Gas Testing and Atmospheric Monitoring

What is atmospheric monitoring?

Atmospheric monitoring uses electronic instruments to measure invisible threats in your work environment. Many fatal gases like carbon monoxide and nitrogen are odourless and colourless. Others, like hydrogen sulfide, cause olfactory fatigue—deadening your sense of smell at high concentrations when you need it most.

Your reliance on electronic instrumentation isn't just best practice—it's a critical life-saving requirement mandated by the Work Health and Safety Act 2011 (WHS Act). Human physiology simply cannot protect you from these hazards.

While frequently associated with confined space entry, you need atmospheric monitoring in various scenarios: hot work where flammable vapours may exist, emergency response zones, process operations in refineries and water treatment plants, and anywhere the atmosphere may be compromised.

The hierarchy of atmospheric hazards

Not all hazards present the same immediate timeframe for catastrophe. Industry standards, including AS/NZS 2865:2009 Confined Spaces, dictate a specific order of testing based on threat immediacy and technological requirements.

1. Oxygen (O₂) - Test first

You test oxygen levels first for two critical reasons. Oxygen deficiency causes immediate asphyxiation, often without warning symptoms. More importantly, the most common sensors used to detect explosive gases (catalytic beads) require oxygen to function correctly.

A lack of oxygen can cause these sensors to provide false "safe" readings in a highly explosive atmosphere. This isn't a theoretical concern—it's a documented failure mode that has contributed to fatalities.

2. Flammable gases and vapours (LEL) - Test second

Flammable hazards are tested second because the risk is fire or explosion—immediate catastrophic events that can result in multiple fatalities and structural failure. You measure concentration as a percentage of the Lower Explosive Limit (LEL).

3. Toxic gases and airborne contaminants - Test last

Toxic gases are tested last because, while potentially lethal, many involve a dose-response relationship over time (Time Weighted Average). Flammability and asphyxiation are immediate acute hazards. Common targets include Hydrogen Sulfide (H₂S), Carbon Monoxide (CO), and Volatile Organic Compounds (VOCs).

Regulatory framework in Australia

The Work Health and Safety Act 2011 imposes a primary duty of care on Persons Conducting a Business or Undertaking (PCBUs) to ensure, so far as is reasonably practicable, the health and safety of workers. This broad duty is operationalised through specific regulations.

WHS Regulations Part 4.3: Confined Spaces

Regulation 34 mandates the identification of all reasonably foreseeable hazards, including atmospheric hazards. You must ensure atmospheric testing is carried out by a competent person using suitable, calibrated equipment.

Where your risk assessment indicates that the atmosphere may change (for example, due to work activities releasing sludge gases), you must implement continuous monitoring throughout the duration of work.

Record keeping requirements

Documentation is a critical component of compliance. You must record the results of atmospheric testing in writing and retain records for 28 days after the work is completed.

If a notifiable incident occurs in connection with the work, you must retain records for at least 2 years. This extended retention period supports investigation and prosecution processes.

Penalties and legal consequences

The legal landscape in Australia has become increasingly punitive regarding safety breaches. Industrial Manslaughter laws are now in effect across most Australian jurisdictions, including Queensland, Victoria, Western Australia, South Australia, the ACT, and New South Wales.

Body corporate penalties have escalated significantly. In NSW, the maximum penalty is $20 million; in South Australia and Victoria, it's $18 million; and in the ACT, it's $16.5 million. Senior officers and PCBUs face imprisonment terms of up to 20 to 25 years.

Even without a fatality, exposing an individual to a risk of death or serious injury (such as ordering a worker into a sewer without a gas detector) constitutes a Category 1 offence with maximum penalties for body corporates of approximately $11.8 million.

Transition to Workplace Exposure Limits (WEL)

Safe Work Australia is transitioning from Workplace Exposure Standards (WES) to Workplace Exposure Limits (WEL). The new WEL framework becomes mandatory on 1 December 2026.

