Before anyone enters a confined space, someone has to test the air. You cannot see or smell most of the gases that make these spaces dangerous, so a confined space gas monitor does the job your senses cannot. It checks the atmosphere for too little oxygen, for flammable gas, and for toxic gases, and it keeps checking while the work goes on. A confined space can hold many different gases, but four come up most often. This guide covers those four gases you need to monitor, the safe levels set by Australian standards, and what a compliant monitoring process looks like.
We wrote it for Australian workplaces, from utilities to electrical contractors, where entering a pit, tank, or switchroom is part of the job. It is also a job for trained, competent workers only. Testing the air or entering a confined space calls for current confined space entry training, not just the right monitor.
Why confined space gas monitoring matters
Workers enter confined spaces far less often than most work areas, but they cause a disproportionate share of workplace deaths. Many victims are would-be rescuers who go in after a workmate and are overcome by the same air. That is why the law treats these spaces seriously. In Australia, confined space work sits under the model WHS Regulations, Part 4.3, and the detail sits in the Confined Spaces Code of Practice published by Safe Work Australia. The Australian Standard AS 2865 covers the practical side of planning and entry. Together they make one thing clear: you test the air first, and you keep testing it.
The four gases a confined space gas monitor checks
A monitor built for entry work checks four things at once. Each one fails in a different way, so all four matter.
Oxygen
Normal air holds about 20.9% oxygen. The safe working range is 19.5% to 23.5%. Below 19.5% the air is oxygen deficient and can cause dizziness, collapse, and death with little warning. Above 23.5% the air is oxygen enriched, which makes materials burn far more easily.
Flammable gas (LEL)
LEL stands for lower explosive limit, the point at which a gas can ignite. The rule is simple: keep the reading below 5% of LEL. LEL stands for lower explosive limit, the point at which a gas can ignite. Below 5% of LEL, work can continue. Between 5% and 10% of LEL, everyone must leave the space, unless a correctly calibrated monitor keeps running there continuously. At 10% of LEL or above, everyone leaves straight away.
Carbon monoxide
Carbon monoxide is toxic and gives no warning of its own. From 1 December 2026 the Australian exposure limit is 20 parts per million as an eight-hour time-weighted average (TWA), with a short-term exposure limit (STEL) of 30 parts per million over 15 minutes. (The previous limit was 30 ppm, so this is a tightening worth noting.)
The TWA and STEL average exposure over time, and a monitor uses them as alarm set points. On site, though, you are more likely to react to the monitor’s low and high alarms. These trigger the instant carbon monoxide reaches a set level, rather than waiting for an average to build. The WatchGas SST Micro comes factory-set at a 35 ppm low alarm and a 200 ppm high alarm. You can adjust both through the SST dock or app.
Hydrogen sulphide
Hydrogen sulphide smells of rotten eggs at low levels, then deadens your sense of smell, so your nose is no guide. It has two exposure limits. Across a full eight-hour day, the average must stay below 10 parts per million. Over any single 15-minute period, it must stay below 15 parts per million. The short-term limit is higher because the body can handle a brief spike, as long as the day-long average stays lower.
Those figures average exposure over time. The monitor also has instant low and high alarms that trip the moment hydrogen sulphide reaches a set level, without waiting for an average to build. The WatchGas SST Micro comes factory-set at 10 ppm low and 15 ppm high. Both match the exposure limits above. You can adjust them through the dock or app.
What the rules require during entry
Gas monitoring is one part of a confined space entry, not all of it. The rules also expect a risk assessment, an entry permit, and control of the air through mechanical ventilation or purging. Monitoring tells you whether those controls are working, before entry and throughout the job.
You have to test the atmosphere properly, and the Confined Spaces Code sets out how. A competent person carries out the testing, using a monitor suited to the gases present and correctly calibrated. Calibrate the monitor at least every 180 days, and run a functional bump test before each day of use. Your site procedure defines who counts as a competent person. The first test happens from outside the space, by lowering a probe through an opening before anyone goes in. As a rule, you test in order: oxygen first, then flammable gas, then toxic gases, because some sensors need oxygen to read correctly. Start by testing around the opening, then work down in roughly one-metre steps to the lowest point of the space. Gases settle at different heights, so a single reading at the entry tells you very little.
If a reading is unsafe
If any gas sits outside its safe range, do not enter. Ventilate the space, let it clear, and retest. Only go in once every reading is safe and holds there, and leave straight away if an alarm sounds.
Ventilation is the control that does the real work. Mechanical ventilation, a portable blower and ducting, sets up quickly and clears a space far better than natural airflow. It is also the better answer than sending someone in on breathing apparatus. Clear the air first, retest, and keep ventilating through the job if the space calls for it.
An unexpected result changes things. If the initial test returns anything you did not expect, the space needs continuous monitoring. That means monitoring for the whole time it is occupied, not just a check on the way in.
Where electrical crews face confined spaces
Confined spaces are not only tanks and silos. Electrical and utility crews meet them often, in underground cable pits and trenches, substation basements, switchrooms, and plant rooms. These spaces can trap the same gases, and adding live electrical work raises the stakes further. The monitoring rules apply here just as they do anywhere else, so the same four gases need checking before and during entry.
Keeping your confined space gas monitor reliable
A monitor only protects you when its readings are true, and two checks keep it honest. A bump test exposes the monitor to a known gas before use, to confirm the sensors and alarms respond. Calibration, done on a set schedule, resets the sensors against a reference gas so the numbers stay accurate. Follow the maker’s intervals and your site rules, and take any monitor that fails out of service. There is one more requirement worth stating plainly: a monitor used where flammable gas may be present has to be intrinsically safe, meaning it cannot spark and ignite the very air it is testing. AS/NZS 60079.29.2 covers the selection, use, and upkeep of these detectors.
Choosing a confined space gas monitor
For entry work you want a monitor that covers all four gases, survives a worksite, and stays simple to use across a full shift. The WatchGas SST Micro, Mini & Pumped are built for exactly this. They detect carbon monoxide, hydrogen sulphide, oxygen, and flammable gas in one compact unit, and it is rated for confined space pre-entry and continuous monitoring. It carries ATEX and IECEx intrinsic-safety certification for hazardous atmospheres, an IP65/68 rating against dust and water, and one-button operation you can manage with gloved hands. It also pairs with a calibration docking station, which keeps the bump-test and calibration routine straightforward.
Where to get more information
For the full legal detail, read the Confined Spaces Code of Practice and AS 2865. Your state work health and safety regulator, such as SafeWork NSW or WorkSafe Victoria, can advise on the rules in your area. As above, anyone who tests the air or enters a confined space should hold current confined space entry training.
Summary
Confined space gas monitoring decides whether it is safe to enter at all. Watch the four gases, hold to the levels set by Safe Work Australia, and keep your monitor bump-tested, calibrated, and intrinsically safe. Get those right and the rest of the entry plan stands on solid ground.