Confined Space Monitoring Procedures and Technologies

Key Takeaways

• Confined space monitoring is necessary to detect atmospheric hazards before entry. Without proper monitoring, workers risk exposure to lethal gases or oxygen-deficient environments.

• OSHA has strict confined space air monitoring requirements, including pre-entry testing and continuous monitoring in some cases.

• Choosing the right confined space monitoring equipment depends on the types of gases present, the space itself, and whether real-time remote monitoring is needed.

• Atmospheric testing isn’t a one-and-done deal; it must be conducted before and during entry and for the entire space while using appropriate procedures to ensure accuracy.

• Confined space monitoring, coupled with proper respiratory protection and safety procedures, ensures a safe working environment for employees.

What Is Confined Space Monitoring?

If you’ve ever worked in industrial settings—whether it's sewers, storage tanks, or tunnels—you know that confined spaces are unpredictable. One second, everything seems fine. The next, you're dealing with an invisible gas buildup that can knock someone out before they even realize what's happening.

Confined space monitoring is exactly what it sounds like: tracking the air inside these enclosed areas to detect dangerous gases, lack of oxygen, or explosive conditions. Some confined spaces have natural ventilation that helps dilute hazardous gases, while others trap them, creating a silent but deadly hazard.

To stay safe, the Occupational Safety and Health Administration (OSHA) standard 1910.146(d)(5)(i) requires pre-entry atmospheric testing and, in some cases, continuous monitoring while workers are inside. The right air monitoring device for confined space work will depend on the environment and the specific risks present.

What Gases Require Monitoring When Entering a Confined Space?

Some gases pose an immediate threat to life and health (IDLH), while others cause long-term harm with prolonged exposure. Before entering a confined space, it is essential to monitor the following gases in the correct order: oxygen, combustibles, and toxic gases.

Combustible Gases

Any confined space with flammable vapors such as propane, methane, or gasoline fumes must be monitored for serious fire or explosion risk. If the concentration of these gases reaches or exceeds the lower explosive limit (LEL), a single spark or heat source could trigger ignition. OSHA requires that combustible gas levels remain below 10% of the LEL before entry into a confined space.

Oxygen (O₂)

Oxygen levels must remain within a safe range for breathing and fire prevention. The OSHA safe range for oxygen levels is between 19.5% and 23.5%. Both low and oxygen levels can pose hazards in confined spaces:

• • Oxygen deficiency (<19.5%) can cause dizziness, unconsciousness, or death.

  • Oxygen enrichment (>23.5%) makes flammable materials ignite faster.

Remember that most gas detectors must have oxygen levels between these parameters to accurately detect the other gases.

Toxic Gases That Can Be Deadly in a Confined Space

These gases vary depending on the worksite, but all pose serious risks:

Some gases, like carbon monoxide and hydrogen sulfide, can be deadly in minutes, while others, like nitrogen dioxide and sulfur dioxide, cause long-term health damage. Since gases have different densities, they do not mix evenly, which means monitoring must be conducted at multiple levels within the space. Carbon monoxide may even be generated from nearby ventilation equipment running on generators, so it’s important to ensure equipment outside the space is not affecting conditions on the inside of the space.

Types of Confined Space Air Monitoring Equipment

The effectiveness of confined space air monitoring equipment depends largely on the type of sensors used to detect gases. Different sensors work in various ways, depending on the gases they are designed to measure.

Types of Sensors in Gas Monitoring Equipment

• • Catalytic Bead Sensors – Used for combustible gas detection, these sensors measure the heat generated when gas oxidizes on a catalyst-coated bead. They are reliable for measuring gases such as methane and propane but require oxygen to function properly.

  • Infrared (IR) Sensors – Common in environments where combustible gases may be present but oxygen levels vary. Infrared sensors detect gases like carbon dioxide and hydrocarbons by measuring how they absorb infrared light. They are more resistant to poisoning compared to catalytic bead sensors.

