Eight ways to improve detection of unseen atmospheric hazards

Occupational Safety and Health Administration (OSHA) standard 29 CFR 1910.146 defines a permit-required confined space as having at least one of the following characteristics: "contains or has a potential to contain a hazardous atmosphere; contains a material that has the potential for engulfing an entrant; or contains any other recognized serious safety or health hazard.

By David D. Wagner, Manager, Customer and Product Services, Industrial Scientific Corporation September 15, 2002
Key Concepts
  • Unseen hazards require the appropriate equipment for detection.

  • An instrument sensor should match a particular hazard in a confined space.

  • Continuous monitoring is an effective way to ensure a safe atmosphere exists.

    Occupational Safety and Health Administration (OSHA) standard 29 CFR 1910.146 defines a permit-required confined space as having at least one of the following characteristics: “contains or has a potential to contain a hazardous atmosphere; contains a material that has the potential for engulfing an entrant; or contains any other recognized serious safety or health hazard.” Clearly, this is not the best location to spend a workday. And yet each day thousands of American workers do.

    Confined spaces exist in almost every industry and in every workplace. At some point, daily duties require workers to enter a confined space and to confront its seen and unseen hazards.

    Some confined space hazards are readily apparent. Seeing the potential for falls or entrapment from cave-ins or falling equipment can be relatively easy. However, unseen atmospheric hazards present the greatest dangers to individuals within a confined space. The lack of breathable oxygen, the presence of deadly poisonous gas vapors, such as carbon monoxide, and the potential for explosion due to dangerous levels of combustible gases all are examples of unseen atmospheric hazards. Portable gas-monitoring instruments are used to detect these potentially life-threatening conditions and alert workers of the potential danger.

    Here are eight important safety tips for selecting, preparing, and using gas detection monitors.

    1.Make sure the sensors in the instrument being used are appropriate for the confined space. Many people refer to a confined space monitor synonymously with a four-gas instrument containing oxygen, combustible gas, carbon monoxide and hydrogen sulfide sensors. While it is true that most confined spaces do potentially contain some hazard related to one of these gases, it is not true for all. For instance, an instrument with carbon monoxide and hydrogen sulfide sensors will be of little use when entering a space that may potentially contain chlorine. Many instruments offer the ability to change the sensors to match the particular hazard to be encountered. These instruments provide greater flexibility and value in a wide variety of applications.

    2. The gas-monitoring instrument must be durable and able to withstand harsh conditions. Although a portable gas detector is certainly a sophisticated piece of electronic equipment, it is just a tool. And as such, it is subjected to the rigors of the environment it is used in just like any other tool. It is dropped, dunked, caught up in other equipment and all the while must be relied upon to provide a potentially lifesaving service. Be certain that the instrument chosen is constructed in a manner that will not let its survival in a tough environment, or yours, be left to chance.

    3.Confined space pre-entry tests require remote sampling of the atmosphere from outside the space. The instrument chosen must have the capability of using a remote sample pump. Whether or not that sampling pump is integral to the instrument or detachable, is strictly based on preference. In either case, the pump must have the capability of drawing an adequate sample flow over the distance required to cover the entire space. The pump also should be able to detect and clearly warn if the sample line is blocked, thereby preventing gas flow to the instrument and its sensors.

    4.Be certain that the sample tube material being used with the pump is of high quality and is compatible with the vapors that you might expect to encounter . Some highly reactive gases, such as chlorine, nitrogen dioxide, or hydrogen chloride may be absorbed into the walls of the tubing and scrubbed from the sample stream. Never use sample tubing made of materials containing silicone rubber compounds. Silicone vapors may off-gas from the tubing and poison catalytic bead-type combustible gas sensors, leaving the instrument unable to detect explosive gases. Sample tubing made of urethane or FEP (Teflon) is generally suitable for most applications.

    5.The gas monitor should be able to operate from a variety of power sources . Rechargeable batteries are generally most suitable for portable monitoring instruments, because the combustible gas sensors used in the detectors consume large amounts of battery power. Extended confined space operations often mean that the rechargeable batteries run out of power before the work is done. The ability to replace the rechargeable battery pack with disposable alkaline or lithium battery cells comes in handy in these situations. Additionally, make sure that the instrument is capable of running the required amount of time when the sample pump is being used. Some sample pumps use the instrument’s battery for power and reduce the run time of the instrument much more than might be expected.

    6.Know the detection limits of the instrument . There are clear differences in the measuring ranges of sensors in various instruments. As a rule of thumb, the monitor should be capable of measuring concentrations approaching the IDLH (Immediately Dangerous to Life and Health) level of the target gas. All catalytic combustible gas sensors require a minimum background oxygen concentration present to respond accurately. Make sure that you are aware of what that level is, and that the instrument chosen is capable of being used with a dilution apparatus to ensure the accuracy of the combustible detector when sampling from inert or oxygen-deficient atmospheres.

    7.Beware of claims that instruments do not need to be tested or calibrated on a regular basis . Know one thing for sure: The only way to ensure that a gas monitor will respond to gas is to test it and to verify its operation with known concentrations of the target gases prior to each use. Who’s to say that someone using the instrument didn’t drop it, dunk it, or get it caught in another piece of equipment causing irreparable damage to one of the sensors? There is no difference in the response from a catalytic or electrochemical gas sensor that is used in a clean atmosphere and a sensor that has failed catastrophically. Regular calibration and functional testing guarantee that the instrument and its sensors are working properly. If calibration and testing are too difficult and cumbersome to do on your own, docking systems or service programs are available which perform these tasks on instruments automatically and document the results.

    8.To monitor continuously or not to, that is the question . Most often, confined space atmospheres are tested before entry to complete the required permits, and then the gas monitor is put away until the next test. Unfortunately the work done in a confined space may create that “unseen hazard” while workers are there. Chemical reactions with solvents during cleaning processes in tanks, or vapors produced during maintenance operations, such as welding, can cause dangerous gases to build up in a confined space during work. Continuous monitoring of the atmosphere in the space during the entire entry ensures that you are not left unaware of possible danger lurking. Using an instrument with the capability of activating a remote alarm will guarantee that a partner watching from outside the space is not left unaware either.

    Edited by James Silvestri, Senior Editor, 630-288-8777, jsilvestri@reedbusiness.com