Gas detection system

01

Set the hazard thresholds the system must detect

OSHA's permit-required confined spaces rule, 29 CFR 1910.146, defines a hazardous atmosphere as one that may expose workers to death, incapacitation, impaired self-rescue, injury or acute illness from flammable gas, vapour or mist above 10% of its lower flammable limit (LFL); combustible dust at or above its LFL, roughly when it obscures vision at 5 ft (1.52 m); oxygen below 19.5% or above 23.5%; a substance above its OSHA dose or permissible exposure limit; or any other condition immediately dangerous to life or health. [1]

Before entry the employer must test conditions in the permit space, and where isolation is infeasible, such as in a sewer, monitor continuously where entrants work; testing must check oxygen first, then combustible gases and vapours, then toxic gases and vapours, and entrants may observe the testing and request re-evaluation. [1] Editorial note: the same thresholds are a useful starting point for fixed-system alarm set points, but the site's hazard assessment and local rules should set the final levels.

Give the supplier the gases to detect and their expected concentrations, alarm set points (low, high, STEL/TWA), fixed or portable use, number and location of detection points, sensor technologies (catalytic bead, infrared, electrochemical, PID), calibration gas for LEL sensors, output and controller (4–20 mA, relays, Modbus, wireless), hazardous-area classification and certification, enclosure rating, environmental conditions, maintenance and bump-test arrangements, data logging, and quantity.

02

Choose the calibration gas and keep the system tested

Industrial Scientific explains that a catalytic-bead LEL sensor responds differently to different combustible gases, so the calibration gas sets the reference for every reading and two correctly working monitors calibrated to different gases can read differently in the same atmosphere. A correlation factor converts the reading when the actual gas is known: an instrument calibrated to methane reading 10% LEL in propane, with a propane factor of 1.62, indicates about 16.2% LEL propane, while a pentane-calibrated instrument would read about 8% LEL in the same atmosphere. Factors cannot be relied on for mixtures, and Industrial Scientific says it "often recommends pentane calibration gas for general industrial environments" where a site is exposed to a mix of hydrocarbon vapours rather than a single gas. [2]

Industrial Scientific says "Gas detection program failures often begin with skipped bump tests, inconsistent training, poor maintenance practices, and limited visibility into device health and alarm data"; sensors drift, so routine calibration is needed, and bump testing confirms that sensors and alarms respond before use. Its analysis of more than one billion data points found that about one in every 2,500 untested instruments failed to respond correctly to gas. [3]

Editorial calculation: one failure in 2,500 untested instruments is 0.04%; across a fleet of 5,000 monitors used without a bump test, about two would fail to respond. [3]

03

Read a fixed detector's outputs, approvals and drift figures

Crowcon's specification for its Xgard Bright fixed gas detector shows the fields a fixed-system quotation should cover. It detects flammable or toxic gases or monitors oxygen, with MPS (Molecular Property Spectrometer), long-life O₂ or infrared sensor options; Crowcon says the MPS flammable sensor is free of poisoning and needs no calibration or scheduled maintenance. It runs on 12–30 V DC at 3 W maximum and gives a 3-wire 4–20 mA output (sink or source) and RS-485 Modbus RTU as standard, with HART 7 optional, plus Alarm 1, Alarm 2 and Fault relays rated 1 A at 24 V DC. Repeatability is ±2% of full scale and zero drift ±2% of full scale per year maximum. Enclosures are aluminium alloy (about 1 kg) or 316 stainless steel (about 3.1 kg), IP65/66, for −40 to +70 °C under ATEX/IECEx in Zones 1 and 2 (Ex db IIC T6 Gb for aluminium) or −40 to +60 °C under UL 1203, Class I Division 1, Groups B, C, D. Warranty is 2 years on the transmitter and 5 years on MPS flammable and long-life O₂ sensors. [4]

Editorial calculation: on a 0–100% LEL range, ±2% of full scale is ±2% LEL, and a year of maximum zero drift adds up to another ±2% LEL, against the 10% LFL hazard threshold in the OSHA confined-space rule; this is why bump tests and zero checks stay in the maintenance plan even where a sensor needs no calibration. Specify the outputs your controller reads, the relay set points, the hazardous-area zone and gas group, and the sensor warranty with the order. [4] [1] [3]

04

Compare what suppliers offer

Suppliers linked to this page list gas detectors or gas monitoring systems; each card links to the page reviewed.

Listings include fixed gas detectors with software, laser methane detectors, portable single- and multi-gas detectors, and online gas monitoring systems; ask each for sensor technology and range per gas, response time, certification, outputs and the calibration gas used.

05

Trace the system construction

This is a functional reference built from the sources cited; it is not one maker's specification.

06

Fill the specification fields with values for your site

A gas detection quotation needs values for your hazards. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

07

Send the gases, levels and upkeep plan together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Gases: [list; expected ranges]
Alarms: [set points; latching]
Use: [fixed points; portable; confined space]
Sensors: [technology; LEL cal gas]
Outputs: [4–20 mA; relays; wireless; software]
Certification: [area classification]
Upkeep: [bump test; calibration; data]

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