QUALITY INTL CERTIFICATIONS4-Gas Monitor Standard — Which Gases & What Alarms
A standard 4-gas monitor carries four sensors: hydrogen sulphide (H2S) and carbon monoxide (CO) in ppm via electrochemical cells, oxygen (O2) in percent volume, and combustibles as %LEL using a catalytic or infrared sensor calibrated on methane. Conventional reference alarms: O2 at 19.5% low and 23.5% high; H2S and CO setpoints per site permit values.
The four sensors every UAE gas monitor carries, and what each alarm means.
The Standard Four Sensors
Walk any UAE industrial site and the monitor clipped to the gas tester's belt is almost always the same configuration: H2S, CO, O2 and combustible gas. The toxic pair — hydrogen sulphide and carbon monoxide — are measured in parts per million by electrochemical sensors, which generate a signal proportional to gas concentration. Oxygen is displayed as percent volume, watching for both deficiency and enrichment. Combustibles appear as %LEL, read through a catalytic bead or an infrared sensor, and the channel is conventionally calibrated on methane.
Why these four? They cover the atmosphere hazards that act fastest: two toxic gases, the oxygen the lungs depend on, and the flammability a single spark turns into an explosion. A 4-gas monitor is not a universal analyser — it does not see every gas that exists — but for confined spaces, hot work and general site gas testing it is the standard tool that permits are written around.
| Sensor | Measures | Technology | Conventional Reference Alarm |
|---|---|---|---|
| H2S | Toxic gas, ppm | Electrochemical | Low alarm around 10 ppm; permits may be stricter |
| CO | Toxic gas, ppm | Electrochemical | Low alarm around 25 ppm, 8-hour TWA basis |
| O2 | Oxygen, % volume | Electrochemical | 19.5% low / 23.5% high (convention) |
| LEL | Combustibles, % LEL | Catalytic or infrared | Low alarm commonly 10% LEL per permit practice |
The setpoints in the table are conventional reference values, not law. The controlling document is always the site's permit, and UAE permits can be stricter than the defaults — read the permit before trusting the monitor's factory settings.
Alarm Setpoints, STEL and TWA Basics
The toxic channels carry more than one limit. The instantaneous reading is what the sensor sees right now; the STEL (short-term exposure limit) tracks a 15-minute average; the TWA (time-weighted average) averages exposure across the shift, conventionally eight hours. Monitors display all three, and permits often reference the TWA when judging whether an atmosphere is acceptable for occupancy while the instantaneous alarm triggers immediate action.
The oxygen alarms are symmetrical: 19.5% low for deficiency and 23.5% high for enrichment, per the common convention. The low alarm protects breathing; the high alarm protects everything else, because oxygen-enriched atmospheres make materials burn fiercely.
The LEL alarm convention matters most for entry decisions. A low alarm commonly set around 10% LEL per UAE permit practice signals the atmosphere is moving toward a flammable condition; the response is withdrawal and ventilation, not silencing the alarm and carrying on. Hot work typically demands even lower combustible readings, and the permit states the number that applies.
Sensor Cross-Sensitivity
Electrochemical sensors are not perfectly selective. A cross-sensitive channel responds to a gas it was not built for — a CO channel nudged by another species present in the atmosphere, an H2S reading where the real hazard is something else. Manufacturers publish cross-sensitivity data, and experienced gas testers read it before trusting a single-channel reading in a mixed atmosphere.
The practical rules are simple. Treat an unexpected reading on one channel as a question, not an answer. Check whether the atmosphere genuinely contains the indicated gas before acting on the number. And never let a monitor with a known cross-sensitivity issue sign off a permit it cannot truly measure. The pattern is predictable, and the manufacturer's data sheet is the reference worth keeping with the instrument. Cross-sensitivity is also why 4-gas monitors supplement, rather than replace, specific instruments for known single hazards such as ammonia in cold stores.
