Personal gas detection
July 1, 2007
Exposure to atmospheric hazards in the workplace is a risk that thousands of employees face every day. Depending on the industry and application, the specific gas hazard is often an identified result of processes or procedures occurring at the workplace. In many cases, the employees at risk are working in more than one location, so it makes sense to equip each worker with a personal detector designed specifically for the detection of the known hazard.
Some common examples of atmospheric hazards are oxygen deficiencies, and accumulations of combustible gases and specific toxic gases such as carbon monoxide (CO) or hydrogen sulfide (H2S). A simple and often inexpensive solution to reduce this risk is to issue a personal detector to each of the employees who work in the affected areas.
Today’s personal gas detectors are small, lightweight and can easily be worn on a belt, clipped on clothing or fitted to a hardhat. Detectors continuously monitor the atmosphere in the area of the worker. When a preset exposure limit is exceeded, the detector will warn the worker with an audible sound, visual light and often a vibrating alarm.
Over the past few years the cost of single-gas detectors has come down significantly. And while the use of personal gas detectors is simple and requires relatively little training, there are some important factors to consider. These include the selection of the right detector and proper maintenance.
Making the right choice
The first and most obvious criterion is to determine the nature of the hazard, and to choose a detector capable of monitoring that specific hazard. The selection process should also take into account the detector limitations and desired features.
The most common types of personal detectors are for oxygen, carbon monoxide and hydrogen sulfide. These detectors generally have few limitations, but there are still a few worth considering.
Most oxygen sensors have a lifespan of two years, meaning that the sensor or the monitor will need to be replaced every two years. Toxic sensors are generally designed to be substance-specific, but some may also respond to other gases. In most applications this is not a problem, but in cases where multiple compounds may be present at the same time, it can result in false readings.
A classic example of such interference is the incidental presence of hydrogen in environments that may contain a carbon monoxide hazard. While hydrogen may be flammable in high concentrations, it is a harmless gas in low concentrations. Most CO detectors will respond to hydrogen and give a reading, which the user mistakenly understands to be a CO hazard. To overcome or reduce this limitation, special CO sensors have been developed with much lower cross-interference to hydrogen.
Most gas detectors use electrochemical sensors for the detection of oxygen and toxic gases, and catalytic sensors for the detection of combustible gases and vapors. All of these sensors have one thing in common: As they age, are consumed or otherwise degrade with use, they may fail to respond appropriately to gas. Most sensors that are used to monitor for atmospheric contaminants can be referred to as “fail-unsafe,” because there is no way to determine if the sensor can respond appropriately to gas without physically applying gas to the sensor.
Today’s sensors are designed to exhibit only a small percentage of degradation per year and it is not uncommon for a sensor to last many years without noticeable loss. It is, however, quite possible that a small percentage of sensors suffer unexpected loss in sensitivity or even fail completely without the user’s knowledge, so it is important to establish a prudent maintenance schedule for personal detectors. This may be as simple as exposing the detector to a known concentration of test gas on a periodic basis to confirm that the instrument responds appropriately. Such a test is referred to as a functional “bump” test.
ISEA (International Safety Equipment Association) states that the safest cause of action is to verify accuracy prior to each day’s use. ISEA also states that under the right conditions, the test interval can be lengthened, but should never exceed 30 days between tests. The risk of sensor failure increases as the time between verifications lengthens. In 2004, OSHA issued a bulletin advising adherence to the ISEA recommendations.
Docking stations are designed to simplify maintenance and recordkeeping for gas detectors. A docking station is a computerized device that accepts a personal gas detector and within seconds is able to test all of its vital systems. Typically, such tests include a fresh air calibration, a quick bump test and a complete test of the alarms, including audible, visual and vibrating alarms.
If a detector does not meet sensor performance criteria for the bump test, the dock will automatically attempt a full calibration. This type of automatic testing not only simplifies maintenance, it also saves time and frequently reduces cost by using less calibration gas during the testing phase. Once all tests have been completed, the dock will store all test results either locally or in a database on a PC. Recorded data goes a long way to showing that the employer is proactively maintaining the instruments and may prove to be of huge value in the event of litigation.
With the increase in usage of personal gas detectors due to reduced costs and ever-improving reliability, workers must be aware of the need for basic maintenance, including periodic verification of sensor accuracy. Integrating an automatic docking station can be a key to an easy-to-manage gas detection program, proper equipment maintenance and seamless recordkeeping.