  • Electrochemical Sensors – Primarily used for toxic gas detection, electrochemical sensors measure gas concentration through a chemical reaction that generates an electrical signal. These sensors are used for gases like carbon monoxide, hydrogen sulfide, and chlorine.

  • Photoionization Detectors (PIDs) – Detect volatile organic compounds (VOCs) and some toxic substances by using ultraviolet light to ionize gas molecules. They provide fast and sensitive readings, making them useful in chemical plants and hazardous waste sites.

Understanding how gas sensors work is only part of the equation. The next step is selecting the right confined space monitoring equipment based on the work environment, potential gas hazards, and the level of monitoring required.

Some confined spaces require personal gas monitors that clip onto clothing or harnesses and immediately alert the wearer to hazardous gas levels, while others benefit from area gas monitors that provide coverage across a larger workspace. In high-risk locations where gas hazards are a constant concern, fixed monitoring systems offer continuous surveillance and automated safety alerts.

Each of these monitoring solutions is designed to work with different types of sensors, providing accurate and real-time data to help workers make informed decisions before and during entry into a confined space.

Procedures for Atmospheric Testing in Confined Spaces

Atmospheric testing in a permit-required confined space follows a structured process, with evaluation testing, verification testing, and duration of testing being key components:

Evaluation Testing

Evaluation testing is the first step in determining whether a confined space contains hazardous atmospheric conditions. The goal is to identify atmospheric hazards that may be present or could develop over time. This process requires the use of gas detection equipment that can measure hazardous substances at levels well below exposure limits.

The results of evaluation testing help establish entry procedures and safety measures to reduce risks before a worker enters the space. A technically qualified professional, such as an industrial hygienist, registered safety engineer, or OSHA consultant, must review the test results to determine whether the space is safe for entry or if additional precautions are needed. The evaluation process must also consider any serious hazards that could arise while work is being performed inside the confined space.

Verification Testing

Once evaluation testing has been completed and a permit space is prepared for entry, verification testing is conducted. This step is meant to confirm that the space now meets the safe entry conditions outlined in the permit. Verification testing follows a strict order of testing:

1. Oxygen Levels – Checked first to confirm that oxygen is within a safe range (19.5% to 23.5%).

2. Combustible Gases – Tested next to determine if flammable gas concentrations are below 10% of the lower explosive limit (LEL).

3. Toxic Gases and Vapors – Monitored last to identify any hazardous substances that could pose a health risk.

The actual gas concentration readings must be documented on the permit, alongside the established safe entry levels. If verification testing identifies unsafe conditions, the space must be ventilated or other hazard controls must be applied before re-testing.

Duration of Testing

The length of time required for atmospheric testing depends on the type of gas detection equipment used and the characteristics of the confined space. Each sensor or detector has a minimum response time, which is specified by the manufacturer. This is the time required for the equipment to detect and display the presence of toxic substances accurately.

When hoses or probe extensions are attached to the monitoring device, additional time must be allowed for air from different depths of the confined space to be drawn into the sensors. If the confined space has layered atmospheres or remote areas away from the entry point, testing must be conducted at different heights and locations to get a full assessment of the atmospheric conditions. Typically the manufacturer will state the extra time needed for accurate measurements depending on the length of any tubing or probes attached.

What Is a Confined Space With Layered Atmospheres?

Some confined spaces have different gases sitting in layers due to their densities. This means the air you breathe at the entrance could be fine, but three feet lower, there's a toxic pocket of hydrogen sulfide or a lack of oxygen. This is why confined space air monitoring equipment should always take readings from various depths, using a pump-style monitor if necessary.

Due to gas stratification, workers may encounter high concentrations of dangerous gases at different levels, making respiratory personal protective equipment like air-purifying respirators essential.

