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The requirement is to monitor a variety of gases in a range of conditions and applications for either occupational exposure levels or gas leak detection levels in accordance with international standards.

BASIC CONCEPTS FOR GAS MONITORING

Concentrations can be expressed in a number of ways, the most commonly found are:

1- % Volume composition: the ratio of the volume of a component to the sum of the volumes of all components of a mixture before mixing. Conversion rate: 1% vol= 10.000ppm

2- Mass concentration (mg/m³): This is the ratio of the mass of a component to the volume of the mixture.  Conversion rate at 20°C and 1atm:  mg/m3 = (ppm) (gram molecular weight of compound)/24.04

3- PPM (parts per million): is the numbers of parts of a component in 1 million parts of the gas mixture.

4- LEL: This only applies to low concentrations of combustibles gases, and it stands for Lower Explosive Limit. It is the volume composition of a flammable gas in air.  Below this limit no explosive atmosphere will be formed.  Concentrations levels for gas detection purposes are expressed in % of LEL.  In other words it is a volume ratio of the gas relative to the LEL.

Details of gas properties including LEL values can be found at:
http://www.cdc.gov/niosh/npg/pdfs/2005-149.pdf

Combustible gases can have very different LEL values and different authorities have different values, typically:

5- TWA: Permissible Exposure Limits (PELs), Recommended Exposure Limits (RELs) and Threshold Limit Values (TLVs) are limits considered safe for most people exposed to a chemical agent 8 hours per day, 40 hours per week, 50 weeks per year, over a working lifetime.  These limits are often referred to as 8-hour time weighted averages (TWA).

6- STEL: Short-term exposure limit varies in different countries and substances from 10 to 15-min time -weighted average exposure.

DETECTION LEVELS

Concentration levels to be detected vary with the purpose intended: Occupational Safety or Detection of a gas leakage.

a-      Occupational Safety: Generally legislation and standards concerned with Occupational Safety require that gases be monitored at levels expressed as Time Weighted Averages (TWA) covering long term (8 hours) and short-term (10 minutes) exposure.

Each country sets its own Occupational Exposure Limits (OELs) generally by reference to official values from other countries.

In Europe the EC is trying to establish a standard set of reference values, and Commission Directive 2000/39/EC includes a first list of indicative OELs. The Directive requires States to establish National OELs taking into account the Community values.  The European Committee of Standardisation (CEN) is currently working on a standard for the assessment of workplace exposures.

Many countries worldwide in setting values would refer to the UK Health and Safety Executive Occupational Exposure Limits (Workplace Exposure Limit WELs) or the MAK values (DFG-Deutsche Forschungsgemeinschaft).  In the USA values are set by OSHA (Occupational Safety and Health Administration, US Department of Labour), NIOSH (National Institute for Occupational Safety, Health- US Dept of Health and Human Services), and ACGIH (American Conference of Governmental Industrial Hygienists).

Table 1. Indicative List of Exposure Levels

(Guide to Occupational Exposure Values – compiled by ACGIH www.acgih.org and EH40-Health and Safety Executive UK)

(Also visit the International Programme of Chemical Safety: and section 6.3 shows ammonia Exposures limits worldwide http://www.inchem.org/documents/hsg/hsg/hsg037.htm#SectionNumber:6.1)

b-   Gas Detection: a typical industry where gas detection levels are applicable is Refrigeration and Air Conditioning.  In this case the position is more defined. For example ASHRAE in the USA, specify the long term Threshold Limit Value (TLV) level as a suitable maximum alarm threshold for gas detectors.

However most Standards derive from the ISO 5149 and so are similar.

In Europe, the EN378 2000 specifies ‘Practical Limits’ for the maximum alarm threshold.  These practical limits are based on various criteria. For halocarbons it is based on half the concentration that can lead to suffocation due to oxygen displacement or which has narcotic or heart sensitisation effects. Typically this can be at levels of 7-8 % (70-80,000 ppm) ie.: R407C 8%, R404A 11%.   For combustible gases 20% of the Lower Flammability Level (LFL) is used. With toxic/combustible gases such as ammonia, the practical limit refers to the toxicological and flammability characteristics. For ammonia a low-level alarm threshold, maximum 500 ppm, and a high level, maximum 30,000 ppm, is recommended.

WHAT LEVEL DO WE DETECT

We want to detect gas leaks as early as possible while complying with legislation and guidelines. Normal industry practise has resulted in typical gas detection ranges for different purposes. We summarise these below with an appropriate sensor type and typical alarm relay set points. Requirements may vary by country.

The gas monitor can be set to alarm at any value within the range.

Download the PDF – FAQ 7. What Gas Concentration to Detect

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