Be Prepared With a Gas Detector That Works By: By William Ball Product Applications and Training Specialist BW Technologies by Honeywell Product Management Group The Training and Safety Officer of a major fire department discovered by chance that a combustible gas sensor in one of the department’s portable multi-gas detectors was not working. The instrument had given no indication at startup the sensor was not working properly. Two firefighters were responding to an incident and during the call one firefighter’s gas detector went into high combustible gas alarm while the other detector did not respond. Upon further testing at the fire station it was determined that the combustible sensor in the suspect instrument was not functioning. The fire department was diligent in maintaining their gas detection equipment. They followed the gas detector manufacturers’ recommendations and calibrated their detectors as often as recommended. How did a sensor pass the instrument self test at start up when it was not working? Unfortunately this question is raised far too often. Life threatening situations are encountered as a routine part of the emergency response professional’s job. Serious injury or even death can be caused by exposure to toxic gases, oxygen deficient environments, or explosions caused by the presence of combustible gases. Most atmospheric conditions that lead to accidents are not detectable by human senses and the only way to ensure atmospheric conditions are safe is to use a gas detector. Everyday workers around the world trust their portable safety gas detector to warn them of possible life threatening atmospheric gas hazards, often with little, if any, understanding of how this important component of their safety equipment works. Properly maintained and functioning personal protective equipment is essential to ensuring a safe workplace. As with any other piece of personal protective equipment it is imperative to use and care for portable gas detectors properly. While calibration adjusts sensor accuracy, between calibrations the only way to confirm that a gas detector is functioning properly and is capable of responding to gas is to expose the instrument to a concentration of target gas high enough to initiate an alarm situation while the instrument is in operating mode in a fresh air environment. This procedure is often referred to as a functional bump test. In the evolution of safety gas detection instrumentation, functional bump testing is a relatively new practice. There was a time when calibration was recommended frequently, even daily, but as sensor technology and sensor performance became better understood, the frequency began to lengthen. As calibration frequency decreased it was just assumed a detector was operating properly between calibration intervals. Today recommended calibration intervals vary, but there is unanimous agreement amongst manufacturers that verifying the function of portable safety gas detectors between calibrations is necessary. Since many users do not understand why functional bump testing is necessary, the practice is often not adopted. In some areas of the world functional bump testing personal gas detectors between calibration intervals is virtually unheard of. Why is functional bump testing necessary? Portable gas detectors can contain various types of sensor technologies with different detection principles. Workplace environments can be harsh and gas detectors are subjected to all kinds of conditions that can affect their operation. Instruments can be physically damaged, sensor ports can become obstructed by dirt and oils, sensors can be damaged by exposure to gas concentrations that exceed their detectable limit, sensors can be exposed to compounds in the atmosphere that can degrade their performance, and, as with any product there is the possibility of a manufacturing defect. Catalytic hot bead combustible sensors are widely used in safety gas detectors. This sensor technology provides response to a wide range of ignitable gases and vapors in concentrations below the Lower Explosive Limit. As with all sensor technologies, the catalytic hot bead sensor has limitations. The sensor contains two coils of very fine platinum wire coated with a ceramic or porous alumina material to form refractory beads. The beads are connected to opposing arms of a balanced Wheatstone Bridge electrical circuit. One bead (active) is additionally coated with platinum or palladium, which enables catalytic oxidation of combustible gases at concentrations below the lower explosive level (LEL) to occur. The opposing bead (reference) is identical in structure except that it is not coated with the catalyst material. Both beads are heated to a specific temperature and in normal air, the Wheatstone Bridge circuit remains balanced. If a combustible gas is present, catalytic oxidation will heat the active bead to a higher temperature than the opposing reference bead, unbalancing the electrical resistance in the Wheatstone Bridge circuit. The difference in the electrical resistance is proportional to the concentration of combustible gas in the atmosphere. One of the limitations of this catalytic bead technology is that the sensor is prone to damage through exposure to airborne contaminants capable of impairing sensor performance, or permanently destroying the sensor. Some airborne substances called poisons can form a solid barrier over the catalyst bead surface and destroy the sensor. Silicone vapors are the most commonly encountered workplace substance capable of Photoionizaton detector sensors can suffer a loss of sensitivity if internal components such as the ultraviolet lamp or sensing electrodes become dirty through everyday use. Performing a functional bump test with a known concentration of gas will enable the user to determine if the PID is responding properly. Even though the gas detector performs diagnostic checks at start up and during operation it is often not possible to detect a problem with sensor response. For example, the detector can not warn users that sensor ports are obstructed by dirt, oil, or some other substance, the capillary pore of an oxygen sensor is blocked, or the catalytic bead of a combustible sensor has been poisoned. Either way, if gas is not able to reach the gas detector sensor, the sensor is not able to detect its presence. Functional bump testing gas detectors is essential to a safety program Manufacturers include a bump testing and calibration recommendation in the detector manual. There are third parties that make recommendations and develop workplace safety standards. The International Safety Equipment Association (ISEA) is an organization dedicated to protecting the health and safety of workers through education and the development of safe work practices. In an effort to clarify the term functional bump test, and to emphasize the importance ISEA has developed the “ISEA Statement on Verification of Calibration for Direct Reading Portable Gas Monitors Used in Confined Spaces”. A functional bump test is defined as, “a means of verifying calibration by using a known concentration of test gas to demonstrate that an instrument’s response to the test gas is within acceptable limits”. ISEA safety recommendations influence standards writing bodies such as the Occupation Safety and Health Administration (OSHA) who make direct reference to the ISEA statement in Safety Health Information Bulletin SHIB 05-04-2004, “Verification of Calibration for Direct-Reading Portable Gas Monitors”. When establishing safe work practices consult the manufacturer’s manual and other sources for pertinent information. The need for functional bump testing personal safety gas detectors is evident and the test is very easy to perform. It takes only seconds and provides gas detection users with confidence that the detector about to be used is working. With the development of automated calibration and functional bump test systems, the process can be as easy as pushing a button. Many automated systems keep a permanent record of each test performed so that in the event of an accident investigation detailed proof of proper maintenance and testing can be easily produced. Records of manual calibration and functional bump testing must also be kept. Personal safety gas detectors are designed to help workers stay safe on the job. Develop a functional bump test policy and implement it as a standard part of any safety program. Be prepared with a gas detector that works!