Authorized Gas Tester (AGT) cheat sheet

Flammable range is bounded by the lower explosive limit (LEL) and upper explosive limit (UEL), both expressed as % v/v of gas in air; concentrations between them can ignite, below the LEL the mixture is too lean, above the UEL too rich.

Flash point is the lowest temperature at which a liquid gives off enough vapour to form an ignitable mixture near its surface; a liquid with a flash point below 23 °C is conventionally classed as highly flammable (e.g. petrol about -43 °C).

Auto-ignition temperature is the temperature at which a vapour-air mixture ignites without an external ignition source; hot surfaces above this value can ignite a flammable atmosphere even with no spark present.

Vapour density relative to air (air = 1) predicts where a gas accumulates: values above 1 (propane 1.5, hydrogen sulfide 1.19, petrol vapour about 3-4) tend to settle in low points, pits and drains; values below 1 (methane 0.55, hydrogen 0.07) tend to rise and collect at high points.

Relative density of a gas is the ratio of its molar mass to that of air (about 29 g/mol); methane at 16 g/mol rises, carbon dioxide at 44 g/mol sinks, so sampling points must follow the expected stratification.

Oxygen is displaced, not consumed by inert gases: nitrogen, argon, carbon dioxide and methane reduce oxygen by simple dilution, so an oxygen meter detects their presence only indirectly as a drop in O2.

Toxic gases have occupational exposure limits; hydrogen sulfide is a special case because at high concentrations it rapidly deadens the sense of smell (olfactory fatigue), so odour is never a valid detection method.

Vapour pressure increases with temperature, so a liquid in a warm vessel or a hot work operation can raise vapour concentration from below the LEL to within the flammable range without any change in the amount of liquid present.

The employer has the primary duty to ensure a safe system of work for confined space entry, including risk assessment, control measures, training, and provision of suitable and sufficient rescue arrangements; these duties cannot be delegated away by permit.

The authorized gas tester tests, interprets and records results, and is responsible for the validity of the readings taken with the instrument they use; the entry supervisor or permit issuer authorizes entry and controls the permit.

A permit to work is a formal written authorization for a specific task, location and time; it is invalidated when conditions change, when the stated validity period expires, or when the task, personnel or atmosphere differ from those recorded.

A stand-by person (attendant) remains outside the confined space, maintains communication, accounts for entrants, and initiates rescue; the attendant does not enter to attempt rescue unless trained and equipped and authorized to do so.

Competence of the gas tester includes knowledge of the instrument, its limitations, the hazards of the space, and the ability to interpret and act on readings, not merely the ability to press buttons.

Records of gas tests, permits, calibration and training are typically required to be retained for a defined period under the applicable national regime; the tester should know the retention rule for their site and jurisdiction.

Roles must be defined before entry: entrant, attendant, entry supervisor, gas tester, and rescue team, each with named individuals and clear authority to stop work.

Any change in the permit conditions (new task, new shift, changed ventilation, alarm event) requires re-testing and re-authorization before work continues.

Risk assessment identifies the hazards of the space and its contents (residues, sludge, previous process, connected services), evaluates the likelihood and severity of harm, and defines controls in a hierarchy: eliminate, substitute, engineer (isolation, ventilation), administrate (permit, procedure), and PPE last.

Isolation before entry includes blanking or spading, disconnecting or locking off all lines, and isolating mechanical, electrical and thermal energy; a closed valve alone is not isolation.

Ventilation is a control, not a guarantee: forced air dilution reduces gas concentration but continuous monitoring is still required because the source may continue to emit.

The permit should record the space, the task, the hazards, the isolation applied, the test results, the control measures, the rescue plan, the names of personnel, and the validity period.

A safe system of work sequences the job: assess, isolate, ventilate, test, authorize, enter, monitor continuously, communicate, exit, and cancel the permit.

Simultaneous operations (hot work near a confined space, adjacent line breaking) can create new hazards; the risk assessment must consider interactions between tasks.

The permit should be cross-referenced to the gas test record and to any hot work permit, so that the two cannot be valid independently when they depend on each other.

Permit cancellation is a positive act: the supervisor confirms all personnel have exited, the space is secured, and the permit is closed and retained for records.

Catalytic bead (pellistor) sensors detect flammable gas by measuring the heat of combustion on a heated bead; they read in % LEL, require oxygen to work, and can be poisoned by silicones, leaded compounds, and sulfides, which reduces sensitivity without an obvious fault.

Infrared (IR) sensors measure absorption of light at a characteristic wavelength; they are specific to a gas or group, do not need oxygen, and are not poisoned by the same agents as catalytic sensors, but they can be blinded by heavy condensation or dirt on the optics.

Electrochemical sensors detect toxic gases and oxygen by generating a current proportional to concentration; they have a limited life, are temperature-sensitive, and can suffer cross-sensitivity from other gases.

Photoionization detectors (PID) measure total volatile organic compounds using a UV lamp; the reading depends on the ionization energy of the compound and the lamp energy, so the response factor must be known to interpret the result.

A combustible gas instrument calibrated on one gas (commonly methane) will read differently on another gas; the response factor must be applied to convert the reading to the actual gas concentration.

Selection criteria include the gases expected, the range needed, the environment (temperature, humidity, dust), the required response time, and whether the instrument is certified for use in a flammable atmosphere (intrinsically safe).

The instrument must be suitable for the space: a sample line pump with sufficient length and flow rate is needed to draw a sample from a remote point, and the line must be purged before the reading is trusted.

No single sensor detects everything: a four-gas instrument typically measures oxygen, a flammable gas, carbon monoxide and hydrogen sulfide, and will not detect other toxic gases such as ammonia or chlorine.

