Authorized Gas Tester (AGT) key terms

The lowest concentration of a flammable gas or vapour in air at which a flame will still propagate through the mixture. Below this figure the mixture is too lean to burn, so readings are usually expressed as a percentage of this value.

The highest concentration of a flammable gas or vapour in air that will still support flame propagation. Above this figure the mixture is too rich to burn, but dilution with air can bring it back into the flammable range.

The band of concentrations between the lower and upper explosive limits within which a gas or vapour mixed with air can ignite. Any reading inside this band represents an immediate fire or explosion risk.

The lowest temperature at which a liquid gives off enough vapour to form an ignitable mixture with air near its surface. Liquids with low flash points generate flammable atmospheres at ordinary working temperatures.

The lowest temperature at which a substance will ignite spontaneously in air without any external ignition source. Hot surfaces, such as exhausts or steam lines, can therefore ignite vapours even when no spark is present.

The mass of a vapour compared with an equal volume of air at the same conditions. Values above one mean the gas tends to sink and collect in low-lying areas, pits, sumps and drains, which affects where samples must be taken.

How heavy a gas is compared with air, usually expressed as a ratio. Gases heavier than air accumulate at low points and lighter gases rise and gather near the roof, so this property drives sampling-point placement.

Layering of gases or vapours of different densities within an enclosed space, so that composition varies with height. A single sample at one level may miss a pocket of flammable or toxic gas elsewhere in the space.

Any energy input capable of starting combustion, such as sparks, flames, hot work, static discharge, hot surfaces or non-intrinsically safe equipment. Identifying and controlling these is central to hot work and flammable-atmosphere planning.

A gas or vapour that harms health when inhaled, even at low concentrations, for example hydrogen sulphide or carbon monoxide. Toxic limits are set separately from flammability limits and require their own measurement and controls.

A regulatory or advisory airborne concentration considered safe for a worker over a defined exposure period, such as an eight-hour time-weighted average or a short-term limit. Gas detector alarm points are often based on these values.

The average concentration of a substance in air over a reference period, usually eight hours, reflecting total inhaled dose rather than peak level. It is used to judge whether long-term exposure to a toxic gas is acceptable.

The maximum average concentration permitted for a short period, commonly fifteen minutes, that must not be repeated excessively during a shift. It protects against sudden peaks of toxic gas that a TWA alone would not control.

An enclosure that is not designed for continuous occupancy and has limited or restricted entry and exit, where serious injury can arise from hazardous atmosphere, engulfment or other hazards. Entry normally requires testing and a permit.

A formal written authorisation that defines the scope of a task, the hazards, the controls required and the conditions that must be met before and during the work. It is signed by authorised people and closed out on completion.

The act of entering a confined space, including any part of the body, to carry out work or inspection. Entry is only permitted under a valid permit, with atmosphere testing, isolation and rescue arrangements in place.

Work that can produce flames, sparks, hot surfaces or sufficient heat to ignite a flammable atmosphere, including welding, grinding and cutting. It requires a specific permit, gas testing and fire precautions.

A permit issued for tasks that introduce ignition sources, stating the area, duration, fire watch, gas test results and precautions required. It confirms the area has been made safe and that monitoring will continue while work proceeds.

A competent person trained and appointed to select and use gas detection equipment, perform tests, interpret results and record readings before and during work. The role carries responsibility for stopping work when conditions fall outside permitted limits.

A systematic process of identifying hazards, evaluating who could be harmed and how, and deciding on controls and residual risk. For gas testing it determines which gases to test for, where, when and with what alarm levels.

A documented combination of procedures, controls, equipment and supervision that makes a task safe. It links the risk assessment to practical steps such as isolation, purging, testing, monitoring, communication and rescue readiness.

Atmospheric testing carried out before anyone enters a confined space, covering oxygen, flammable gas and likely toxic gases. It must be done from outside, at representative levels, and repeated if conditions change or after a break.

Ongoing measurement of the atmosphere with detectors worn or positioned in the space while work proceeds. It provides early warning of changing conditions, such as gas ingress or oxygen depletion caused by the work itself.

An atmosphere containing less than about 19.5 per cent oxygen by volume, which can cause impaired judgement, breathlessness and loss of consciousness. It may result from displacement by other gases or from consumption by rusting, combustion or biological activity.

An atmosphere containing more than about 23.5 per cent oxygen by volume, which sharply increases the speed and severity of combustion. Clothing and hair can absorb oxygen and burn fiercely, so enrichment is a serious fire hazard.

Replacing air in a vessel or space with an inert gas such as nitrogen or carbon dioxide to exclude oxygen and prevent combustion. The resulting atmosphere is immediately dangerous to life because it will not support breathing.

Displacing an unwanted gas or vapour from a space by flushing with air, steam or inert gas before entry or hot work. Effective purging depends on adequate flow, time and venting, and must be verified by testing afterwards.

A detector element that burns flammable gas on a heated catalyst and measures the heat released to indicate concentration as a percentage of the lower explosive limit. It needs oxygen to work and can be poisoned or desensitised by certain substances.

A detector that measures absorption of infrared light at characteristic wavelengths to quantify a gas. It can work in oxygen-free atmospheres and is used for flammable gas and carbon dioxide measurement, but is gas-specific and can be affected by humidity.

A cell in which the target gas reacts at an electrode to produce a small current proportional to concentration. It is widely used for toxic gases and oxygen, has a finite life, and responds slowly at low temperatures.

A brief check in which a known test gas is applied to a detector to confirm that sensors respond and alarms activate. It verifies function but not accuracy, and is normally done before each use or shift.

Adjusting a detector's response against a certified reference gas of known concentration so that readings are accurate. It is performed at defined intervals and after sensor replacement, and differs from a bump test or functional check.

A verification that a detector's alarms, display, battery and sampling system operate correctly, without necessarily exposing sensors to gas. It complements bump testing and calibration in an equipment assurance programme.

Setting the detector to read zero in clean, fresh air or with a certified zero gas, so that the baseline is correct. A poor zero setting causes inaccurate readings, particularly for oxygen and toxic sensors.

The concentration at which a detector sounds a warning, chosen in advance from risk assessment, exposure limits or a percentage of the lower explosive limit. Two-stage alarms give early warning before conditions become dangerous.

A chosen location from which an atmosphere sample is drawn, selected because gas is likely to accumulate there. Correct placement uses knowledge of gas density, ventilation and the geometry of the space.

An instrument that draws a sample through a probe and hose to a remote sensor using a pump. Sample lines must be purged and checked for leaks, and the response time depends on hose length and flow rate.

Electrical apparatus designed so that its energy and temperature cannot ignite a flammable atmosphere, allowing safe use in hazardous areas. Certified gas detectors and torches used for testing must meet this requirement.

The planned means of recovering a casualty from a space, including trained rescuers, breathing apparatus, communications and access equipment. Untrained emergency entry is a common cause of multiple fatalities and must never be attempted.

A competent person stationed outside a confined space to maintain communication, monitor conditions, operate equipment and summon or assist rescue. They must not enter the space and must remain at the entry point until relieved.