Basics of Gas Detection Instruments

Written on Thursday, April 9, 2009

Basics of Gas Detection Instruments

Portable detection instruments are vital in protecting you from the hazards of toxic and combustible gases in any work environment. Typical sensor options include detection of combustible gas, carbon monoxide, hydrogen sulfide, and oxygen deficiency/enrichment. Instruments may be able to detect all (multi-gas detectors) or just one of these (single-gas detectors). Depending on your specific application, you can choose the appropriate gas detector.

Gas detection is most commonly used in industrial applications such as oil/gas/petrochemical, chemical, utilities, steel, general manufacturing and the fire service.


Parts Per Million (PPM) – is used for the so-called toxic gases such as Hydrogen Sulphide and Carbon Monoxide. These two gases require separate sensors because their lethal concentrations are so different – H2S is deemed dangerous above 15 ppm, while CO may be survivable at over 200ppm although most monitors will alarm at only 35ppm.

Explosive Limit – The explosive limit of a gas or a vapor is the limiting concentration (in air) that is needed for the gas to ignite and explode.

There are two explosive limits for any gas or vapor, the lower explosive limit (LEL) and the upper explosive limit (UEL). At concentrations in air below the LEL there is not enough fuel to continue an explosion; at concentrations above the UEL the fuel has displaced so much air that there is not enough oxygen to begin a reaction. Concentrations of explosive gases are often given in terms of percent of lower explosive limit (%LEL).

Threshold Limit Value (TLV) – refers to the duration of exposure a worker can have to a particular atmosphere without having adverse health effects.

Three types of TLV’s for chemical substances are defined:

1. Threshold Limit Value – Time Weighted Average (TLV-TWA): average exposure on the basis of a 8h/day, 40h/week work schedule

2. Threshold Limit Value – Short Term Exposure Limit (TLV-STEL): spot exposure for a duration of 15 minutes, that cannot be repeated more than 4 times per day

3. Threshold Limit Value – Ceiling (TLV-C): absolute exposure limit that should not be exceeded at any time

Permissible Exposure Limit (PEL) – is a figure defined by the OSHA as the legal limit of exposure for an employee. This means that they may be knowingly exposed to a known hazardous gas for no longer than a legislated maximum time.

Immediately Dangerous to Life or Health (IDLH) – IDLH given as a percentage or PPM represents the maximum exposure level at which you could survive for 30 minutes without breathing apparatus and without incurring debilitating symptoms which would hinder your escape or even irreversibly damage your health.
For More Information:

OSHA’s Workshop, “What to Look for in Gas Detection and Measurement Instrumentation”, can provide further information on choosing the right portable gas detection instrument.

The workshop can be found here: OSHA

Written by: Carissa Kelley

2 thoughts on “Basics of Gas Detection Instruments

  1. Very well written article. I’m really impressed by you knowledge of the subject. Fell free to write more.

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