May 2024 Vol. 79 No. 5

Rehab technology

Tips for proper gas monitoring during manhole rehab

Manhole rehabilitation is important but dangerous work. Among the potential hazards are toxic and flammable sewer gases and oxygen-deficient atmospheres.  

For companies that have performed countless manhole rehabs without incident, it may be easy to underestimate the risks or become lax about gas monitoring, also known as air monitoring. Yet fatalities happen every year. Undetected toxic gases are the most common cause.  

A gas that poses the greatest risk during sanitary sewer work is hydrogen sulfide (H2S), which is produced from the breakdown of organic materials, including human waste. The sulfuric acid that forms when H2S is released from wastewater can eat away at manhole rungs, liners and grout. 

Even at relatively low concentrations, H2S can cause a worker to lose consciousness before he realizes what’s happening and has a chance to exit the manhole. Very high concentrations can cause death in less than 10 seconds.  

Despite its infamous rotten-egg odor, people can no longer smell H2S if they’re exposed to high concentrations or to low concentrations for long periods. This “olfactory fatigue” can happen instantly, so relying on smell is an extremely poor detection strategy.  

Proper atmospheric testing and monitoring are essential to mitigate the risks to workers posed by toxic gases and hazardous atmospheres. Following these five best practices can help avert a needless tragedy.  

Pump-driven pre-entry testing 

A designated Competent Person should test the manhole atmosphere from outside the manhole with a direct-reading monitor before anyone enters and before the space is ventilated. 

A pump-driven multi-gas monitor is a good choice for this testing. These monitors actively draw gases from within the manhole across the sensors. Diffusion monitors without an attached pump, however, test only the air that passes the sensors through normal air movement. A pump-equipped monitor has an attached wand and draw hose, which are incrementally lowered into the manhole while the Competent Person holds the instrument. This allows for the results to be accurately and timely recorded. 

It’s acceptable to use a diffusion gas monitor for pre-entry testing, but only if it’s fitted with a supplemental pump from the manufacturer.  

Test at multiple levels 

Different gases have different molecular weights, so the atmosphere at the middle or bottom of a manhole may be very different from the atmosphere at the top. Hydrogen sulfide and carbon dioxide, for example, are heavier than air.  

During pre-entry testing, it’s critical to draw air from multiple levels so workers don’t encounter dangerous surprises as they work their way down. Remote sampling at every four feet in the direction of travel is an approach that’s consistent with OSHA requirements and the recommendations of most gas detector manufacturers. 

Continue to monitor 

Testing prior to worker entry is essential, but monitoring shouldn’t stop there, since the atmosphere inside a manhole can change quickly. Continuous monitoring must be performed unless the Competent Person determines that periodic monitoring is sufficient. Making that determination can be tricky; if there is any doubt, continuous monitoring is the better and safer choice.  

The same gas monitor can be used for both pre-entry testing and continuous monitoring (the attached pump should be removed from a diffusion monitor). Diffusion gas detectors worn by workers near the face are acceptable for continuous monitoring. 

The monitoring equipment must have an alarm. Most multi-gas detectors have an audible, sensory (vibration) and visual alarm. If the atmosphere becomes unsafe and evacuation is necessary, the gas detector will alert. Gas monitors can be set to alarm if the concentration of a gas approaches a dangerous level.  

Not all hydrocarbons can be detected on all monitors. If a worker in the manhole starts to feel symptoms, he or she should evacuate the space even if the monitor isn’t alarming.   

The success of continuous gas monitoring hinges on workers knowing how to use the monitors properly and how to react to alerts and alarms, so training is vital. Frequent reminders, such as during toolbox talks, not only refresh workers’ knowledge but also reinforce the importance of adhering to safety protocols. 

Bump test before each shift 

Gas monitors should be bump tested at the start of each shift to confirm that they’re working properly. The test usually takes less than a minute, so there’s no good reason to skip it. 

A bump test uses a calibration gas to confirm that gases can reach the sensors and that the device’s alarms are functional. It doesn’t test for accuracy. If the results aren’t within the acceptable range, the monitor must be calibrated before use. 

It’s important to use a calibration gas from the pump manufacturer; concentrations vary among manufacturers, since the sensors are unique to the pump design. The gas should not be expired. 

Calibrate per the manufacturer’s recommendations

Calibration puts the monitor through a cycle of tests to determine whether the sensors are set to the proper range and give accurate readings.  

Sensor readings will “drift” over time, so it’s important to calibrate as frequently as the manufacturer recommends and according to the manufacturer’s guidelines. Sensor drift can be caused by environmental conditions such as extreme temperatures, humidity and pressure, wear and tear, and manufacturing faults in the sensor. Drifts are detectable only through proper calibration. 

A bonus of performing regular calibration and bump testing: -they may increase the lifespan of the sensors. 

Proper gas monitoring, along with necessary training, ventilation, PPE and rescue equipment, is imperative for companies that want their workers to go home safely at the end of the day. It minimizes the risk of manhole workers succumbing to hazardous atmospheres and, also, reduces ill-advised rescue attempts. Would-be rescuers make up more than 60 percent of confined space fatalities. 


United Rentals, (844) 873- 4948, 

ABOUT THE AUTHOR: Joe Wise is a regional customer training manager for United Rentals Trench Safety Specialty Business, who has been with the company for 20 years. In his role, he provides strategic oversight to Competent Person training programs in confined space and excavation safety. With a team of safety training professionals, who deliver scheduled training across North America, he oversees design and development of existing programs to enhance classroom and online learning experiences for worker safety education. 

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