Sadly, stories of workers being overcome by toxic fumes while working underground in sewers make the news more often than they should. In addition to protective gear, crew workers should use a gas detector to detect any harmful toxic gas in a sewer before entering. 

Perhaps it’s easy to assume you’ll be fine without a gas detector or confined-space gear because you’ve climbed into a pipe without those things and come out unscathed hundreds of times before. Familiarity breeds complacency. Someday, when you least expect it, a simple hand-held device could help save your life. 

That brings us to sorting through all the available options to choose the one that best suits your needs. 

Should you choose a portable gas detector capable of detecting the one hazardous gas you’re most likely to encounter, or a multi-gas meter, capable of detecting several gases simultaneously?

“Sewer contractors usually know which gases they’re likely to encounter,” says Rick Wanek, industry market manager, portable instruments with Pittsburgh-based Dräger Safety Inc. “If they run into anything unusual, like chlorine used to treat wastewater, they’re usually informed before they get to the site.”

Bells and whistles

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 light through the sample.

Electrochemical sensors read electrical currents generated by the interaction of specific chemicals and a target gas. Catalytic sensors ignite small samples of gases, such as methane, and measure the resistance between two filaments.

“In a sewer setting you would want to detect four standard gases — methane, oxygen, hydrogen sulfide and carbon monoxide,” Wanek says. Why oxygen? Because a meter capable of detecting methane does so thorough electronic combustion of microscopic methane samples, which requires oxygen.

Anything but ordinary

Occasionally, sewer workers may be required to work in confined spaces containing an additional gas. “Typically, the contractor will be warned beforehand,” Wanek says. “It may be more economical for contractors to buy additional single-gas meters to deal with specific problems related to the situation in which they’re working. Generally workers are reluctant to carry two meters, but that may be the least expensive way to go.”

Alternate testing devices can also be used. Dräger tubes, for example, are glass ampoules containing a number of reagents that can detect specific gases in various 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 like carrying a video camera running continuously, while using the tubes is more like taking a snapshot of a limited environment, such as a sewer, shaft, tank or other confined space.”

Chip off the old block

Another solution is a chip system, a higher-tech version of the tubes in which a portable unit can be outfitted with the choice of more than 55 gas-specific chips. However, like the tubes, the chip system provides only a snapshot of gas concentrations.

Portable meter technology continues to advance. “The most dramatic change is in the microprocessors used to analyze the readings,” Wanek says. “For example, if you’re exposed to 50 ppm of a certain gas each day, and 60 ppm for a very short time, the microprocessor is able to calculate what is considered an acceptable exposure for the day through time-weighted averages.”

A good portable gas detector, however, is no substitute for safe work practices, notes Wanek. “The detector will alert you to the hazard,” he says. “But regulations dictate that you need a spotter to help get you out of there if there’s trouble.”