Gas detection and monitoring

Most of the gases that kill oil and gas workers cannot be seen, and the most dangerous of them cannot be trusted to be smelled. Hydrogen sulphide deadens the sense of smell at high concentrations, methane is odourless, and oxygen deficiency gives no warning at all. Gas detectors are therefore the eyes and nose of everyone who works on a lease. This lesson explains the detectors used on oil and gas sites, how their sensors work and fail, how to check them before use, what alarm set points mean, how to sample before entering an area and how fixed detection protects facilities.

HSE-244 Lessons 6 and 7 covered gas detectors, bump tests, calibration and fixed monitoring for hydrogen sulphide in depth, and HSE-211 Lesson 6 covered atmospheric testing for confined space entry. This lesson builds on both and concentrates on what is particular to oil and gas sites: multi-gas monitors, flammable gas sensors and the mixed hydrocarbon atmospheres they must read, and the way monitoring fits into site routines.

The four-gas monitor

The standard personal monitor in the oil and gas industry is the four-gas monitor. It measures flammable gas as percent of the LEL, oxygen as percent by volume, hydrogen sulphide in ppm and carbon monoxide in ppm. It sounds, flashes and vibrates when a reading passes an alarm set point, and it records readings and alarms in a data log that can be downloaded. Many operators require every person on a sour site, and often on every site, to wear one, clipped in the breathing zone on the collar or chest. Additional sensors are available for sulphur dioxide (produced when H2S burns, for example at flares), volatile organic compounds and benzene (photoionization detectors), and other gases where a hazard assessment calls for them.

How the sensors work, and how they fail

Each sensor has a different principle, and each can be fooled. Knowing the limits is part of using a monitor safely.

Flammable gas sensors deserve extra care on oil and gas sites, because they are usually calibrated on one gas but asked to read mixtures of many. A sensor's response to a gas other than its calibration gas is described by a response or correction factor in the manual. A catalytic sensor calibrated on methane, for example, typically under-reads heavier hydrocarbons such as pentane, so the real percentage of the LEL may be higher than the display shows. Some sites calibrate on pentane or propane for that reason, accepting that the monitor will then over-read methane, which errs on the safe side. The program later in this lesson shows how a correction factor changes the reading.

Bump tests and calibration

A monitor that has not been checked cannot be trusted. A bump test, also called a function check, exposes the sensors briefly to a known gas to confirm that they respond and that the alarms work. Industry practice, and most manufacturers, call for a bump test before each day's use; many operators require it before each shift or before entering a hazardous area. A calibration adjusts the monitor's readings to match a certified test gas and is done at the interval the manufacturer specifies or whenever a bump test fails. Docking stations do both automatically and keep the records.

If a monitor fails a bump test, it must not be used until it has been calibrated and passes. A monitor that has been exposed to a very high concentration of gas, dropped, submerged or exposed to silicone sprays should be checked before further use. Watch the display for sensor error messages, low battery warnings and calibration due dates; a monitor that shuts off halfway through a task leaves the wearer unprotected.

Alarm set points

Each monitor has alarm set points for each gas. A low alarm warns that a hazard is developing; a high alarm means leave now. For toxic gases there are often also time-weighted average and short-term exposure alarms based on the data log. The site, not the worker, sets the levels, based on the law, the hazard assessment and the operator's standards. The values below are typical of oil and gas practice, but they vary, and some operators use lower H2S set points that reflect the ACGIH threshold limit values discussed in HSE-244.

When a monitor alarms, the response must be automatic. Stop work, warn others nearby, and move upwind and, for heavy gases, uphill to fresh air, using the wind socks to choose the direction. Do not stay to finish the task, and do not silence and ignore the alarm. Report the alarm so that the cause can be found. Do not return until the area has been tested and cleared by a competent person. If someone has collapsed, do not rush in to help without respiratory protection; Lesson 6 and HSE-244 explain why rescuers become casualties.

Sampling before entering

A personal monitor tells you about the air where you are standing. Before entering an area where gas may have collected, such as a tank top, a cellar around a wellhead, an excavation, a building or a low-lying area, sample it from outside first. Pumped monitors draw air through a sample tube so that you can test before entering. Test at several heights, because methane rises while heavier gases sink, and allow enough time for the sample to travel through the tube to the sensors; manufacturers give a draw time per length of tube. Test oxygen first, because catalytic flammable sensors give falsely low readings when oxygen is low, then flammable gas, then toxic gases. HSE-211 covers sampling for confined space entry in detail.

Area monitors and fixed detection

Personal monitors protect individuals; area and fixed monitors protect places. Portable area monitors are set up around a work zone during jobs such as well servicing, fracturing, pipeline tie-ins and tank cleaning, often linked by radio so that an alarm at one sounds at all of them and at the supervisor's station. Fixed detectors are installed at batteries, compressor stations, gas plants and well sites with sour gas, wired to a control system that can sound alarms, start ventilation and shut in wells or isolate equipment automatically. Fixed systems must be inspected, bump tested and calibrated on schedule like portable ones; a fixed detector that has been bypassed or left in fault is an impairment and must be managed as one.

Reading a monitor fleet

Supervisors are responsible for making sure that every monitor on site works. The program below checks an invented fleet of monitors for a day's work at Pinecrest: it flags monitors whose last bump test is more than one day old, whose calibration is past due or whose last bump result was outside 10 percent of the test gas value. It also shows how a methane-calibrated LEL sensor reading should be corrected when the gas is mostly pentane, using an illustrative correction factor; real factors come from the manufacturer.

from datetime import date
today = date(2026, 3, 9)
TEST_H2S = 25.0  # ppm in the bump gas (illustrative)
fleet = [  # id, last bump, last calibration, calibration interval days, H2S bump reading
    ("MON-01", date(2026, 3, 9), date(2026, 2, 20), 90, 24.0),
    ("MON-02", date(2026, 3, 6), date(2026, 1, 15), 90, 25.5),
    ("MON-03", date(2026, 3, 9), date(2025, 11, 30), 90, 23.8),
    ("MON-04", date(2026, 3, 9), date(2026, 3, 1), 90, 19.0),
]
for mid, bump, cal, interval, reading in fleet:
    problems = []
    if (today - bump).days > 1:
        problems.append(f"bump {(today - bump).days} days old")
    if (today - cal).days > interval:
        problems.append(f"calibration {(today - cal).days - interval} days overdue")
    if abs(reading - TEST_H2S) / TEST_H2S > 0.10:
        problems.append(f"bump reading {reading} ppm outside 10%")
    print(mid, "OK" if not problems else "REMOVE: " + "; ".join(problems))

# Correcting an LEL reading for gas type (illustrative factor).
display = 12.0          # %LEL shown, sensor calibrated on methane
factor_pentane = 2.0    # multiply reading when the gas is pentane (check manual)
print(f"Displayed {display}% LEL, corrected for pentane about {display * factor_pentane:.0f}% LEL")

MON-01 OK
MON-02 REMOVE: bump 3 days old
MON-03 REMOVE: calibration 9 days overdue
MON-04 REMOVE: bump reading 19.0 ppm outside 10%
Displayed 12.0% LEL, corrected for pentane about 24% LEL

Three of the four monitors should not be used today: one was not bumped this morning, one is overdue for calibration and one gave a bump reading 24 percent low, which suggests a failing H2S sensor. The correction calculation shows why sites take mixed hydrocarbon atmospheres seriously: a display of 12 percent LEL could, for a heavy hydrocarbon vapour, mean an atmosphere at about 24 percent LEL, already above a typical high alarm.