Gas detection coverage mapping

Detector layouts are only defensible when the coverage they achieve is quantified. This module builds design-basis spherical gas clouds from the credible release inventory and tests, cloud by cloud, whether the proposed detector layout would see them, producing a geographic coverage score across the monitored area.

Open the Gas Detection Mapping module

What this calculator returns

Design-basis cloud radius for the selected target gas volume
Geographic coverage percentage for the detector layout
Uncovered zones highlighted on the plot-plan
Effect of adding, moving or removing individual detectors

Required inputs

Plot-plan extents and monitored area boundary
Detector coordinates and detection type
Design-basis cloud volume or target gas quantity
Voting arrangement (1ooN or 2ooN)

Calculation method

The design-basis cloud is represented as a sphere of the target flammable volume, positioned on a grid across the monitored area.

At each grid position the layout is tested against the voting requirement: a cloud is detected when enough detectors fall inside its envelope.

The fraction of grid positions detected gives the geographic coverage, and the undetected positions are mapped as gaps.

Governing equations

r = ( 3 Vcloud / (4 pi) ) ^ (1/3)

Radius of the equivalent spherical design-basis cloud.

Coverage = N_detected / N_total

Geographic coverage as the detected fraction of cloud positions.

Nomenclature

Vcloud: design-basis flammable cloud volume, m3
r: equivalent cloud radius, m
N_detected: cloud positions satisfying the voting requirement
N_total: total cloud positions tested on the grid

Assumptions and limitations

Spherical, homogeneous design-basis clouds rather than CFD-resolved dispersion shapes.
Detectors are assumed available and correctly calibrated.
Coverage is geographic; scenario-weighted coverage requires release frequency data.

Reference practice

Consistent with the coverage-based approach in ISA TR84.00.07 for fire and gas system effectiveness.

Worked example

A design-basis flammable cloud of 15 m3 screened against point detectors in a congested module.

Step	Value	Basis
Cloud volume	15 m3	Set from the release and congestion basis for the area
Equivalent radius	1.53 m	r = (3V / 4 pi)^(1/3) for the spherical design cloud
Coverage test	Detector inside the cloud envelope	A grid of cloud centres is swept over the module footprint
Voting	1ooN or 2ooN	2ooN coverage needs two detectors inside the same cloud, which drives detector count sharply up

A design cloud only 1.5 m in radius means detector spacing, not detector sensitivity, controls coverage; small clouds and 2ooN voting are what force dense layouts.

Illustrative numbers only — rerun the module with the project basis of design before using any result.

Common questions

What is geographic coverage?

The fraction of grid positions where a design-basis cloud placed at that position would be detected by the required number of detectors.

What coverage target is common?

Around 90 percent geographic coverage for 1ooN detection is a widely used goal, with a lower target where 2ooN voting is required.

Do line-of-sight detectors change the assessment?

Yes. Open-path detectors are assessed on beam intersection with the cloud and path-averaged concentration, not on a single point location.

Related calculators

Indoor Release — Indoor release concentration and ventilation calculator
Gas Dispersion — Atmospheric gas dispersion calculator
Noise Mapping — Plant noise mapping calculator