Portable gas detectors are essential safety equipment for contractors who face potential exposure to hazardous gases. Choosing the right equipment can help save money while ensuring workers are alerted to critical workplace dangers
Contractors who work in a wide range of environments understand the need to protect themselves with sensitive portable monitors capable of detecting hazardous gases.
A wealth of options, however, can make selection a complicated matter. Should contractors buy meters capable of detecting the one gas they’re most likely to encounter, or buy multi-gas meters, capable of detecting up to five gases simultaneously? When outfitting an entire crew with similar protection, cost becomes an even more important factor.
“In many cases, contractors called onto a site are usually informed of the type of gas hazards they may encounter,” says Rick Wanek, industry market manager, portable instruments with Pittsburgh-based DrÃ¤ger Safety Inc. (Draeger). “Often the way portable detectors are selected is to either ask the people on the site what they’re using and buy that, or to go out and buy the cheapest detector for the job.”
Contractors who actually need to enter a mine have a relatively easy selection task. Any gas detector used in a mine must be certified by the Mine Safety and Health Administration in the U.S. or by the respective provincial authority in Canada. Further, many other settings require the meters to be approved by Underwriters Laboratories Inc. or the Canadian Standards Association.
Although Wanek says occasional counterfeit equipment reaches the market, the perpetrators are quickly eliminated. “In order to sell, they have to advertise, and organizations like Underwriters Laboratories are quick to defend their trademark,” he notes.
The testing is both rigorous and expensive. That’s one reason there are only a handful of suppliers in the market – the cost of testing eliminates casual competitors. For MSHA approval, for example, the meter considered for approval must function to spec the first time it’s tested. If not, the prototype unit is returned and must be altered in some fashion – either through software adjustments or recalibration – and the next approval process begins from scratch.
The meters make use of a number of technologies appropriate to each gas they’re designed to detect. A multi-meter may use multiple methods to detect a range of gases. Infrared sensors detect gas concentrations by shining infrared spectrum light through the sample. Electrochemical sensors read the electrical currents generated by the interaction of specific chemicals and a target gas. More complex catalytic sensors ignite small samples of gases such as methane and measure the resistance between two filaments.
DrÃ¤ger portable detectors run from about $200 for single-gas units to around $1,500 for sophisticated multi-gas units.
“The mining industry leans toward multi-gas meters,” says Wanek. “In a coal mine you would at least want to have methane detection, but you could easily add oxygen and carbon monoxide sensors, and where diesel engines are used, nitrogen dioxide. Some mines also have issues with hydrogen sulfide.”
Why oxygen? Because a meter capable of detecting methane does so through electronic combustion of microscopic methane samples – which requires the presence of oxygen to complete the combustion process.
A meter capable of detecting those five gases would also cover contractors who work in the oil patch or in natural-gas fields.
“After that point, it may actually be more economical for contractors to buy additional single-gas meters to deal with specific problems related to the field in which they’re working,” says Wanek. “Generally workers are reluctant to carry two meters, but that may be the least expensive way to go on a case-by-case basis.”
Wanek notes portable monitors are getting smaller. The smallest portable meter offered by the company measures just 3 inches long by 2.5 inches wide and a half-inch thick. The larger ones measure about 2 by 6 inches and weigh just 6 or 7 ounces.
“The meters could actually be much smaller,” says Wanek. “But we need to keep some meter real estate open for the display, and the necessary labels that accompany the meters.”
Meters alert the user to threshold levels of gas by sounding an alarm, either a vibration or an actual sound. The DrÃ¤ger units provide a two-alarm system: a low-level alarm that indicates the gas levels are creeping toward the threshold and a secondary alarm that warns the user that the threshold is being passed.
“At that point, you simply leave or evacuate and only the professionals stay behind,” says Wanek.
Acceptable thresholds for various gases can change from jurisdiction to jurisdiction. Those thresholds can be programmed into the instruments either by the manufacturer or the user.
“Ours have a standard default setting for the gases, but the user can change those presets or change the type of alarms with a software interface,” says Wanek. “That software is necessarily password protected, because you don’t want the average worker to change those settings. A beeping unit is annoying because it’s meant to annoy you and alert you to a problem. You don’t want anyone to hit a snooze button in a critical situation. If the threshold is set low, you may occasionally get a false alarm, but better to have some false alarms and know the meter is working.”
For contractors who only occasionally work in environments where they’re likely to encounter gas, alternative testing devices can be used. DrÃ¤ger-tubes, for example, are glass ampoules containing a number of reagents that can detect specific gases in various work environments.
“You pump the surrounding atmosphere through the tubes to determine peak concentrations,” says Wanek. “However, they’re best used in limited areas. Carrying a portable gas meter is essentially like carrying a movie camera running continuously, while using the tubes is more like taking a snapshot of a particular and limited environment, for example in sewers, shafts, tanks or other confined spaces. We often find tubes used to measure concentrations of specific gases like benzene or other toxic or carcinogenic gases that are associated with a specific work site.”
Up to five tubes can be mounted in a harness designed to test for five gases simultaneously. The drawbacks to the system? Contractors can use each tube only once, so new tubes must be purchased for each test.
The DrÃ¤ger Chip Measurement System is a higher-tech version of the tubes in which a portable unit can be outfitted with the contractor’s choice of 55 different gas-specific chips. However, like the tubes, the chip system provides only a snapshot of gas concentrations. The meter does, however, record up to 50 measurements for later analysis.
While portable meter technology continues to advance, the basic measurement techniques remain largely the same. “The most dramatic change is in the micro-processors used to analyze the reading,” says Wanek. “For example, if you can be exposed to 50 ppm of a certain gas each day, and 60 ppm for a very short time, the micro-processor is able to calculate what is considered an acceptable exposure for the day through time-weighted averages.”
Contractors, of course, want value for money and prefer to hang onto their meters for as long as they can. Typically, a portable meter is good for at least five years, Wanek says. Meters used in mines can have a shorter life than that because they’re often knocked around and exposed to dust.
FOR THE LONG HAUL
Proper maintenance can ensure the units function effectively for as long as possible. Units should be cleaned each morning to ensure that the display is clear and permanent filters should be cleaned per the manufacturer’s recommendations to ensure the free flow of sample atmospheres. Removable filters should be replaced if dirty. Cracked or broken units should not be used under any circumstances.
After cleaning, the meters should also be “bump-tested” each day by exposing them to a set sample of gas at a pre-determined concentration. The bump-testing kits, sold by DrÃ¤ger and other companies, act as mini-calibration tests to ensure the unit is reading properly.
“If you’re getting low response, or no response, you should check the filters,” says Wanek. “If the problem persists, the unit shouldn’t be used in the field.”
The portable units are powered by batteries. While they can run on alkaline batteries, Wanek says it’s often simpler to use rechargeable batteries, which can be gang-charged each night to maximum power, rather than rely on alkalines, which need to be swapped out frequently to ensure they cover the entire workday.
FINAL WORD ON SAFETY
A good portable gas detector, however, is no substitute for safe work practices, notes Wanek. “If you’re entering a confined space, it’s not something you should ever do alone,” he says. “The detector will alert you to the hazard, but regulations dictate that you need a spotter to help get you out of there if there’s any communication indicating trouble.”