Gas testing and atmospheric monitoring

Because confined space atmospheres can kill without warning, the only reliable way to know whether the air is safe is to measure it. Gas testing before entry, and monitoring during entry, are the core atmospheric controls in every confined space rule. This lesson explains how portable gas detectors work, the order in which gases are tested, how to test a space from outside and at different levels, how bump testing and calibration keep detectors trustworthy, and how to respond when an alarm sounds. Operating a detector and interpreting the results is a skill that requires hands-on training on the instruments your employer uses; this lesson gives the awareness behind that training.

The portable gas detector

The most common instrument for confined space work is a multi-gas detector, often called a four-gas meter because it usually measures oxygen, flammable gases (as percent LEL), carbon monoxide and hydrogen sulfide. Many models can take additional sensors for gases such as carbon dioxide, ammonia, chlorine, sulfur dioxide, nitrogen dioxide or volatile organic compounds. The choice of sensors must match the hazards identified in the assessment. A four-gas meter will not detect a solvent vapour, ammonia or carbon dioxide unless it has a sensor for it.

The sensors work in different ways:

• oxygen and many toxic gas sensors are electrochemical cells, in which the gas reacts at an electrode and produces a current proportional to its concentration;
• flammable gas sensors are usually catalytic bead (pellistor) sensors, which measure the heat produced when gas burns on a heated bead, or infrared sensors, which measure how much infrared light the gas absorbs;
• photoionization detectors (PIDs) measure many organic vapours at low ppm levels;
• infrared sensors are also used for carbon dioxide.

Each type has limitations. Catalytic bead sensors need oxygen to work and can read falsely low in oxygen-deficient air; they can be poisoned by silicones, lead compounds and some other chemicals, and damaged by very high gas concentrations. Infrared flammable sensors do not respond to hydrogen. Electrochemical sensors can be affected by other gases (cross-sensitivity), by extreme temperatures and by age. Detectors also have a warm-up period and a response time. Knowing these limits is part of being trained to test.

The testing sequence

Before entry, the atmosphere is tested in a fixed order: oxygen first, then flammable gases, then toxic gases.

• Oxygen first, because many combustible gas sensors need adequate oxygen to give a correct reading, and because an oxygen reading below normal shows that something has displaced or consumed the air.
• Flammable gases second, because the threat of fire or explosion is more immediately life-threatening, and affects more people, than most toxic exposures.
• Toxic gases third, because they are measured against exposure limits that assume oxygen and flammables are already controlled.

Modern multi-gas detectors measure all their gases at the same time, so in practice the sequence means reading and interpreting the results in this order, and stopping if oxygen or flammables are out of range.

Testing from outside, and at every level

The first test is always made from outside the space, without breaking the plane of the opening. A detector with a pump and a length of sample tubing, or a probe on a pole, draws air from inside the space to the instrument. Lowering a diffusion detector (one without a pump) on a string is a poor substitute: it gives readings only near where it hangs, and it may take a long time to respond.

Gases stratify according to their density. Methane and hydrogen rise to the top; hydrogen sulfide, carbon dioxide, propane and solvent vapours sink to the bottom; carbon monoxide and nitrogen mix fairly evenly. A space must therefore be tested at several levels, typically every 1.2 m (4 ft) or so in a vertical space, and at the top, middle and bottom at minimum, as well as in corners, sumps, side passages and anywhere gases could collect. In a long horizontal space such as a sewer or a tunnel, it is tested ahead of the entrants as they advance.

Bump tests and calibration

A gas detector is only useful if it responds correctly. Two checks keep detectors reliable:

• A bump test (or functional test) briefly exposes the detector to a known concentration of test gas to confirm that the sensors respond and the alarms work. It does not adjust the readings. Industry guidance, including the widely used position of the International Safety Equipment Association, and most manufacturers recommend a bump test before each day's use, and many confined space programs require one before every entry.
• A full calibration adjusts the detector so its readings match known gas concentrations. It is done at the interval the manufacturer specifies, after a failed bump test, after exposure to high concentrations or sensor poisons, and after sensor replacement.

Detectors also need a fresh-air zero in clean air, charged batteries, clean filters and intact tubing. Records of bump tests and calibrations should be kept, and many detectors store them automatically for download to a docking station.

Correction factors for flammable gases

A catalytic bead flammable sensor is calibrated on one gas, usually methane or pentane, but it responds differently to other gases. If the gas in the space is not the calibration gas, the reading must be corrected using a factor supplied by the manufacturer. For example, if a sensor calibrated on methane responds only half as strongly to a particular solvent vapour, a reading of 8 per cent LEL might mean the true concentration is about 16 per cent LEL. Correction factors vary by instrument and gas, so they must come from the manufacturer's data for the specific detector. Programs that work with many different vapours often calibrate on pentane or use a PID to get a more appropriate response.

Continuous monitoring during entry

An atmosphere that was safe at the start can change. Gas can be released by sludge disturbed by a shovel, by a leaking valve, by heat, by coatings and cleaning chemicals, by welding, by a change in weather pushing sewer gas along a line, or by a process upset elsewhere. For this reason, the atmosphere must be tested as often as necessary during the entry, and continuous monitoring is required or strongly recommended wherever conditions could change.

Canadian rules require testing as often as necessary before and during the entry. Alberta requires continuous monitoring where the atmosphere could change unpredictably, and British Columbia requires continuous monitoring where a flammable atmosphere above 20 per cent of the LEL could develop and in other higher-hazard situations.

In practice, continuous monitoring means each entrant, or the work area, has a detector running and alarming throughout the entry, and the attendant may also have a detector sampling the space or a remote readout of the entrants' detectors. The detector must be positioned near the breathing zone and where gases are expected, not left in a toolbox or on the surface.

Responding to alarms

Gas detectors have alarm set points, typically a low alarm and a high alarm for each gas, plus alarms for over-range readings, low battery and sensor faults. Typical set points are around 19.5 per cent for low oxygen and 23 or 23.5 per cent for high oxygen, 10 per cent LEL for flammables, and toxic alarms at or below the applicable exposure or ceiling limits. The program and the permit state the set points used.

The rule for entrants is simple: when an alarm sounds, leave the space immediately, without stopping to finish the job or to check the reading. The attendant orders evacuation and does not allow re-entry until the cause is found, the space is re-evaluated and the atmosphere is acceptable again. A detector that alarms for an unknown reason, or shows a fault, is treated as a real alarm until proven otherwise.

Recording test results

Test results are recorded on the entry permit or an attached log: the time, the location in the space, each gas reading, the instrument used and its last bump test or calibration, and the name of the tester. During long entries, readings are recorded at the intervals the permit specifies. Records show that the atmosphere was acceptable, help investigators if something goes wrong, and reveal trends, such as hydrogen sulfide creeping up through the morning, that should prompt action before an alarm.

Who may test

Canadian rules require testing by a competent, qualified or adequately trained person, depending on the jurisdiction. Ontario requires the person performing the tests to have adequate knowledge, training and experience; Alberta requires a competent worker.

Whoever tests must know how the instrument works, its limitations, the sequence, the sampling technique, the acceptable values and what to do when readings are out of range.