Gas Detector Coverage Area: How Many You Need and Where

Ask how many gas detectors a room needs and you will usually hear a number per square metre. It is a useful cross-check and a poor starting point, because gas does not leak from floor area — it leaks from things: a gas train, a flange group, a regulator, a flexible hose. This guide sets out how detector count, spacing and height are normally worked out for fixed point detectors in plant rooms, why the leak source should come first, and how the answer changes with the gas.

The figures below are widely used rules of thumb. They are there to sanity-check a design, not to replace the selection and installation guidance in IEC 60079-29-2 or a site-specific gas-hazard assessment. AgControlli supplies the instruments; the count and positions remain the responsibility of the system designer.

1. Start with the gas, not the room

The height at which a detector works is set by how the gas behaves in air. Compare the gas with air (density 1.0):

Gas	Density vs air (approx.)	Where it collects	Mount	Detector
Hydrogen (H₂)	0.07	Highest point, roof pockets	At the highest point of the room	AG-GDC-150
Methane / natural gas (CH₄)	0.55	Ceiling and roof apex	Within about 300 mm of the ceiling, above the leak source	AG-GDC-100
Ammonia (NH₃)	0.6	Rises, then spreads	High level plus a head at the machinery	AG-GDT-310
Carbon monoxide (CO)	0.97	Mixes with room air	Breathing zone, about 1.5 m	AG-GDT-300
Hydrogen sulphide (H₂S)	1.2	Sumps, wet wells, trenches	Low	AG-GDT-320
Carbon dioxide (CO₂)	1.5	Pits and low areas	Low, in the space where it can displace air	AG-GDT-340
Chlorine (Cl₂)	2.5	Floor level	About 300 mm above the floor	AG-GDT-330
Oxygen deficiency (N₂, Ar, CO₂ displacing air)	—	Wherever air is displaced	Breathing zone; add a low head where a heavy gas can blanket the floor	AG-GDO-400

Two consequences follow. A room with natural gas and a combustion engine needs detectors at two different heights, because methane rises while carbon monoxide stays in the breathing zone. And a detector at the wrong height is worse than no detector, because it reports a healthy room while the gas collects somewhere else.

2. The source rule comes before the area rule

There are two ways to count detectors, and they give different answers:

• The area rule. One point detector for roughly every 100–150 m² of floor for methane at ceiling level, with a head-to-head spacing in the order of 10–15 m. It is fast and it scales with room size.
• The source rule. At least one detector over every credible leak source — each gas train, each regulator or valve group, the point where the fuel main enters the room, and the ventilation extract path.

Where they disagree, the source rule governs. The area rule exists to catch gas that has travelled from a source you did not foresee; it cannot protect a source that sits between two detectors’ areas.

Worked example: a gas engine hall

An illustrative case, generalised from a real enquiry. A hall measures about 15.3 m × 9.9 m with a 6.0 m ceiling — roughly 150 m² of floor and 900 m³ of volume — and houses three gas engines in a row, 4.7 m apart, running on natural gas.

Method	Result
Area rule (150 m² ÷ 100–150 m² per head)	1 to 2 methane detectors
Source rule	3 above the engine gas trains, 1 where the fuel main enters, 1 in the ventilation extract path = 5 methane detectors
Carbon monoxide (exhaust joints, breathing zone)	2 along the operating aisle at about 1.5 m
Total	5 × AG-GDC-100 + 2 × AG-GDT-300 = 7 detectors

Counting by area alone would have under-detected this hall by a factor of three. The five methane heads sit 4.7 m apart along the ceiling line — well inside the 10–15 m spacing limit — so the area check is also satisfied, and the 18 m room diagonal is covered.

3. Roof shape, beams and airflow

• Roof geometry. Light gas collects wherever the ceiling traps it: roof apexes, the pockets between beams, the underside of a mezzanine. Each bay or pocket may need its own head. A flat ceiling is the easy case.
• Ventilation. Detection assumes the ventilation is running. A hall of this size at 12 air changes per hour moves about 10,800 m³/h; confirm the design air-change rate, because the detector in the extract path depends on where the extract actually is.
• Obstructions. Do not mount a head behind ducts, cable trays or equipment that would shield it from the gas path or from the airflow.

4. Height, access and testing

A ceiling-mounted methane head in a 6 m hall sits at roughly 5.7 m. Catalytic-bead sensors need scheduled bump testing and calibration to stay reliable — commonly a bump test each quarter and a calibration every six months, set by the site safety policy and the detector manual — and that regime is the safety control for this sensor type. At 5.7 m every test needs a platform or a mobile access unit. Decide how the heads will be reached before the detectors are fixed, not at commissioning; ask us whether a remote-head arrangement suits your application.

5. Alarm stages and what the detectors report to

Both the AG-GDC-100 and the AG-GDT-300 have two field-adjustable alarm stages. For methane the usual starting point is 20 % and 40 % of the lower explosive limit (LEL; methane’s is 5 % by volume in air), driving ventilation boost at the first stage and gas shut-off at the second. The detectors report to a BMS or PLC directly, so a separate gas alarm panel is not required.

6. “Addressable” gas detectors: how many can share a cable

Detectors with an RS-485 Modbus RTU output are addressable in the practical sense: each one has its own slave address (1–247), so many detectors share one cable pair and the controller polls each in turn. This is what keeps wiring simple in a hall with several detectors. The AG-GDC-100 and AG-GDT-300 are rated for a cable run of up to 1000 m, and 4–20 mA is available as an alternative or in parallel — confirm the output configuration at order. For bus-wiring rules (daisy-chain topology, termination, number of devices) see our Modbus RTU vs BACnet MS/TP guide.

7. A short checklist for the specifier

• List the gases and their densities relative to air; set a mounting height for each.
• List every credible leak source and put at least one detector over each.
• Check the result against the area rule and the spacing limit; add heads where the check fails.
• Account for roof pockets, beams, airflow and the extract path.
• Plan access for bump testing and calibration before fixing the heads.
• Choose the output (4–20 mA, Modbus RTU or both) and confirm it at order.

Detectors referred to in this guide

The gas detection range covers methane, hydrogen, carbon monoxide, carbon dioxide, ammonia, hydrogen sulphide, chlorine and oxygen deficiency, each with two field-adjustable alarm stages and a 4–20 mA and Modbus RTU output. The AG-GDC-100 and AG-GDT-300 carry an Ex d IIC T6 marking (manufacturer states CE, ATEX). Send us the gas, the room and the leak sources, and we will confirm the right model, output and lead time.