In the confined work spaces found in chemical plants, paper mills, refineries, underground mines and utility passageways, the air may be contaminated with toxic or combustible gases or suffer from a lack of oxygen. Regulations call for the monitoring of these environments.
Every day, workers who are just doing their jobs can come into contact with airborne contaminants that are harmful or even fatal. This especially is true of workers who must enter confined spaces to perform job tasks.
To protect workers, employers are required by law to have a way to monitor the air before entry and during the entire time employees occupy the confined space. Employers must ensure a safe and healthy worksite to maintain production and to protect their workers.
Each person entering the confined space should be equipped with a portable gas monitor worn where it can be checked easily and frequently. It’s a must-have, life-saving tool that can be used in a wide range of industrial settings.
Gas Monitor Design
Portable gas monitoring systems may be designed for a single substance, or can be fitted with multiple sensors. Some measure up to six gases and include piercing audible and attention-getting 360-degree visual alarms.
An alternative or adjunct to personal monitors is a transportable area monitor. Look for one with area monitoring via diffusion or with a sampling pump, wireless communications capabilities and a waterproof housing with continuous operation of around 60 hours. More run time helps increase productivity by minimizing new checks of the atmosphere for each shift change or when the space may be unattended. A multiple sensor design often is driven by OSHA requirements, which specify confined space monitoring by following this detection sequence: oxygen, combustible gases and any potential toxic contaminants (see 29 CFR 1910.146(c)(5)(ii)(C)).
Portable gas monitors are lightweight with dimensions similar to a mobile phone for ease of use. They run on either rechargeable or replaceable batteries.
Sensors Are the Heart of a Gas Monitor
As you might imagine, the sensor is the most important component inside a gas monitor. When facing an unknown gas hazard, you need a sensor that provides dependable detection of combustible atmospheres. It quickly should respond to explosive gases and have a high level of sensitivity to combustible organic vapors in a confined space. The monitor you choose should include sensors for:
➤ O2 – To check the oxygen level for deficiency or enrichment.
➤ Combustible gas – To test for levels of flammable or explosive gases.
➤ Toxic gas levels – Typical confined space monitors have hydrogen sulfide (H2S) and carbon monoxide (CO) sensors; additional toxic sensors may be used based on the application.
Gas monitors also provide an alarm at the appropriate real-time concentration, short-term exposure limit (STEL) or time-weighted average (TWA) for the monitored substance.
Electronic gas monitors with four or five different sensors (multi-gas monitors) may be sufficient for many confined space applications. Some situations may require monitoring capabilities for additional substances. As a general rule, each type of sensor is best suited for certain substance categories.
Electrochemical (EC) oxygen sensors measure the volume of oxygen in air over the range of 0-25 percent. They can provide a relatively accurate alarm point at a volume of 19.5 percent to protect workers from reduced oxygen levels. EC toxic gas sensors can detect, monitor and accurately measure CO, H2S, SO2, Cl2 and many other toxic gases at parts per million (ppm) levels, making them suitable for monitoring STELs and TWAs specified by OSHA and other agencies.
Catalytic (CAT) oxidation sensors for combustible gases can detect a wide range of substances and can be calibrated to measure combustible gas concentrations in the range of 0-100 percent of the lower explosive limit (LEL). While not specific to any one gas or vapor, they are sensitive to several different ones within the same general group of substances.
Infrared (IR) combustible gas sensors are not subject to poisoning, do not require oxygen to operate and, like CAT sensors, can measure a wide range of combustible gases and vapors over the range of 0-100 percent LEL. They cannot measure some inorganic combustibles like hydrogen, and although more expensive up front, last much longer.
Photo ionization detector (PID) sensors are used to detect low concentrations of volatile organic compounds in the air, such as fuels like gasoline and diesel, and to measure solvents such as acetone, xylene, etc. Because of their ppm sensitivity, they can be used for STEL and TWA measurement applications. Their detection capabilities are not specific to any one compound, and their relative responses to different compounds can vary widely.
Other sensor characteristics to consider based on your workplace needs include:
➤ Monitor design that allows gas intake from both the top and front of the monitor.
➤ The contaminant concentration measurement range, minimum detection limit and resolution.
➤ Environmental conditions during use (range of temperature, pressure and relative humidity).
➤ Sensor replacement procedures (on site versus manufacturer replacement).
➤ Warranty period.
Intrinsic safety is a key feature of the electronic circuitry in a gas monitor. This means the available electrical and thermal energy in the gas monitor always is low enough that ignition of the hazardous atmosphere cannot occur. Among other things, electrical sparks are not produced and even under fault conditions, the temperature of a component cannot reach a level that could cause autoignition of a combustible atmosphere.
