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Understanding Environmental Surveillance

Battery-powered, direct-reading instruments are classified as two groups—single-gas instruments or multiple-gas instruments—

typically monitoring one or a combination of the following atmospheric conditions:

1. oxygen deficiency or enrichment;

2. the presence of combustible gas; and

3. the presence of certain toxic gases.

Depending on the capabilities of the

 instrument, monitoring can be conducted simultaneously for oxygen and combustible gas, or for oxygen, combustible gas and toxic gases. These devices are commonly referred  to as 2-in-1,3-in-1,4-in-1 or 5-in-1 alarms.

No matter which type of instrument is used to check environmental gas concentrations, regular monitoring should be performed because a contaminant’s level of combustibility or toxicity might increase even if it initially appears to be low or non-existent. In addition, oxygen deficiency can occur unexpectedly.

Atmospheric Composition

To determine the composition of an atmosphere, reliable instruments should be used to draw air samples. If possible, do not open the entry portal to the confined space before this step has been completed. Sudden changes in atmospheric composition within the confined space could cause violent reactions, or dilute the contaminants in the confined space, giving a false low initial gas

concentration. When testing permit-required spaces for acceptable entry conditions, always test in the following order:

1. oxygen content

2. flammable gases and vapors

3. potential toxic air contaminants

Comprehensive testing should be conducted in

various locations within the work are a. Some

gases are heavier than air, and tend to collect at

the bottom of a confined space. Others are

lighter, and are usually in higher concentrations

near the top of the confined space. Still others are the same molecular weight as air, so they can be found in varying concentrations throu-ghout the space. This is why test samples should be drawn at the top, middle and bottom of the space to pinpoint varying concentrations of gases or vapors (see Figure 1).The results of the atmospheric testing will have a direct impact on the selection of protective equipment necessary for the tasks in the area. It may also dictate the

duration of worker exposure to the environment

of the space, or whether an entry will be made

at all. Substance-specific detectors should be

used whenever actual contaminants have been



Combustible Gases

In order for combustion to occur, there must be

three elements:


2. oxygen to support combustion

3. heat or a source of ignition

This is known as the fire triangle, but if you

remove any one of the legs, combustion will not

occur (see Figure 2).

The percentage of combustible gas in the air is

important, too. For example, a manhole filled

with fresh air is gradually filled by a leak of

combustible gas such as methane or natural gas,

mixing with the fresh air. As the ratio of gas to air changes, the sample passes through three ranges: lean, explosive and rich (see Figure 3).In the lean range, there isn’t enough gas in the air to burn. On the other hand, the rich range has too much gas and not enough air. However, the explosive range has just the right combination of gas and air to form an explosive mixture. Care must be taken, however, when a mixture is too rich, because dilution with fresh air could bring the mixture into the flammable or explosive range. An analogy is the automobile that won’t start on a cold morning (a lean atmosphere

because the liquid gasoline has not vaporized

sufficiently), but can be flooded with too much

gasoline (a rich atmosphere with too much

vaporization). Eventually, when the right mixture of gas and air finally exists (explosive), the car starts.


How Combustible Gas Monitors Work

To understand how portable combustible gas

detection instruments work, it is first important

to understand what is meant by the Lower

Explosive Limit (LEL) and Upper Explosive Limit

(UEL). When certain proportions of combustible

vapors are mixed with air and a source of ignition is present, an explosion can occur. The range of concentrations over which this reaction can occur is called the explosive range. This range includes all concentrations in which a flash will occur or a flame will travel if the mixture is ignited (see

Figure 3).The lowest percentage at which this

can happen is the LEL; the highest percentage is the UEL. Most combustible instruments display gas concentrations as a percentage of the LEL. Some models have gas readouts as a percentage by volume and others display both percent of LEL and percent combustible gas by volume. What’s the difference? For example, the LEL of methane (the major component in natural gas) is 5 percentby volume, and the UEL is 15 percent by volume. If we slowly fill a room with methane, when the concentration reaches 2.5 percent by volume, it is

 50 percent of the LEL; at 5 percent by volume it is

100 percent of the LEL. Between 5 and 15 percent

by volume, a spark could set off an explosion.

Different gases need different percent by volume concentrations to reach 100 percent of the LEL (see Figure 4).Pentane,for example, has an LEL of 1.5 percent. Instruments that measure in percent of the LEL are easy to use because, regardless of the gas, you are most concerned with how close the concentration is to the LEL.

Single-Gas Monitors for Oxygen Deficiency

Oxygen indicators measure atmospheric concentrations of oxygen. Concentrations are generally measured over a range of 0 to 25 percent oxygen in air, with readings being displayed on either digital readout or an analog meter. Oxygen indicators are calibrated with uncontaminated fresh air containing a minimum of 20.8 percent oxygen. With some models, an

alarm is activated when oxygen levels drop below 19.5 percent.


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Fax: (773) 538-8080

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