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OSHA Hazard Information Bulletins
The Use of Combination Oxygen and Combustible Gas Detectors


January 18, 1990

MEMORANDUM FOR:

REGIONAL ADMINISTRATORS

THRU:

  • LEO CAREY
  • Director
  • Office of Field Programs

FROM:

  • BARRY WHITE
  • Director
  • Directorate of Technical Support

SUBJECT:

  • Hazard Information Bulletin on the Use of Combination Oxygen and Combustible Gas Detectors

The Directorate of Technical Support Issues Hazard Information Bulletin (HIB) in accordance with OSHA Instruction CPL 2.65 to provide relevant information regarding recognized or misunderstood health hazards, inadequacies of materials, devices, techniques, and engineering controls. HIB's are initiated based on information provided by the field staff, studies, and reports and concerns expressed by safety and health professionals, employers, and the public. Information is compiled based on a thorough evaluation of available facts, literature and in coordination with appropriate parties. HIB's are used as vehicles for the dissemination of technical information to OSHA.

The purpose of this bulletin is to alert field personnel to some of the shortcomings/limitations in the use of combination oxygen and combustible gas detectors. Recently the Boston Regional Office brought to our attention a fatality in which an employee, from a small welding firm, was cutting up a 4,000 gallon tank that had contained leaded gasoline, when the tank exploded.

The ensuing investigation uncovered that the tank had been repeatedly steam cleaned and purged with nitrogen gas prior to the explosion. Although combustible gas readings were taken prior to welding, with the last reading indicating below the LEL, it was evident that improper use of the instrument and an incomplete understanding of its limitations by the operator were the cause of the fatality.

In tank removal operations, such as the above, it is common practice to purge a tank containing flammable vapors with either carbon dioxide or an inert gas, such as nitrogen. When the oxygen content falls to about 10% or below, a false combustible gas indicator reading can occur.

The combination oxygen and combustible gas meter is used to test atmospheres for sufficient oxygen content for life support and/or the presence of combustible gases or vapors posing a potential flammability/explosion hazard. Common examples of locations where this instrument is used include storage tanks, confined spaces, manholes, tank cars, ships and shipyards, tunneling, pumping stations and hazardous waste sites.

The combustible gas indicator is designed to measure combustible gas or vapor content in air. This instrument is capable of detecting the presence of any gas or vapor which, when combined with oxygen in free air, presents a potential hazard due to flammability/explosion. The combustible gas indicator will not indicate the combustible gas content in atmospheres containing less than 10% oxygen.

It should be noted that each instrument has its own set of operating procedures and instructions. Consequently, it is impractical to list, in this memo, all precautions and warnings for each oxygen/combustible gas detector in use. However, by way of illustration, other precautions and warnings not already discussed above, and specifically applicable to the MSA MicroGard Portable Alarm, are as follows:

  1. The instrument should not be used where the oxygen concentration exceeds that of fresh air (oxygen enriched atmosphere) when sampling for gases like acetylene and hydrogen.
  2. Certain materials such as silicon, silicates (such as in certain hydraulic fluids) and organic lead (such as in leaded gasoline) will poison the combustible gas sensor thereby giving erroneously low readings.
  3. Combustible gas readings, either negative or greater than 100% LEL, may indicate an explosive concentration of gas beyond the accurate response range of the combustible gas sensor.
  4. Pressurized or low pressure samples will give erroneous oxygen percent readings.
  5. Acid gases, such as carbon dioxide, will shorten the service life of the oxygen sensor.
  6. The instrument will not indicate the presence of combustible airborne mists or dusts such as lubricating oils, coal dust or grain dust.

The above caution list is by no means comprehensive. For a more complete listing of cautions and warnings regarding the MSA MicroGard Portable Alarm the instruction manual must be referenced.

The safe and effective performance of any oxygen/combustible gas detector requires that the operator know the correct use of the instrument to detect explosive concentrations of combustibles. It is important that the instrument response be appraised in light of the limitations and guidelines given in the instrument manual. The instrument should be operated only after the instructions, labels, cautions and warnings, and any other literature accompanying the instrument are carefully read and understood.

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