Q&A: Gas monitor calibration and ventilation in confined spaces

How often must you calibrate and perform bump tests for gas monitoring equipment used to determine if a confined space has a hazardous atmosphere, and what are the requirements for forced air ventilation when a hazardous atmosphere exists?

OSHA defines a hazardous atmosphere under 29 CFR 1910.146(b) as “an atmosphere that may expose employees to the risk of death, incapacitation, impairment of ability to self-rescue (that is, escape unaided from a permit space), injury, or acute illness from one or more of the following causes:

1. Flammable gas, vapor, or mist in excess of 10 percent of its lower flammable limit (LFL);
2. Airborne combustible dust at a concentration that meets or exceeds its LFL; [Note: This concentration may be approximated as a condition in which the dust obscures vision at a distance of 5 feet (1.52 m) or less.]
3. Atmospheric oxygen concentration below 19.5 percent or above 23.5 percent;
4. Atmospheric concentration of any substance for which a dose or a permissible exposure limit is published in subpart G, Occupational Health and Environmental Control, or in subpart Z, Toxic and Hazardous Substances, of this part and which could result in employee exposure in excess of its dose or permissible exposure limit; [Note: An atmospheric concentration of any substance that is not capable of causing death, incapacitation, impairment of ability to self-rescue, injury, or acute illness due to its health effects is not covered by this provision.]
5. Any other atmospheric condition that is immediately dangerous to life or health. [Note: For air contaminants for which OSHA has not determined a dose or permissible exposure limit, other sources of information, such as Material Safety Data Sheets that comply with the Hazard Communication Standard, § 1910.1200 of this part, published information, and internal documents can provide guidance in establishing acceptable atmospheric conditions.]”

OSHA resources for gas monitor use and confined space safety

The Occupational Safety and Health Administration (OSHA) has several published documents to support employers and employees with using, maintaining, and calibrating direct-reading portable gas monitors (DRPGMs) and testing confined space atmospheres. Listed below are several.

• Calibrating and Testing Direct-Reading Portable Gas Monitors. This OSHA Safety and Health Information Bulletin (SHIB 11-26-2024), provides workers and employers guidance on calibrating and testing direct-reading portable gas monitor.
• OSHA Fact Sheet: Procedures for Atmospheric Testing in Confined Spaces.
• OSHA QuickCard™ Publication 3214-07R, 2013). This document explains what workers should do before entering a confined space, such as an underground vault, tank, storage bin, silo, or manhole.
• 29 CFR 1910.146 Appendix B provides additional guidance, general control measures, and procedures for atmospheric testing. Also, you should follow the manufacturer’s recommendations and instructions for calibrating, testing, and operating the DRPGM. You should have received such recommendations and instructions with your gas monitoring equipment.
• 29 CFR 1910.146(c)(5) establishes the requirements for forced air ventilation, specifically 29 CFR 1910.146(c)(5)(ii)(E) states that “[a]n employee may not enter the space until the forced air ventilation has eliminated any hazardous atmosphere.” Therefore, any time testing or monitoring reveals a hazardous atmosphere within the confined space, forced air ventilation is required. Testing for hazardous atmospheres must be conducted daily before any employee enters the confined space and periodically as necessary to ensure that the continuous forced air ventilation is preventing the accumulation of a hazardous atmosphere.
• 29 CFR1910.146(c)(5)(i)(A) through (F) provide the requirements for continuous monitoring. The employer can demonstrate that continuous forced air ventilation alone is sufficient to maintain that permit space safe for entry.

Gas monitor safety key compliance takeaways

• Follow manufacturer calibration schedules: OSHA requires adherence to manufacturer recommendations for calibrating direct-reading portable gas monitors (DRPGMs) as outlined in OSHA SHIB 11-26-2024.
• Conduct bump tests before use: While not explicitly required by OSHA, industry best practices and guidance support performing bump tests before each use to verify sensor functionality.
• Maintain adequate records: OSHA does not mandate recordkeeping for bump tests or calibrations under 29 CFR 1910.146, but maintaining documentation supports compliance and may be required by equipment manufacturers for warranties.
• Use multi-gas detectors: Confined space monitoring must address oxygen levels, flammable gases, and toxic substances as specified in 29 CFR 1910.146(b).
• Ensure sufficient ventilation: Forced air ventilation is required to eliminate hazardous atmospheres before entry and must continue as necessary to prevent reaccumulation (29 CFR 1910.146(c)(5)(ii)(E)).
• Train workers on equipment use:  OSHA’s 29 CFR 1910.146(g)(1) mandates training for employees to recognize confined space hazards and properly operate gas monitoring equipment.

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