Preparation is the Key

A new OSHA information bulletin provides advice, information to increase safety with properly calibrated gas monitors.

OF all the dangers inherent to confined space work, it's the invisible threat of atmospheric hazards that poses the greatest risks to employees. Dangerous conditions involving unsafe oxygen levels, as well as toxic and combustible gases, have caused the most accidents in this type of work environment. These hazards also have contributed to confined spaces' having a much higher fatality to injury rate than most other types of workplace accidents.

Recognizing atmospheric hazards is problematic in that they are not visible, nor accurately detectable through any of the four other human senses. Atmospheric conditions within confined spaces also can change rapidly, adding an element of unpredictability and elevating the level of danger. For these reasons, the only way to safely enter and perform work in a confined space is by using a gas monitoring instrument that will alert the worker to potential dangers within. It should be noted, however, that possessing and properly using a gas detection instrument are not enough to ensure safe working conditions. Regular instrument calibration also must be part of preparing for the potential dangers of confined space work. Only then can you be certain the instrument will function properly and respond accurately to hazardous concentrations of gas.

This is the focus of a Safety and Health Information Bulletin released by OSHA on May 4. While the document introduces no new standards or regulations, it presents detailed information designed to advise and assist employers in providing a safe working environment.

Confined Space Atmospheric Hazards
Understanding OSHA's bulletin on instrument calibration is tantamount to understanding the nature of confined spaces as outlined in its 29 CFR 1910.146. The standard defines a confined space as any space that is large enough and configured so that an employee can bodily enter and perform assigned work; and has limited or restricted means for entry or exit; and is not designed for continuous employee occupancy. In addition to meeting this definition, a permit-required confined space also presents potential health and safety hazards. OSHA states that a permit-required confined space contains or has a potential to contain a hazardous atmosphere; contains a material that has the potential for engulfing an entrant; has an internal configuration such that an entrant could be trapped or asphyxiated; or contains any other recognized serious safety or health hazard.

It is imperative that oxygen levels within confined spaces be tested and monitored to determine whether the level is either too low or high for a worker to safely enter. Generally speaking, the oxygen content of ambient air is 20.9 percent by volume. Oxygen concentrations above 23.5 percent represent an extreme fire hazard where flammable materials burn rapidly and combustion of typically non-combustible materials can occur. On the other hand, if levels fall below 19.5 percent, workers can be at risk of everything from disorientation and impaired judgment to mental failure, fainting, and death from asphyxiation.

Two of the most common toxic gases, hydrogen sulfide and carbon monoxide, also can cause asphyxiation in confined space atmospheres. Symptoms from low-level exposure to hydrogen sulfide and carbon monoxide include eye irritation and mild headache, respectively. At the other end of the spectrum, however, confined space workers can suffer immediate unconsciousness and death in minutes from exposure to these toxic gases.

Hydrogen sulfide and carbon monoxide also are among the many gases that are combustible when combined with oxygen in certain proportions and an ignition source. Gas explosions originating from confined space work are often catastrophic and deadly, affecting employees working both in and around the confined space.

Detecting the Hazards
The potential for these hazards to cause harm can be significantly reduced and, many times, virtually eliminated if confined space requirements are carefully followed. OSHA's 1910.146 states that before an employee enters a permit-required confined space, "the internal atmosphere shall be tested, with a calibrated direct reading instrument, for the following conditions in the order given: (1) oxygen content, (2) flammable gases and vapors and (3) potential toxic air contaminants."

Use of the word "calibrated" here represents a clear understanding that inaccurate readings and compromised safety are much more likely to come from a gas monitor that has not been properly tested by exposing its sensors to a known concentration of gas. OSHA's May 4 bulletin states these inaccurate gas concentration readings could lead to injury or death, and that avoiding this is the primary reason for calibration.

The Need for Calibration
OSHA refers to the instrument's response to calibration gas exposure as its "reference point," or the point where all atmospheric gases will be measured and compared. When an instrument's reference point shifts, its readings also shift and become inaccurate. This is known as "calibration drift" and can be caused by chemical degradation of sensors, drift in electronic components, exposure to extreme environmental conditions, exposure to high concentrations of target gases, or exposure to poisons and inhibitors. When calibration drift occurs, as it does in all instruments over time, the device is still capable of measuring gas concentrations. The problem, however, is in the accuracy of the numeric reading. Performing a full calibration resets the instrument's reference point and ensures accurate readings, which are vital to worker safety.


Industries where confined spaces exist:
  • Agriculture
  • Oil and gas
  • Pulp and paper
  • Steel and coke manufacturing
  • Transportation
  • Food and beverage
  • Petrochemical
  • Public utilities
  • Telecommunications
  • Water and wastewater treatment

Ultimately, the user should refer to the instrument manufacturer for recommendations on when and how often an instrument should be calibrated. However, OSHA's bulletin does include a position statement from the International Safety Equipment Association, a trade association for manufacturers of protective equipment, which states that "A bump test or full calibration of direct-reading portable gas monitors should be made before each day's use in accordance with the manufacturer's instructions, using an appropriate test gas." A bump test is a method of testing an instrument's sensors with a concentration of gas high enough to make the instrument go into alarm. If the instrument fails to alarm or its reading does not coincide with the measure of gas as indicated on the test cylinder, a full calibration must then be performed to adjust the instrument's reading.

Focus on Safety
Like other safety procedures, training employees on the correct use of the instruments and the proper methods of calibration will go a long way toward creating safe working environments. All too often, however, preparing to safely work in confined spaces is an afterthought, and the employee's focus is directed entirely to the task at hand. While the importance of using gas monitors in confined spaces is understood for the most part, bump testing and calibrating the instruments is sometimes seen as either a necessary evil or as a tedious and time-consuming task.

Fortunately, instrument manufacturers have responded to the concerns of the end user and have designed automated instrument management and maintenance stations that can do everything from battery charging and bump testing to full calibration, data downloading, and recordkeeping. While these systems can provide convenience and peace of mind, workers still need to be trained on instrument use and how to perform pre-entry testing and continuous monitoring of the confined space atmosphere.

Regardless of whether instrument maintenance is performed automatically or manually, following manufacturer and OSHA recommendations ensures employees are prepared to safely work in confined spaces and that their gas monitoring equipment is also up to the task.

This article originally appeared in the August 2004 issue of Occupational Health & Safety.

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