Get into the habit of bump-testing

To ensure a gas monitor will perform as intended in the presence of gas, most manufacturers of portable gas monitors and regulatory agencies agree that a functional, or "bump," test should be carried out on an instrument prior to each day's use. Manufacturers offer confined space entry kits that provide all of the equipment necessary for this operation, and docking stations can bump-test as well as record the events.

However, many workers remain puzzled about how often they should bump-test or when to subject the instrument to a full calibration. Manufacturers' recommendations can vary with language that cites the need for "periodic" or "frequent" testing and intervals between calibrations up to 180 days. Questions abound: Is a monitor with a 180-day calibration interval better than one recommending 30 days? What do OSHA or other governing agencies require? These are simple questions that involve safety, reliability, ease of use and maintenance costs for a company.

In actuality, there is no "one-size-fits-all" answer to the question about testing an instrument. Sensor technology, applications and functions of the monitor must be considered.

Sensor technologies used in today's gas monitors have improved significantly over the past decade. Advances such as electrochemical over solid state, enhanced poison resistance and less cross-interference have resulted in a more reliable performance. This evolution is one reason why calibration intervals have lengthened. Still, gas monitors must operate in harsh conditions and are not impervious to damage. Along with physical shock to the instrument, sensors can be damaged by gas concentrations that exceed the detectable limit. Filters and sensor ports can become obstructed by liquid, dirt and dust, producing no change in readings even though the atmospheric conditions may have fluctuated. Proper bump testing and verification of accuracy between calibrations can prevent a false sense of security.

The frequency of this testing may depend on the monitor itself. Most, if not all, confined space monitors used today are direct-reading and detect for oxygen, lower explosive level, hydrogen sulfide and carbon dioxide at a minimum. Catalytic bead combustible sensors are particularly prone to contamination that inhibits response to the target gas. Oxygen sensors are prone to failure when they do not respond to atmospheric changes. Monitors that employ these sensors should be tested prior to each day's use.

In 2004, OSHA posted a bulletin on "Verification of Calibration for Direct-Reading Portable Gas Monitors," available at www.osha.gov/dts/shib/shib050404.html. OSHA stipulates that although the advisory bulletin is not a standard and creates no legal obligations, it can assist employers in providing a safe and healthful workplace.

Portable gas monitors are designed to protect workers against potentially life-threatening occupational environments. Verifying the proper performance is an essential part of any gas monitoring safety program. Following this protocol will ensure confidence in the workers, as well as everyone's responsibility for keeping them safe.