The change from "Standard" to "Limit" reinforces that these values represent a hard ceiling that must not be exceeded. A critical aspect involves Hydrogen Sulfide (H₂S)—the current Time Weighted Average is 10 ppm, but Safe Work Australia is reviewing a proposal to significantly lower this limit to potentially 1 ppm, with compliance costs for the water industry projected at over $1.24 billion.

Understanding atmospheric hazards

Gas behaviour is governed by laws of diffusion, density, and chemical reactivity. To effectively monitor an atmosphere, you need to understand the physical and chemical properties of the hazards you face.

Oxygen (O₂)

The earth's atmosphere normally contains approximately 20.9% oxygen by volume. Atmospheres with less than 19.5% oxygen are considered unsafe.

At 15-19%, you may experience impaired coordination. Below 12%, respiration increases and judgment is severely compromised. Below 6%, loss of consciousness is immediate, followed by death. Causes include displacement by other gases (nitrogen, methane), consumption by rust inside steel vessels, or microbial activity in sewers.

Atmospheres with more than 23.5% oxygen present a severe fire hazard. Oxygen enrichment dramatically lowers the activation energy required for ignition. Materials that wouldn't burn in normal air (like fire-retardant clothing) can burn furiously in enriched atmospheres.

Flammable gases and the Lower Explosive Limit (LEL)

The Lower Explosive Limit is the minimum concentration of a combustible gas in air capable of propagating a flame. Below this level, the mixture is too "lean" to burn.

Gas detectors display flammability as a percentage of the LEL. For example, the LEL of Methane is 5% by volume. A monitor reading "10% LEL" actually detects 0.5% Methane by volume. Standard alarm set points are typically 5% or 10% LEL—at 10% LEL, evacuation is mandatory.

Toxic gases and exposure standards

Toxic gases interact with your body to cause harm through inhalation, absorption, or ingestion. You manage exposure via three primary metrics:

Key toxic gases

Hydrogen Sulfide (H₂S) has a "rotten egg" smell at low levels, but it deadens the olfactory nerve at higher levels (approximately 100 ppm). It's heavier than air and pools in low areas. High concentrations cause immediate respiratory paralysis.

Carbon Monoxide (CO) is a byproduct of incomplete combustion with roughly the same density as air. It binds to haemoglobin 200 times more effectively than oxygen, causing chemical asphyxiation.

Atmospheric stratification

Gases don't mix instantly. Their accumulation depends on vapour density relative to air (Air = 1.0). Lighter gases like Methane (0.55) and Hydrogen (0.07) accumulate at the top of confined spaces. Heavier gases like Hydrogen Sulfide (1.19), Propane (1.56), and petrol vapours (3.0-4.0) sink to the bottom.

This is why testing at a single point is insufficient. You must conduct stratified sampling—testing every 1.2 metres (4 feet) in the direction of travel to detect layers of different gases.

Sensor technology: mechanisms and limitations

A "gas detector" is a housing for multiple distinct sensor technologies. Understanding how these sensors work—and more importantly, how they fail—is the hallmark of a competent gas tester.

Catalytic bead sensors (Pellistors)

The catalytic bead sensor is the industry standard for detecting combustible gases (LEL). The sensor contains two platinum wire coils embedded in ceramic beads in a Wheatstone bridge electrical circuit. When combustible gas meets the detector bead, the catalyst facilitates oxidation (burning), raising the bead's temperature and electrical resistance.

Critical limitation: Because detection involves actual combustion, oxygen is required for the reaction. A catalytic bead sensor typically requires at least 10-15% oxygen to provide an accurate reading. In an oxygen-deficient atmosphere (below 10%), the sensor may read "0% LEL" even if the atmosphere is 100% methane—a potentially fatal false negative.

Sensor poisoning: Certain chemicals can permanently deactivate the catalyst. Silicones (found in hair products, lubricants, cleaners), lead compounds, and phosphates melt and coat the bead, preventing gas contact. Sulfur compounds and halogenated hydrocarbons can temporarily reduce sensitivity.

