Never trust your senses to decide if the air in a confined space is safe. You cannot see or smell many toxic gases and vapors, nor can you determine the level of oxygen present.
Testing the Atmosphere | Permit-Required Confined Spaces
It is important to understand that some gases or vapors are heavier than air and will settle to the bottom of a confined space. Also, some gases are lighter than air and will be found around the top of the confined space. Therefore, it is necessary to test all areas — top, middle, bottom — of a confined space with properly calibrated instruments to determine what gases are present.
If testing reveals oxygen-deficiency or the presence of toxic gases or vapors, ventilate the space and retest before entry. Allow sufficient time for diffusion during the ventilation/purging process. If ventilation is not possible and entry is necessary, such as for an emergency rescue, workers must wear appropriate respiratory protection.
Atmospheric testing is required to evaluate the hazards of the permit space and to verify that acceptable entry conditions exist. The person performing the testing must know how to operate and read the test instrument. A monitor should be calibrated before every use or as required by the manufacturer to ensure that the device provides accurate measurements.
Draw air samples through a weep hole or other small entry port leading into the space. When combustible or flammable gases could be present, use a non-sparking probe. 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 space could cause violent reactions or dilute the contaminants, giving a false low initial gas concentration.
Be sure to allow enough time for the instruments to respond to full scale. Assess as many space conditions as possible from the outside; but if entry is necessary for some part of the assessment, respiratory protective equipment may be needed for entrants’ safety. All entrants must be permitted to observe the testing and review the results before they enter the space.
For safety’s sake, take at least three sets of readings:
1. Before ventilation,
2. After ventilation, and
3. The entrant’s reading during the initial entry survey.
Additional or continuous monitoring may be needed. Because of the way test instruments operate, atmospheric monitoring must be performed in a specific sequence.
Oxygen tests must always be made first because most combustible gas meters are oxygen-dependent. Too little oxygen may cause a low combustible gas reading. Too much oxygen can cause a combustible gas meter to explode if gases and vapors are present in ignitable quantities.
Oxygen concentrations are generally measured over a range of 0 to 25 percent oxygen in air, with readings being displayed on either an electronic 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 and above 23.5 percent.
§ Flammable and combustible gases
Flammable and combustible gases are measured next because the risk posed by fire or explosion is more immediate and life-threatening than exposure to toxic gases and vapors.
§ Toxic gases and vapors
Toxic gases and vapors, which are commonly found in confined spaces, are measured last. A toxic sensor requires that the specific toxic substance be identified in advance. Each substance has a specific level to ensure entrant safety and the sensors are specific to these levels. Substance specific detectors should be used whenever actual contaminants have been identified.
The results of the atmospheric testing will have a direct impact on the selection of protective equipment necessary for the tasks in the confined space. It may also dictate the duration of worker exposure to the environment of the space, or whether an entry will be made at all. Evaluation and interpretation of this data and development of the entry procedure should be implemented, based on the evaluation of all serious hazards.
The atmosphere of a permit space which may contain a hazardous atmosphere should be tested for residues of all identified contaminants using permit-specified equipment. This will determine if residual concentrations at the time of testing and entry are within the range of acceptable entry conditions. Results of testing (i.e., actual concentration) should be recorded on the confined space permit in the area provided adjacent to the acceptable entry condition.
The measurement of values for each atmospheric substance should be made for at least the minimum response time of the test instrument specified by the manufacturer.
When monitoring for entries involving a descent into atmospheres that may be stratified, the space should be tested a distance of approximately four feet (1.22 m) in the direction of travel and to each side. If a sampling probe is used, the entrant’s rate of progress should be slowed to accommodate the sampling speed and detector response.
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