General Gas Knowledge Database

General Gas Knowledge Database

General Gas Knowledge Database


This document is an attempt to collect, all in one place, the pertinent properties of some of the airborne contaminants for which Air-Met is able to provide real time measuring and monitoring devices. These devices include portable and fixed gas detectors using different detection principles and gas detector tubes. Exposure monitoring using portable sampling pumps and appropriate sample collection media is covered more than effectively through the SKC catalogue and website, so is not included here.

This list is not comprehensive but consists of gases and vapour-forming liquids commonly encountered in industry. The information provided has been collated from a variety of sources and whilst every attempt has been made to ensure its accuracy and currency, no assurances can be given that this is the case. The intention is to provide a broad guide as a “first step” for recommending and choosing appropriate instrumentation, further detailed investigation and cross-checking is strongly encouraged.

Each substance is listed on a separate page for ease of revision and the information is divided into the following sections:

A more detailed explanation of each section follows.

Common name

This is the name most commonly used in industry. It is not necessarily the “correct” chemical name and in some case might be an acronym or abbreviation.


Many substances are known by other names and this section lists some of them. Again, abbreviations and acronyms are also listed.


This “Chemical Abstract Service number” is a unique identifier for every different chemical and is often used in references. It is useful when seeking further information and to avoid confusion with similar substances and substances which are different but given the same abbreviated name.

Chemical formula

Many substances are frequently referred to using their chemical formula, e.g. carbon dioxide = “C O two” (CO2). For organic compounds, wherever possible, the formula given is in a format which gives an indication of its structure rather than just its molecular make-up.

Vapour density

Gases and vapours of volatile liquids all have a density relative to air, which is always 1. This parameter gives an indication of whether the gas or vapour will, in the early stages of a leak or spill, collect on the floor, rise to the ceiling or just “hang about”. All gases and vapours eventually will tend to form a uniform mixture within a certain area but initially, when detection of leaks and spillsis critical, where to place fixed sensors and/or take measurements with portable devices is vitally important. Vapour density is a useful guide to this decision process.

Safe Work Australia Exposure Standards

Where relevant, the Safe Work Australia Exposure Standards (ES) are listed. These are sourced from: and should always be checked for currency as they are subject to review and change as new information comes to hand. In some cases an ES has two values, a TWA (time weighted average) and a STEL (short term exposure limit). The TWA is based on an 8-hour exposure time and the STEL is usually based on a 15-minute exposure time. Although the values listed are used as the basis for setting alarm levels, it is strongly recommended to read the comprehensive explanatory notes on the above website for more detailed information on this subject before an alarm set-point decision is made.

Flammable limits

Again, where relevant, the flammable limits, as published in AS/NZS 60079.20.1:2012, are listed. The LEL (lower explosive limit) is the minimum concentration of a gas or liquid vapour in air, capableof forming a self-sustaining flame if ignited. Ignition can occur below this concentration but the mixture will be too “lean” and will not form an explosive

mixture. Similarly the UEL (upper explosive limit) is the maximum concentration of a gas or liquid vapour in air, capable of forming a self-sustaining flame if ignited. Mixtures with a concentration above this figure are considered to be too “rich” to form an explosive mixture. In terms of monitoring, UEL is not important but the parameter is provided to

give some indication of how hazardous a flammable vapour or gas is – a wide flammable range indicates a greater chance of an ignitable mixture being formed. A few substances have a UEL of 100% by volume, indicating that they might decompose explosively without the presence of oxygen.

Equipment group

The equipment group refers to the use of electrical equipment in areas where flammable atmospheres might be present and where it is important that the equipment does not become a source of ignition. Standards exist (e.g. the AS/NZS 60079 “Explosive atmospheres” series) setting out the requirements for the design, construction, installation and use of such equipment. Independent bodies, such as ANZEx and IECEx, exist to issue certificates of compliance to these Standards. Referred to as “Ex” certificates, they require equipment to be marked with symbols indicating what level of explosion protection is provided and in which potentially hazardous areas they can be used safely. e.g. If a flammable gas or vapour has an Equipment Group rating of IIA, then devices used to monitor these substance should have a IIA or better (e.g. IIB or IIC) certification and marking.

Temperature classification

The same as for Equipment Group applies to Temperature Classification. For example, for a gas or vapour having a T4 Temperature Classification, equipment should be certified and marked T4 or higher (e.g. T5 or T6).

Chemical/physical properties

This section sets out briefly the type of chemical, whether it is a gas or liquid at room temperature,whether it has a detectable odour and whether it is flammable.
Detection of substances by odour is notoriously unreliable due to individual differences in olfactory sensitivity (some people can smell things that other people can’t and at different levels) and olfactory fatigue (loss of the ability to smell a certain substance after prolonged exposure to it). This information is provided more for the sake of help with identification.

Hazardous properties

Using information sourced mainly from the ACGIH “Documentation of the Threshold Limit Values and Biological Exposure Indices” and the ILO “Encyclopaedia of Occupational Health and Safety”, this section provides a plain language summary of any exposure effects which have been shown to be deleterious to health.


An indication of where the substance is likely to be found and what are some of its major uses in industry.

Detection methods available

A listing of the some of the available types of instruments offered for detecting, measuring and monitoring the substance; along with an indication of the measuring range(s) available. Expert advice should be sought regarding which method is most suitable for each different application.

Suggested alarm levels

Alarm levels used in detection and monitoring instruments offered are largely based on the data given but, if listed, are suggestions only – site conditions and the motivation for requiring a warning that a gas concentration has exceeded (or fallen below) a certain level, can dictate that alarm set points be different to those suggested.
Not all substances have suggested alarm levels listed and even if they do, it is strongly recommended to seek professional advice before choosing alarm set points.

List of Substance
Acetone Hexane Ozone
Acetylene Hydrogen Pentane
Ammonia Hydrogen Bromide Phosphine
Arsine Hydrogen Chloride Propane
Benzene Hydrogen Cyanide Refrigerant gases
Butane Hydrogen Fluoride Silane
Carbon Dioxide Isophorone diisocyanate (IPDI) Sulphur Dioxide
Carbon Monoxide Methane Toluene
Chlorine Methylene bis(40cyclo-hexylisocyanate) (HMDI) Toluene-2,4-diisocyanate (TDI)
Cholorine Dioxide Methylene bisphenyl isocyanate Xylene
Diborane Methylene Chloride  
Ethyl Alcohol Nitric Oxide  
Ethylene Nitrogen Dioxide  
Ethylene oxide (EtO) Nitrous Oxide  
Hexamethylene diisocyanate (HDI) Oxygen