Infrared (IR) sensors

Infrared technology is increasingly used for LEL detection and is the standard for Carbon Dioxide (CO₂). Gases absorb infrared light at specific wavelengths—the sensor passes IR light through a sample chamber to a detector.

Advantages: IR sensors don't require oxygen to function, making them the only safe choice for testing inert atmospheres (nitrogen-purged tanks). They're immune to catalytic poisons and fail-safe—if the light source fails, the unit faults rather than reading zero.

Limitations: Diatomic molecules like Hydrogen (H₂) don't absorb IR light. An IR LEL sensor will read 0% LEL in a hydrogen-rich atmosphere—a critical blind spot. They're also calibrated for specific hydrocarbons and may respond differently to others.

Electrochemical sensors

Used for toxic gases (CO, H₂S, Cl₂, NH₃) and Oxygen, these operate like a micro-fuel cell. Gas diffuses through a membrane onto a working electrode, generating an electron flow (current) proportional to gas concentration.

Cross-sensitivity: A major limitation is the sensor's response to non-target gases. Standard Carbon Monoxide sensors are cross-sensitive to Hydrogen. In battery charging rooms where H₂ is present, a CO sensor might read high levels of CO that are actually H₂. Specialized "H₂-Compensated" CO sensors are required in these environments.

The chemical components (electrolyte and electrodes) are consumed over time. Oxygen sensors typically last 2 years; toxic sensors may last 2-4 years.

Photoionization Detectors (PID)

PIDs detect Volatile Organic Compounds (VOCs) at very low concentrations (parts per million or billion). A UV lamp emits high-energy photons that ionise gas molecules, creating a current. They're used for detecting solvents, fuels (benzene, toluene), and other organic vapours that might be toxic at levels far below their LEL.

Operational procedures and sampling strategies

Possessing a gas detector doesn't guarantee safety. You must rigorously follow correct operational procedures to ensure the data you collect is valid.

Pre-entry testing vs. personal monitoring

Personal monitoring (diffusion) involves a detector worn on your chest within the "breathing zone" (a 300mm hemisphere around your face). It relies on natural air currents to bring gas to the sensor, monitoring your immediate environment.

Pre-entry testing (pumped) requires a motorized pump to draw air through a hose before entering a confined space. You must test the atmosphere from the outside. Diffusion monitors cannot be used for pre-entry testing of deep spaces as they cannot sample remotely.

The "rule of thumb" for sample lines

When using a pump and hose, gas takes time to travel from the probe tip to the sensors. The industry standard rule is 2 seconds per metre of hose plus 2 minutes (or the device's T90 response time).

For a 10-metre hose: 10m × 2s/m = 20 seconds travel time, plus 120 seconds stabilization = 140 seconds total wait time at each sample point. If you drop the hose in and immediately look at the screen, you're reading the clean air that was already in the hose, not the toxic air at the bottom of the tank.

Stratified sampling protocol

Because gases stratify, the "Top, Middle, Bottom" approach is mandatory:

1. Approach: Test the atmosphere around the entry point (outside)
2. Top: Insert the probe slightly into the space to test for light gases (Methane, Hydrogen)
3. Middle: Lower the probe to the mid-point to test for neutral density gases (CO) and general air quality
4. Bottom: Lower the probe near the floor (without sucking up liquid) to test for heavy gases (H₂S, Propane, solvent vapours)
5. Record: Results from all levels must be recorded on the entry permit

Zeroing the instrument

"Zeroing" sets the baseline for your sensors. This critical step is often performed incorrectly. You must only zero in confirmed clean air (outside the plant or office).

The contaminated zero trap: If you zero the instrument while standing in a cloud of exhaust or near a minor gas leak, the instrument sets that contamination level as "0". When you move to clean air, the unit will read a negative value. More dangerously, if you enter a space with that specific concentration of gas, the monitor will read "0", masking the hazard.

