Table of Contents

Introduction

The four-gas monitor is the single most common personal gas detector in industrial safety. It combines the four hazards that appear most often across oil & gas production, refining, petrochemical plants, wastewater and confined-space work: flammable gas (LEL), oxygen (O2), hydrogen sulfide (H2S) and carbon monoxide (CO). For EHS teams across Russia and the Middle East, selecting the right 4-gas monitor is not a commodity purchase — it is the instrument a worker carries into the hazard.

This buyer’s guide explains what a 4-gas monitor actually measures, the specifications that matter in hot, dusty, explosive climates, and the trade-offs between pump and diffusion models. It ends with a practical checklist for procurement.

What a 4-Gas Monitor Measures — and Why

LEL (catalytic bead or NDIR): measures combustible gas as a percentage of the lower explosive limit, warning before an explosive mixture can form.
O2 (electrochemical): warns of oxygen depletion (<19.5 %) or enrichment (>23.5 %), both dangerous in confined spaces.
H2S (electrochemical): the sour-gas killer in oil & gas — toxic at ppm levels and capable of rapid, un-warned collapse.
CO (electrochemical): incomplete-combustion and combustion by-product poisoning common in vehicle-fired enclosed spaces, boilers and petrochemical units.

These four gases together cover the majority of confined-space and field-entry fatalities. A well-specified 4-gas monitor is the baseline; adding a fifth sensor (for example a PID for VOCs, or SO2 in a sulphur plant) is a separate decision driven by the process.

Pump (Draw) vs Diffusion

The first real buying decision is sampling method. A pumped monitor draws sample through a hose, letting the worker pull air from a vessel, duct or below-grade point without entering it. A diffusion monitor relies on natural air movement across the sensor and is worn close to the breathing zone.

Feature	Pump (draw) monitor	Diffusion monitor
Best use	Confined space entry, pre-entry testing	Continuous personal area monitoring
Reach	Pulls sample through a probe/hose	Senses only air at the instrument
Response	Fast, point-selectable	Responds to ambient conditions
Mechanics	Pump consumes battery	No pump, longer battery life
Weight / cost	Heavier, higher cost	Lighter, lower cost

In sour oil & gas operations, pumped units are the norm for pre-entry and below-grade checks; diffusion units are worn for continuous walk-around protection. Many fleets run both.

Specifications That Matter in Harsh Climates

Explosion protection: in explosive atmospheres the instrument must carry the right Ex marking. The Hangwei HD620 is rated Ex ia IIC T4 Ga (intrinsically safe), appropriate for Zone 0/1 locations.
Ingress protection: dust and sand are relentless in Gulf-region plants; an IP65 rating keeps out dust and water jets.
Battery runtime: a long, swappable pack matters on shift work. The HD620 uses a 5000 mAh pack for roughly 20 hours of operation.
Alarms: audible, visual and vibration — and in noisy plants, loud audible alarms and a clear 2.4-inch colour display make a real difference.
Localised voice: spoken alarms in the operator’s language reduce reaction time. The HD620 supports English and Mandarin voice prompts.

Certification & Market Context

For Russia and the EAEU, the instrument must meet the EAC mark under Technical Regulation TR CU 012/2011 for equipment intended for explosive atmospheres. Across the Middle East, oil & gas operators and national regulators commonly accept ATEX (EU 2014/34/EU, EN 60079 series) and IECEx certificates. Procurement should confirm the required marking against the end customer’s approvals before order, not after.

Data Logging, Records & Compliance

Modern 4-gas monitors log readings and alarm events. EHS teams use this data to prove bump-test and calibration compliance, to investigate near-misses and to demonstrate duty-of-care to operators and auditors. When several units are in a fleet, ask the manufacturer how records are downloaded and whether the format meets your audit expectations.

Setting Alarm Thresholds

A monitor is only as protective as its setpoints. The low and high alarms should be set against the site’s occupational exposure limits and the process hazard analysis, not left at factory defaults. Oxygen alarms typically warn on depletion below roughly 19.5 % and enrichment above roughly 23.5 %; LEL alarms commonly pre-warn at 10 % LEL and high-alarm at 20 % LEL; toxic setpoints are chosen against the applicable exposure limits for H2S and CO. Document the setpoints and who can change them, and re-check them after each calibration.

Hangwei 4-Gas Monitoring Range

Pumped four-gas: HD620 Four-In-One Pump Gas Detector

The HD620 measures H2S, CO, O2 and LEL in one intrinsically safe (Ex ia IIC T4 Ga) unit, is IP65, runs about 20 hours on a 5000 mAh battery, shows results on a 2.4-inch colour screen and combines audible, visual and vibration alarms with English/Chinese voice prompts.

Diffusion four-gas: HD620 Diffusion Four-In-One Detector

Single-gas dedicated monitoring: HD600 Single-Gas Detector with LCD

FAQ

What four gases does a standard 4-gas monitor detect?

Flammable gas as % LEL, oxygen (O2), hydrogen sulfide (H2S) and carbon monoxide (CO). These are the hazards most commonly responsible for confined-space and field-entry incidents.

Is a pumped or diffusion monitor better for confined space entry?

A pumped monitor is preferred for pre-entry testing because it draws air from inside the vessel through a probe before anyone enters; a diffusion unit suits continuous personal protection once entry is authorised.

What Ex rating does a 4-gas monitor need in an explosive atmosphere?

For Zone 0/1 locations an intrinsically safe mark such as Ex ia IIC T4 Ga is typical, as on the Hangwei HD620. Always match the marking to the actual zone and gas group of the site.

How often should a 4-gas monitor be bump tested?

Best practice is a bump test before each day’s use, and a full calibration at least every six months or after a failed bump test, sensor replacement or known over-exposure.

Can a 4-gas monitor detect VOCs or benzene?

Not in the standard four channels. VOCs require a PID channel; specify a PID-instrument or a fifth sensor when benzene or solvent vapours are a real process hazard.

Get Expert Guidance

Hangwei Sensors (Henan Hangwei Sensing Technology Co., Ltd.) supplies OEM/ODM multi-gas detectors and detector-plus-controller packages, backed by 20+ years of manufacturing, 20 patents and 10 production lines. We support distributors and end users across Russia, Central Asia and the Middle East, and can tailor sensor combinations, voice languages and certification documentation to the project.

Pumped: HD620 Four-In-One Pump Gas Detector

Diffusion: HD620 Diffusion Detector

Custom multi-gas solutions: Custom Gas Sensor & Detector

Contact Hangwei Sensors

Email: hw@hangweisensors.com · Phone: +86 371 6667 0886 · Website: https://www.hangweisensors.com/