2S: OSHA's general industry limit is a 20 ppm ceiling with a 50 ppm peak allowed for up to 10 minutes in limited cases. The construction and shipyard limit is 10 ppm TWA. NIOSH recommends a 10 ppm ceiling for 10 minutes, and ACGIH's TLV is 1 ppm TWA and 5 ppm STEL.
• CO: OSHA's general industry limit is 50 ppm TWA. NIOSH recommends 35 ppm TWA with a 200 ppm ceiling, and ACGIH's TLV is 25 ppm TWA.

Factory default setpoints vary by manufacturer and may not match your limits. Review and document the alarm settings for each instrument, especially if your company follows ACGIH or NIOSH values.

Bump Testing and Calibration

Gas sensors drift and can fail without visible signs. OSHA's Safety and Health Information Bulletin on verifying calibration of direct-reading portable gas monitors recommends a bump test or calibration check before each day's use, following the manufacturer's instructions. If the monitor fails, it must be fully calibrated and retested, or removed from service.

• Bump test: exposes sensors to gas above alarm setpoints to confirm sensors respond and alarms activate.
• Calibration check: confirms readings are within the manufacturer's accuracy tolerance.
• Full calibration: adjusts the instrument to match a known gas concentration, at the frequency the manufacturer specifies.

Test more often after a drop, after exposure to high gas concentrations, or after exposure to substances that can poison or inhibit sensors, such as silicones or certain solvents.

Confined Space Testing Order

OSHA's permit-required confined spaces standard requires the atmosphere to be tested with a calibrated direct-reading instrument before entry. Appendix B of the standard explains that testing should be done in this order:

1. Oxygen first, because most combustible gas sensors need enough oxygen to give accurate readings.
2. Combustible gases second, because the threat of fire or explosion is more immediate and life-threatening than most toxic exposures.
3. Toxic gases and vapors third.

Most four-gas monitors show all readings at once, but the order still matters for interpreting results. Test at several levels, since some gases are heavier than air and settle low while others rise, and allow the time the manufacturer specifies for sample tubing and pump draw.

How to Use a Four-Gas Monitor Step by Step

1. Inspect the instrument. Check the battery, housing, filters, sensor openings and calibration date.
2. Start up in fresh air. Let the monitor complete its self-test and zero in clean air.
3. Bump test. Confirm every sensor responds and all alarms work, and record the result.
4. Test before entry. Sample from outside the space at the top, middle and bottom, using a pump and tubing where needed.
5. Monitor continuously. Keep the monitor in the breathing zone while work is underway.
6. Respond to alarms. Leave the area immediately on alarm and do not re-enter until the cause is found and controlled.
7. Record and maintain. Log readings, download data where available, and calibrate on schedule.

Common Mistakes to Avoid

• Skipping the daily bump test.
• Zeroing the monitor in contaminated air.
• Clipping the monitor on a belt instead of near the breathing zone.
• Assuming a four-gas monitor detects every hazard. It does not measure other toxic gases or vapors unless fitted with additional sensors.
• Ignoring LEL sensor limits, such as reduced accuracy in low oxygen or for gases different from the calibration gas.

How SMS360 Helps

SMS360's Audits & Inspections module supports daily bump test and calibration checklists completed on mobile devices, with failed checks turned into follow-up actions. Work Permits records atmospheric test results on confined space and hot work permits, and Training Management tracks who is trained to use the monitors.

See OSHA's bulletin on verifying calibration of portable gas monitors and 29 CFR 1910.146 on eCFR.

Frequently Asked Questions

What four gases does a four-gas monitor detect?

Oxygen, combustible gases measured as percent of the lower explosive limit, hydrogen sulfide and carbon monoxide.

What are typical four-gas monitor alarm settings?

Oxygen alarms are commonly set at 19.5 percent and 23.5 percent, and LEL alarms at or below 10 percent. H2S and CO setpoints should be based on the exposure limits your company follows and the manufacturer's guidance, since defaults vary.

How often should a four-gas monitor be bump tested?

OSHA guidance recommends a bump test or calibration check before each day's use, following the manufacturer's instructions, and more often after drops or exposure to high gas levels or sensor poisons.

Why is oxygen tested first in a confined space?

Most combustible gas sensors need adequate oxygen to read accurately, so oxygen is checked first. Combustibles are tested next, then toxic gases.