Confined Space Entrant · Practice Tool
Atmospheric Hazards
A gas monitor does not alarm at the lower explosive limit. It alarms at a fraction of it, and entry is prohibited at a larger fraction, long before the atmosphere could actually ignite.
Put a gas's LEL in and this works out both numbers. The LEL comes off the safety data sheet for the product you are actually working near.
Read this first. A practice tool, not an authority. Your entry permit, your gas monitor and your written procedure decide whether a space is safe to enter. Nothing on this page can tell you that, and a monitor reading below an alarm threshold is not on its own a reason to enter.
Step 1
Alarm Threshold
Ten percent of the lower explosive limit. This is where the LEL alarm on a monitor sounds.
Alarm sounds at
LEL × 0.1 = A10
The LEL is a property of the substance, not of the space. Read it off the safety data sheet for the product in your workplace. This page does not know it and will not supply it.
From the safety data sheet. The course's six are in the table further down.
Alarm sounds at
LEL × 0.1
—
10% of the LEL
Worked examples from the course material
| LEL | A10 | Load |
|---|---|---|
| 2.5 | 0.25% (acetone) | |
| 12.5 | 1.25% (carbon monoxide) |
10 CSE - Student Manual - V2.2 250519, section 2.3.2. 'Up to 10 % of the LEL: LEL alarm will sound.' The manual notes this is a best practice followed by most air monitor manufacturers. Both rows below are that section's own worked answers.
Step 2
Entry Prohibited Past
Twenty percent of the lower explosive limit. Past this, entry is prohibited unless it is an emergency.
Entry prohibited past
LEL × 0.2 = A20
This is a hard stop in the Regulation, not a judgement call, and it is not the point at which the atmosphere becomes flammable. It is the point at which the margin is gone.
Carried down from step 1. Type over it to work this step on its own.
Entry prohibited past
LEL × 0.2
—
20% of the LEL
Worked examples from the course material
| LEL | A20 | Load |
|---|---|---|
| 2.5 | 0.50% (acetone) | |
| 12.5 | 2.50% (carbon monoxide) |
10 CSE - Student Manual - V2.2 250519, section 2.3.2. 'Past 20 % of the LEL: Prohibited against entry unless in an emergency', citing OHS Regulation 5.31(c) and (d).
Reference
Flammable ranges
The six substances the course uses. Below the LEL the mixture is too lean to burn, above the UEL too rich. In between it will ignite.
Each of these has its safety data sheet published alongside this page.
| Substance | LEL | UEL | 10% of LEL | 20% of LEL | Description |
|---|---|---|---|---|---|
| Acetone | 2.5% | 12.8% | 0.25% | 0.50% | Used in the manufacture of adhesives, flame retardants. |
| Acetylene | 2.5% | 100% | 0.25% | 0.50% | Oxygen depletion (asphyxiant). Explosive potential. |
| Allyl chloride | 3.2% | 11.2% | 0.32% | 0.64% | Used as cleaner for grease, adhesive, and paint. |
| Carbon monoxide | 12.5% | 74% | 1.25% | 2.50% | By-product of incomplete combustion. |
| Gasoline | 1.4% | 7.6% | 0.14% | 0.28% | |
| Hydrogen sulfide | 4% | 44% | 0.40% | 0.80% | By-product of decomposition. |
LEL and UEL are often written LFL and UFL. They mean the same thing.
The acetone and carbon monoxide figures in the 10% column are the manual's own worked answers. The rest apply the same arithmetic to the manual's own LEL values.
Source. 10 CSE - Student Manual - V2.2 250519, section 2.3.2, 'Flammable ranges'. The LEL, UEL and description columns are the manual's; the two percentage columns apply the manual's own thresholds.
Reference
Exposure limits and vapour density
Flammability is one hazard. A gas well below its LEL can still be poisoning you, and where it sits in a space depends on whether it is heavier or lighter than air.
Vapour density is relative to air at 1. Anything above 1 collects low, which in a confined space means where you are standing.
| Substance | 8-hour TWA | STEL / Ceiling | Vapour density |
|---|---|---|---|
| Acetone | 250 ppm | 500 ppm | 2.0 |
| Acetylene | N/A | Simple asphyxiant | 0.907 |
| Allyl chloride | 1 ppm | 2 ppm | 2.6 |
| Carbon monoxide | 25 ppm | 100 ppm | 0.97 |
| Gasoline | 300 ppm | 500 ppm | 3–4 |
| Hydrogen sulfide | N/A | C 10 ppm | 1.537 |
8-hour TWA is the time weighted average concentration which may not be exceeded over a normal 8-hour work period.
STEL is the time-weighted average over a 15-minute period, limited to no more than four such periods in an 8-hour shift with at least one hour between any two. A C before a figure is a ceiling limit, which may not be exceeded at all.
If the work period is more than 8 hours in a 24-hour day, the 8-hour TWA limit must be lowered to a revised time weighted average.
Source. 10 CSE - Student Manual - V2.2 250519, section 2.3.3, 'Exposure Limits'. Reproduced as printed.