A Field Guide to Atmospheric Monitoring on Commercial Dive Sites

I’ve seen this exact moment on the deck. A dive team is set for a confined entry at a wastewater outfall in NSW, and the supervisor drops a pumped multi-gas monitor into the culvert.

 

Seconds later, the H2S reading jumps past safe limits. The job pauses, nobody enters, and the instrument makes the decision, not anyone’s nose.

 

If you work on Australian dive projects, you already know how fast conditions can turn. Sea chests, culverts, cofferdams, tanks, and reservoir assets can shift from safe to lethal within minutes.

 

Oxygen can fall, flammables can rise, and contaminants like hydrogen sulfide (H2S) and carbon monoxide (CO) can build without warning. The fix isn’t bravado, it’s a repeatable test and response routine.

Quick Reference

Use these numbers and actions as a fast pre-start check, then back them up with permits and logs.

 

• Safe oxygen sits between 19.5% and 23.5% by volume. Below 19.5%, air-supplied respiratory protective equipment (RPE) is required if entry can’t be avoided.
• Keep flammables below 5% LEL during work. LEL means Lower Explosive Limit. At 5 to 10% LEL, remove workers unless continuous LEL monitoring is active. At 10% LEL or above, egress immediately.
• Always test from outside first. Sample side-to-side and top-to-bottom because H2S tends to sit low while methane can collect high.
• CO limits are tightening. The current WES is 30 ppm (8-hour TWA). TWA means time-weighted average. On 1 December 2026, the new WEL drops that to 20 ppm. Update your alarm setpoints now.
• Bump test every portable detector before each day’s use. Any unit that fails must be calibrated or pulled from service.
• Use IECEx or ANZEx-certified instruments. ATEX alone isn’t natively recognised in Australia without a Conformity Assessment Document.

How Air Monitoring Fits the Job

Air monitoring is not the same task as checking breathing gas quality. Both matter, but they use different tools and different limits.

 

Breathing air for diving must meet EN 12021 limits, including CO at or below 5 ppm. Air monitoring focuses on the atmosphere around divers and topside crews, especially around confined or partially enclosed spaces.

 

Australia’s Confined Spaces Code of Practice expects testing for oxygen, flammables (as %LEL), and harmful contaminants such as H2S and CO. A competent person must use a suitable detector that’s correctly calibrated.

 

This intersects with dive work more than people admit. Wet and dry bells, sea chests, culverts, caissons, tanks, ducts, trenches, and reservoir infrastructure can all meet the confined space definition.

 

High-risk dive work in Australia also ties back to AS/NZS 2299.1, and state regulators, including SafeWork NSW, reference it in guidance and enforcement.

Three Ways Monitoring Prevents Incidents

Monitoring buys decision time, and that time protects people and keeps projects moving.

1. Early Warnings Stop Silent Killers

 

Oxygen deficiency, H2S, and CO can rise with little or no sensory warning. H2S can also shut down your sense of smell at higher concentrations, which tricks experienced workers as easily as new ones.

 

Continuous monitoring during entry catches changes caused by disturbed sludge, hot work, backflow, or shifts in intake air. One alarm at the right time beats a rushed rescue every day.

2. Compliance Reduces Stops and Liability

Today’s Workplace Exposure Standards (WES) still apply, including H2S at 10 ppm (8-hour TWA) and 15 ppm STEL. STEL means short-term exposure limit, usually assessed over 15 minutes.

 

Safe Work Australia will replace WES with Workplace Exposure Limits (WEL) on 1 December 2026. CO tightens from 30 ppm TWA to 20 ppm, so your alarm setpoints, briefing cards, and permit language should be updated well before then. Staying ahead of regulatory changes is a familiar challenge across high-risk industries; the same discipline applies whether you’re managing compliance in a warehouse or on a dive site.

 

Certification matters too. IECEx or ANZEx markings on instruments cut the back-and-forth with auditors, principals, and site safety teams.

3. Better Sampling Means Fewer Aborted Entries

Outside-first testing, stratification-aware probes, and stable readings reduce guesswork. That means fewer false stops when conditions are genuinely safe, and faster calls to abort when they’re not.

 

Sensors to Bring and Why

 

Start with oxygen and flammables, then add toxic and VOC sensors based on what the site can generate.

Oxygen (Electrochemical Sensor)

 

Treat 19.5% to 23.5% by volume as the safe band. Below 19.5%, you’re in oxygen-deficient territory and must control the risk with ventilation and suitable air-supplied RPE if entry still goes ahead.

 

When you ventilate, don’t use oxygen to “improve” the atmosphere. Oxygen enrichment increases fire risk and can push you past 23.5%.

Flammables (%LEL)

You’ll usually choose between catalytic bead and infrared sensors. Catalytic bead sensors cover a wide range of flammables, but they can be poisoned by silicones and sulfur compounds and they need oxygen to work properly.

 

Infrared LEL sensors resist poisoning and still function in low-oxygen environments, but they’re generally blind to hydrogen. Match the sensor to the likely fuel, not just what you’ve always bought.

 

Keep your action thresholds aligned to the Code of Practice. Below 5% LEL is acceptable, 5 to 10% LEL requires removal unless continuous monitoring is operating, and 10% LEL or above means immediate egress.

Toxic Gases: H2S and CO

H2S is heavier than air, so sample low and don’t assume ventilation has mixed the space evenly. Treat any alarm seriously, because brief spikes can occur when sludge or scale is disturbed.