To sell a gas monitor as an intrinsically safe device, the manufacturer must have it tested and certified by one or more independent testing labs.
Hazardous areas are separated by classes, divisions and groups to define the level of safety required for equipment in these locations. The most common intrinsic safety approval ratings on portable gas monitors are Class I; Division 1; Groups A, B, C and D; and Temperature Rating T4.
Make sure that when choosing a gas detector that might be used in a work environment where explosive, combustible or flammable vapors are present, the detector design has been tested and verified to be intrinsically safe by a recognized testing lab (UL, etc.). This verification assures users that the detector will not cause an explosion or combustion when used in such locations, even if something goes wrong in the electronic circuitry of the detector.
An external feature of a gas monitor’s internal electronics is the user interface. Some considerations in selecting a gas monitor that is user friendly include:
➤ Is its operation intuitively obvious?
➤ Are controls easy to understand?
➤ Is it easily operated by a user fitted with gloves and equipment?
➤ Are there adequate security features to prevent tampering?
➤ Is the display large enough to be easily read?
➤ Besides warning properties, are there functions that support documentation and reporting, such as built-in data logging?
A bright, flashing LED lamp is an excellent visual alarm, and a loud multi-tone horn sound is a good audible alarm. A strong vibrating alarm the user can feel also might be necessary in case of loud background noises or if the monitor is not attached to the user where it easily can be seen. Alarm thresholds should be able to be individually adjusted to comply with company policy or other standards.
Once an alarm is detected, a large, easy-to-read digital display is necessary to verify the concentration of a dangerous gas. In case of low light conditions, the display should be adequately backlit.
A signal output interface facilitates the downloading of gas concentrations from a built-in data logger or event recorder. Various interface types are available, including those that use a USB cable to transfer data to a PC, and infrared interfaces that perform a similar function.
Some gas monitors have the ability to store thousands of events and gas concentrations over many hours of sampling. Other significant events you might want to have recorded can include switching the gas monitor on or off, gas and battery alarms, error codes, configuration changes, fresh air calibrations and bump tests conducted.
In addition, various software packages are available for PCs and Mac computers to store and manipulate data and generate reports.
Typically, a thermoplastic polymer is used for the gas monitor housing. Other features related to the housing to consider when evaluating a portable gas monitor are:
➤ Size and weight;
➤ Portability accessories;
➤ Protection from radio frequency interference;
➤ Protection from dust and water ingress;
➤ Chemical resistance; and
➤ Resistance to physical shock damage.
An IP65 or IP67 rating may be needed for adequate protection from dust, rain and water spray. In harsh environments, a polymer housing may be rubber coated or a rubber boot may be added for greater protection from aggressive chemicals and temperature extremes.
As part of good operating practices, gas monitors should be bump tested every day before use. A bump test is defined as a qualitative function check where a challenge gas is passed over the sensor(s) at a concentration and exposure time sufficient to activate all alarm indicators to present at least their lower alarm setting.
Note that a bump test is not required to provide a measure of calibration accuracy. Additional considerations in bump testing include checking the manufacturer’s expiration date on the bump station gas cylinder, and documenting proper instrument calibration and other functions.
Detector tubes can be valuable tools in determining the type of sensor that should be used in a gas monitor, as well as in checking a confined space before entry. A detector tube is a hermetically sealed glass tube containing an inert carrier material mixed with one or more reagents that undergo a colorimetric reaction when specific contaminants are drawn into the tube. The length of the color change in the tube, or the intensity of color change as compared to a standard, indicates the amount of contaminant present.
OBSERVE PROPER SAMPLING PROCEDURES
Normally, a sampling hose and probe are attached to the gas monitor to allow the air inside a confined space to be sampled. The air sample is brought into the gas monitor by a sample pump. Some gas monitors have a built-in pump and some have an external pump. This could be either a manual pump or one that is battery-powered with automatic volume control.
Follow the manufacturer’s recommendations and all rules and regulations published by the employer, which should be consistent with guidelines published by OSHA, NIOSH, OHSB (Canada) the American Conference of Governmental Industrial Hygienists and other recognized authorities. When the atmosphere in the confined space has reached the immediately dangerous to life and health level, do not enter without a properly fitted self-contained breathing apparatus and take other appropriate precautions.
Checking the air in a confined space for oxygen, combustibles and toxics before entry is essential in protecting a worker’s health and safety. Consider including portable gas monitors as part of your organization’s confined space safety management plan.
Ed Ligus has been working with gas detection devices in the safety market for over 30 years. He has worked as a technical service representative, product manager for detector tubes and marketing manager for gas detection products. He currently is a portable gas detection product manager with Draeger Safety Inc. in Pittsburgh. In that position, he works to support and promote the Draeger line of detector tubes, the chip measurement system and single-gas portables in North America.