Hazardous Gases in Confined Spaces
The most common dangerous gas situations in the workplace arise during worker entry into confined spaces, including pipes, vessels and even pit excavations. Without adequate ventilation, microbial activity or gas leaks can cause an accumulation of the following gas hazards:
Lack of Oxygen
This is commonly caused by microbes using up the oxygen, especially in wet conditions. Also, the air may be displaced by natural gas leaks, carbon dioxide from groundwater, or when inert gases like nitrogen or argon are purposefully used to flush a vessel to prevent explosion. Lack of oxygen quickly causes unconsciousness and is deadly within minutes. Therefore, when entering a confined space, it is critical to carry an oxygen sensor that alarms when the normal air concentration of 20.9% drops below 19%, as an early warning.
Combustible gases such as natural gas or liquid fuel vapors can be ignited by electronic equipment or dropped metal tools, causing an explosion. Low concentrations of combustible gases cannot cause an explosion because the gas is to lean. Combustion can only occur when the amounts are above the Lower Explosive Level, or LEL, which is typically between 1% and 5% by volume. Combustible gas sensors are commonly refered to as LEL sensors, and are required for entering confined spaces.
Hydrogen Sulfide (H2S)
Hydrogen sulfide is formed by anaerobic microbes after oxygen used up in a confined space. It causes a “rotten egg” smell, which can be detected at very low levels before it is toxic. However, the nose quickly becomes desensitized to the gas above 10 ppm, respiratory failure starts above 30 ppm, and rapid unconsciousness and death above 500 ppm. Because of workers loss of smell, and the high toxicity of this gas, H2S sensors are used in all confined space entry monitors.
Carbon Monoxide (CO)
Carbon monoxide is an odorless gas that is formed by incomplete combustion and some microbial processes. The 8-hour workplace exposure limit is 50 ppm, and above about 800 ppm, unconsciousness and death follow. This gas is most common around combustion sources and is easily measured by low-cost CO sensors.
4-Gas Monitors for Confined Space Entry Compliance
Nearly all countries have regulations requiring the above 4 gases to be measured before a worker can enter a confined space. To comply with these regulations and thus ensure worker safety, 4-gas monitors are available that have all four sensors in compact, portable instruments. One example is the mPower MUNI, which weighs only 230 g, has audio, visual, and vibration alarms at user-adjustable concentration set-points, and runs a full 24-hours on a single battery charge.
More Toxic Gases
Other toxic gases may be present in selected industrial operations, such as ammonia refrigeration, chlorine bleaching, sulfur dioxide from coal combustion, nitrogen oxides from diesel engines and aviation, phosphine for fumigation and many volatile organic compound (VOC) solvents. The POLI multi-gas meter is an advanced model that offers all of the standard features and 4 sensors (O2/LEL/H2S/CO) plus options for over 20 other toxic gas sensors including VOCs. Such units allow the monitors to be used not only for confined spaces but also by hazmat teams responding to spills and leaks in the open air. Convenient logging of data allow for record-keeping to maintain proof of compliance during safety audits.
Werner Haag, Ph.D.
mPower Chief Chemist
Dr. Haag has over 25 years experience in the industrial gas detection instruments and has authored the book “The PID Handbook: Theory and Applications of Direct-Reading Photoionization Detectors”
mPower has one of the most experienced service and applications support teams in the business.