 

CO commonly comes from engines, generators, and compressors. Its current WES TWA of 30 ppm drops to 20 ppm under WEL, so adjust alarms and documentation before December 2026.

 

If your site needs IECEx-rated multi-gas units with pump kits for remote sampling on barges and pipe risers, it’s worth checking that the model you choose can handle long sample lines, has clear flow-fault alarms, and comes with local support for calibration gases and docking. For options proven in gas transmission environments from a single Australian supplier, see ProDetec’s gas detectors for options proven in gas transmission environments from a single Australian supplier.

VOCs (PID Sensor)

Photoionization detectors (PIDs) are useful when solvent vapours or other organics are possible, such as from coatings, cleaners, or residues. They don’t detect methane and several other low-ionization alkanes, so don’t treat a PID as a flammability tool.

 

If you use a PID, remember it reports in a reference gas and needs correction factors for estimates. Use PTFE or metal sampling lines for VOC work because long soft plastic runs can absorb vapours and skew results.

Maintenance and QA Checks

 

Trust comes from checks, not labels. Bump test every portable detector before each day’s use, and remove any unit that fails until it’s calibrated and passing again.

 

Full calibration intervals commonly sit between 6 and 12 months, but shorten that cycle in hot, humid, or wastewater-heavy environments where sensors drift faster.

 

Keep traceable records of bump tests, calibrations, alarms, serial numbers, test gases used, and operator IDs. Check pumps, filters, probes, and hoses, and note the maximum sample-line length used on each permit.

 

When a site has tight tide windows and shutdown constraints, you often need crews that can mobilise quickly, integrate with permit systems, and run confined-entry testing without slowing the job. If you want that capability backed by ADAS-certified teams experienced in pipelines, culverts, and reservoir entries, Harcan Marine offers commercial diving services in Australia with ADAS-certified teams experienced in pipelines, culverts, and reservoir entries.

Sampling Locations and Timing

Test from outside before entry, then switch to continuous monitoring whenever conditions can change.

Pre-Entry Sampling

Insert a probe from outside the space and sweep side-to-side and top-to-bottom. Let the reading stabilise after each move, then record values in a consistent order: oxygen, then LEL, then toxics.

 

This sequence helps you avoid misleading readings and keeps ignition risk controlled during testing.

During Operations

Monitor the bell and adjacent voids, not just the point of entry. Watch for off-gassing when disturbing sludge, breaking crusts, jetting, or opening isolation points.

 

Be clear on roles. The person watching atmospheric readings is not doing breathing-gas quality checks, and compressor and bailout gases still need separate EN 12021 verification.

Topside and Deck

Test compressor intakes, engine rooms, plant rooms, and any staging enclosure where air can stagnate. If hot work is planned nearby, increase test frequency and widen the sampling zone.

Choosing and Certifying Gear in Australia

Good equipment is only useful if it’s certified correctly, supported with accessories, and easy to log on site.

 

• IECEx or ANZEx marking on the device, such as Ex ia IIC T4 Ga. Avoid ATEX-only units unless you can obtain a Conformity Assessment Document.
• Pumped sampling for remote or vertical entries. Check flow-fault alarms. Diffusion is suitable for personal monitoring in a ventilated breathing zone.
• Sensor suite: O2 plus LEL plus H2S plus CO at minimum. Add PID when VOC risk is credible.
• Environmental specs: IP67 or better, appropriate operating temperature range, onboard data logging, and Bluetooth for downloads where permitted.
• Accessories: docking and bump stations, calibration gases, filters, and sample probes sized for your typical openings.

A 12-Step Procedure You Can Brief

A short standard operating guideline (SOG) makes the work repeatable from toolbox talk to permit close-out.

 

1. Run a toolbox talk covering hazards, thresholds, and actions on alarms.
2. Bump test all units and confirm calibration due dates.
3. Zero instruments in clean air or with filtered zero gas, as required by the manufacturer.
4. Perform outside-first remote sampling in the order of O2, LEL, then toxics.
5. Sweep top, middle, and bottom, allowing readings to stabilise at each level.
6. Record readings, locations, times, and the tester’s ID on the permit.
7. Ventilate or purge, then retest until the atmosphere is within limits.
8. If 5 to 10% LEL persists and work must continue, switch to continuous LEL monitoring and control ignition sources.
9. Maintain continuous monitoring during entry and record any alarms.
10. If any limit is exceeded, stop work, egress, and reassess controls before re-entry.
11. After exit, retest and close the permit with final readings.
12. Download logs and file them with the permit pack for audit trail.

Conclusion

Consistent testing, clear action points, and clean records turn invisible hazards into controlled work. When the numbers move, you’ll already know who calls it, what happens next, and how you’ll prove it later.

FAQs

These are the questions that usually come up when crews start tightening their testing routine.

Do I need a pumped monitor for every job?

Use pumped units when you need to sample from outside a space or at distance. Diffusion monitors suit personal, continuous checks in a ventilated breathing zone.

What is the correct order of atmospheric tests?

Test oxygen first, then flammables as a percentage of LEL, then toxics like H2S and CO. Keeping the order consistent reduces confusion and prevents misleading readings.

How often should we bump test and calibrate?

Bump test before each day’s use. Calibrate to the manufacturer’s guidance and your site conditions, which commonly means every 6 to 12 months. If a bump test fails, calibrate immediately or remove the unit from service.

Do PIDs replace flammability sensors?

No. PIDs detect many volatile organic compounds, but they can’t see methane and several simple alkanes. Use them as an extra channel of information, not a substitute.