Best Practices for Atmospheric Testing Procedures in Confined Spaces

Atmospheric testing in confined spaces must be accurate, thorough, and performed at the right intervals to detect potential dangers. The following best practices help improve testing reliability and reduce exposure risks:

1. Conduct Testing in the Correct Sequence: Atmospheric testing must follow a specific order (oxygen, combustible gases, and toxic gases and vapors) to prevent misleading readings and allow workers to interpret results correctly.

2. Perform Multi-Level Sampling: Testing must be conducted at the top, middle, and bottom of the space to account for the layering of gases.

3. Allow Time for Sensor Response: Moving too quickly during sampling can lead to false-negative readings, where hazardous gases are present but not detected due to insufficient exposure time.

4. Continuously Monitor the Atmosphere: Activities such as welding, chemical cleaning, or material movement can release gases, while ventilation changes can cause hazardous accumulations. Continuous or periodic testing helps detect dangerous fluctuations in gas levels.

5. Retest After Ventilation or Environmental Changes: If ventilation is used to remove hazardous gases or improve oxygen levels, the atmosphere must be retested before workers re-enter the space. 

6. Maintain and Calibrate Equipment: Gas detectors must be regularly calibrated and bump-tested to confirm they are functioning correctly.

Understanding Centralized Confined Space Monitoring

Traditional confined space monitoring relies on workers using handheld gas detectors, but centralized confined space monitoring takes worker safety to another level by using remote, real-time monitoring systems. It uses a combination of wireless personal gas detectors, area monitors, and fixed sensors to collect real-time atmospheric data. The information is then transmitted to a remote monitoring center, like the one shown in the image above, which may be located onsite or at an offsite operations hub. 

But does this type of monitoring system comply with OSHA standard §1910.146(c)(5)(ii)(C)?

A remote monitoring center can meet OSHA’s standards for confined space entry, provided specific conditions are satisfied. OSHA allows the use of test or monitoring equipment that displays results at a central location for pre-entry atmospheric testing as long as:

• • There is minimal delay between collecting the test sample and displaying the results.

  • The entrant can return to the confined space entry point quickly after observing the pre-entry test.

  • No hazardous atmosphere develops between the time of testing and when the worker enters.

When these conditions are met, remote monitoring centers provide a compliant and effective method for atmospheric testing in confined spaces, offering real-time detection and enhanced worker safety.

At the end of the day, gas testing equipment must be trusted and verified prior to making entry. All too often someone may think, “The gas meter is malfunctioning” if it’s alarming. The truth is, it may be alerting the user of a hazardous environment. Trust but verify the data and prove the gas detector is functioning reliably with bump testing, calibrating regular maintenance. 

Confined Space Monitoring FAQs

Which employee is responsible for monitoring activities inside and outside the confined space?

The attendant is responsible for monitoring activities outside the confined space, keeping track of atmospheric conditions, and communicating with entrants. The entrant must also wear a personal gas monitor to detect hazards inside the space.

How often do you monitor a confined space?

Pre-entry atmospheric testing must be conducted before a worker enters, and continuous or periodic monitoring is required if conditions could change. Monitoring should be ongoing when work processes (e.g., welding, chemical use) or ventilation changes may affect gas levels.

What is a safe LEL level in confined space?

The Lower Explosive Limit (LEL) must be kept below 10% for safe entry into a confined space. If LEL levels exceed this threshold, the space is considered hazardous and requires ventilation or additional controls before entry.

How to conduct air monitoring in a confined space?

Use a direct-reading gas detector to test for oxygen levels, combustible gases, and toxic gases, following the correct sequence: oxygen first, then flammables, then toxins. Sampling should be done at multiple heights (top, middle, bottom) since gases stratify in confined spaces.

What are the toxic gases in confined space?

Common toxic gases include carbon monoxide (CO), hydrogen sulfide (H₂S), chlorine (Cl₂), ammonia (NH₃), sulfur dioxide (SO₂), nitrogen dioxide (NO₂), and phosphine (PH₃). These gases can cause respiratory distress, poisoning, or even death at elevated levels.

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