If fresh air cannot be guaranteed, use a cylinder of impurity-free "Zero Air" to calibrate the zero point.

Maintenance, calibration, and verification

Gas detectors are precision instruments that drift over time due to sensor degradation, environmental factors, and physical shock. You need a rigorous maintenance regime to maintain accuracy and compliance.

Bump testing (functional check)

A bump test is a qualitative check to ensure your unit is working. You expose the sensors to a known concentration of gas (from a calibration cylinder) that exceeds the alarm set points.

Verify that: (1) the sensors respond to the gas, (2) the readings rise to the expected level within tolerances, and (3) the audible and visual alarms activate.

Frequency: Prior to each day's use. This is strongly recommended by AS/NZS 60079.29.2 and mandated by many site procedures. A unit that fails a bump test must not be used. A detector could be dropped, the filter blocked, or the sensor poisoned yesterday—without a bump test, you enter the hazard zone with a non-functional device.

Calibration

Calibration is a quantitative adjustment of the sensor's response curve. Using a precise gas standard, the instrument's internal reference points are adjusted to match the known gas concentration, correcting for "sensor drift" (the natural loss of sensitivity over time).

Frequency: Every 6 months in Australia, as per manufacturer recommendations and AS/NZS 60079.29.2. Some high-risk sites may require quarterly calibration. Calibration must be performed by a competent person or accredited service provider.

Maintenance of sampling equipment

The pump and hose system requires specific attention. Particulate and hydrophobic (water-stop) filters prevent damage to the pump and sensors. You must check filters for blockages before use—a blocked filter can burn out the pump motor or prevent gas from reaching the sensor.

Leak testing: Before a pumped test, block the inlet with your finger. The pump should stall and alarm "Blockage." If it continues running, there's a leak in the hose, and the unit is sucking in ambient air rather than the sample.

Competency and training requirements

In Australia, operating a gas detector is a recognised skill that requires formal training. It's not sufficient to simply hand a worker a monitor.

Authorised gas tester

The industry standard competency is MSMWHS217 - Gas test atmospheres. This unit covers the physics of gases, operation of specific equipment (including bump testing and calibration checks), sampling techniques (stratification), and interpretation of results.

While there are no formal prerequisites, it's often undertaken in conjunction with confined space entry training. Industry best practice suggests refresher training every 2 to 3 years to stay abreast of legislative changes and new technology.

The role of the competent person

The WHS Regulations require risk assessments and atmospheric testing to be conducted by a "competent person"—someone who has acquired through training, qualification, or experience the knowledge and skills to carry out the task.

The competent person must sign the confined space entry permit, verifying that the atmosphere is safe and specifying the monitoring intervals (continuous or periodic).

Industry-specific risks

References and Further Reading

Safe Work Australia: Confined Spaces - Code of Practice provides comprehensive guidance on atmospheric testing requirements and procedures.

Work Health and Safety Act 2011 and WHS Regulations Part 4.3 establish the legal framework for atmospheric monitoring in confined spaces.

Australian Standard AS/NZS 2865:2009 - Confined spaces details the requirements for risk assessment, isolation, and atmospheric monitoring, including the competency of the gas tester.

Australian Standard AS/NZS 60079.29.2 covers the selection, installation, use, and maintenance of gas detectors, including the requirement for daily bump testing.

Safe Work Australia: New Workplace Exposure Limits (WEL) provides information on the transition from Workplace Exposure Standards to Workplace Exposure Limits, effective 1 December 2026.

Safe Work Australia: Maximum Monetary Penalties outlines the significant financial and custodial penalties for WHS breaches, including Industrial Manslaughter provisions.

MSMWHS217 Gas test atmospheres - National Training Package unit for authorised gas tester competency.

A national review of hydrogen sulphide exposure, limits and controls in the water industry discusses the proposed reduction in H₂S exposure limits and implications for compliance.