Calibration establishes the instrument response against a known gas standard; it is performed with certified gas of known concentration and the reading is adjusted to match, with the result recorded.

A bump test briefly exposes the instrument to a gas above the alarm set point to confirm that the sensor responds and the alarm operates; it does not adjust the reading and is not a substitute for calibration.

A functional check confirms the instrument operates: battery, display, pump flow, alarm sound and light, and sensor response, without necessarily using gas.

Calibration frequency is set by the manufacturer, the site procedure, and the observed stability of the sensor; a common regime is bump test before each use and full calibration at defined intervals.

Calibration gas must be within its expiry date, matched to the sensor's target gas, and the regulator and tubing must be leak-free; a depleted cylinder gives a false pass.

The zero reading must be set in clean air or with a zero-gas cylinder; setting zero in a contaminated atmosphere biases every subsequent reading.

Records of calibration and bump tests should include date, time, instrument serial number, gas used and concentration, result, and the name of the person performing the test.

An instrument that fails a bump test or calibration must be taken out of service, tagged, and not used until repaired and recalibrated.

Test the atmosphere before entry and after any change in conditions; the result is valid only for the conditions at the time of testing, so continuous monitoring is required during occupancy.

Test from outside the space, using a sample line, and sample at multiple levels (top, middle, bottom) because gases stratify by relative density; a single point sample can miss a pocket of gas.

Test oxygen first, then flammables, then toxics: an oxygen-deficient or enriched atmosphere distorts the reading of a catalytic flammable sensor, so oxygen must be known before interpreting the LEL reading.

The order of testing reflects the risk: oxygen deficiency and enrichment are immediately life-threatening, flammable gas can ignite, and toxic gas causes acute or chronic harm.

Acceptance criteria are set by the site procedure and the applicable standard; typical entry criteria are oxygen between 19.5% and 23.5%, flammable gas below 10% LEL, and toxics below the occupational exposure limit.

Purge and ventilate before testing; a reading taken during purging reflects the mixture at that moment, not the atmosphere that will exist when entry begins.

If the atmosphere fails the criteria, do not enter: ventilate further, re-test, or use respiratory protection with a permit that specifically authorizes it.

Continuous monitoring during occupancy with the alarm set at the acceptance thresholds allows evacuation before the atmosphere becomes dangerous.

Hot work includes welding, cutting, grinding, and any operation producing sparks, flame, or hot surfaces; it requires a hot work permit and a gas test before and during the work.

The flammable atmosphere must be tested before hot work and continuously during it, with the instrument alarm set at a defined threshold, commonly 10% LEL for hot work in a confined space.

Hot work near a confined space can ignite gas escaping from the space; the test must cover the space, the surrounding area, and any low points where heavier-than-air gas may collect.

The test must consider the gas being produced by the work itself: welding generates heat and sparks, and cutting can release flammable gas from coatings or residues.

Hot work in a space that previously contained flammable liquid or gas requires the space to be cleaned, purged, and tested, and the test must confirm the absence of flammable atmosphere at all levels.

A hot work permit and a confined space entry permit are separate authorizations; where both apply, both must be valid and cross-referenced.

If the flammable gas reading rises to the alarm threshold, stop hot work, withdraw, ventilate, and re-test before resuming.

Hot work should be stopped and the area re-tested if conditions change: ventilation failure, a change in the process, or a rise in the reading.

Normal air contains about 20.9% oxygen; an oxygen-deficient atmosphere is generally defined as below 19.5% and an oxygen-enriched atmosphere as above 23.5% by volume.

Oxygen deficiency causes impaired judgment and coordination before the victim is aware; at around 16-19% symptoms may be subtle, and below 10% loss of consciousness is rapid, so the victim cannot self-rescue.

Oxygen enrichment makes materials burn vigorously: clothing, hair, and grease that would not normally ignite can burn fiercely, and a spark in an enriched atmosphere can cause a serious fire.

Oxygen is displaced by inert gases and consumed by combustion, rusting, and biological activity; a drop in oxygen may indicate the presence of an inert gas that is itself an asphyxiant.

An oxygen reading above 23.5% may indicate a leaking oxygen line, a welding oxygen supply, or a chemical reaction; the source must be found and isolated before entry.

Oxygen must be tested before flammables because a catalytic flammable sensor's reading depends on oxygen concentration; an oxygen-deficient sample can give a falsely low LEL reading.

Oxygen monitors must be calibrated in clean air or with a certified oxygen standard; a sensor that has been exposed to a high oxygen concentration may read high for a period afterwards.

Oxygen enrichment and deficiency are both entry-stopping conditions; neither can be corrected by respiratory protection alone without a specific permit and equipment.

Records should capture the space, the date and time, the instrument used and its calibration status, the gases tested, the results at each level, the acceptance criteria, and the tester's name and signature.

Communication during entry is continuous: the attendant maintains contact with entrants by voice, radio, or signal line, and a missed communication is treated as an emergency.

The rescue plan must be specific to the space and the task, must be rehearsed, and must be available before entry; a generic plan that has not been tested is not adequate.

Rescue equipment (tripod, harness, lifeline, breathing apparatus, resuscitator) must be at the entry point and checked before entry, not fetched after an incident.

The attendant must not enter to rescue an unconscious entrant without breathing apparatus and training; most confined space fatalities involve would-be rescuers.

Emergency response should include raising the alarm, isolating the space, calling the rescue team, and providing first aid; the sequence and contacts should be recorded on the permit.

Records of gas tests and permits support investigation after an incident and demonstrate that the safe system of work was followed; they should be legible, contemporaneous, and retained.

Debrief after any alarm, near miss, or rescue activation; the findings should feed back into the risk assessment and the permit procedure.