Choosing LEL Sensor Technology: Catalytic vs Infrared
The combustible channel comes in two flavours, and the environment chooses. Catalytic bead sensors detect gas by burning a tiny sample on a heated element — robust and familiar, but susceptible to poisoning by contaminants, and they need sufficient oxygen present to work properly. Infrared sensors read how the gas absorbs light instead, so they tolerate dirty, contaminant-heavy atmospheres better and suit applications where the atmosphere may be inerted or oxygen-limited.
For UAE duty the practical guidance is straightforward: dusty, contaminant-rich industrial environments favour infrared LEL sensors; general site gas testing is well served by either technology, with catalytic units widespread. Whatever the choice, the channel is calibrated on methane, so readings for other combustible gases follow the manufacturer's correction factors — a methane-calibrated meter does not read every hydrocarbon equally, per standard references.
Calibration and bump discipline applies to both technologies; a monitor is only as good as its last verification. QIC (Quality International Certifications), a UAE inspection and calibration company since 1998, is among the providers calibrating these instruments across the Emirates.
Confined Space Pre-Entry Checks and Common Misuse
Before anyone enters a confined space, the 4-gas monitor goes in first. The conventional sequence: verify the monitor was bump tested and is in calibration; test the space's atmosphere at top, middle and bottom — gases and vapours stratify by weight — against the permit's limits; only then consider entry, with the monitor staying on the entrant and its alarms unsilenced. Ventilation changes the atmosphere, so re-test after it runs and whenever conditions inside the space change.
Common misuse undermines all of this, and the classics repeat in every incident review: zeroing the monitor in a contaminated atmosphere so every later reading is false; relying on an expired bump or calibration as today's verification; testing only at the opening when the hazard sits at the bottom; silencing an alarm to finish the shift; and reading %LEL as percent volume. The monitor is a measurement instrument — it is only as trustworthy as the discipline of the person carrying it.
Checklist
- Confirm the monitor is bump tested and in calibration before gas testing work.
- Test the space at top, middle and bottom — gases stratify by weight.
- Compare readings against the permit's stated limits, not the monitor's defaults.
- Keep the monitor on the entrant for the whole occupancy.
- Never silence an alarm to continue work — withdraw and ventilate.
- Zero the instrument only in genuinely fresh air.
- Check cross-sensitivity data before trusting single-channel readings in mixed atmospheres.
- Apply correction factors for gases other than the methane calibration gas.
- Prefer infrared LEL sensors in dusty, contaminant-heavy environments.
- Log every pre-entry test result with time, location and instrument ID.
Frequently Asked Questions
What gases does a 4-gas monitor detect?
Four: hydrogen sulphide (H2S) and carbon monoxide (CO) as toxic gases in ppm through electrochemical sensors, oxygen (O2) in percent volume covering deficiency and enrichment, and combustible gas shown as %LEL through a catalytic or infrared sensor, conventionally calibrated on methane. Together they cover the atmosphere hazards UAE permits care about most.
What are standard alarm levels for H2S and CO?
Conventional reference values are a low H2S alarm around 10 ppm and a low CO alarm around 25 ppm on an 8-hour TWA basis, with high alarms set per site standard. Oxygen alarms sit at 19.5% and 23.5% by convention. Always defer to the permit's stated setpoints — UAE sites can be stricter than these defaults.
Can a 4-gas monitor be used for confined space entry?
Yes — it is the standard pre-entry tool: test the atmosphere before entry and keep monitoring while the space is occupied. Ventilate and re-test if any reading strays outside the permit's accepted ranges, and evacuate on alarm. The monitor must be bump tested and in calibration for the results to count on the permit.
Which sensor fails first on gas monitors?
Conventional field experience points to the electrochemical cells — especially the oxygen sensor, which consumes its internal materials in normal operation and so depletes with age. Catalytic LEL sensors can also fail suddenly when poisoned by contaminants in dirty environments. Daily bump tests catch both failure modes before a permit depends on them.
What is LEL and how does the sensor measure it?
LEL is the lower explosive limit — the lowest concentration at which a gas-air mixture can ignite. The monitor shows it as %LEL, where 100% means the gas has reached that limit. Catalytic sensors burn a sample on a heated bead; infrared sensors read light absorption instead. Both are conventionally calibrated on methane, per standard references.