Smaller Gas Detection Sensors = Smaller Gas Detectors

Manufacturers have seen a definite trend in sales shifting from single-gas to multi-gas monitors as the price and size of the latter decline.

• By William Ball
• May 01, 2007

ADVANCEMENTS in safety gas detection sensor technology in the past few years have allowed dramatic reductions in the size and cost of personal multi-gas monitors, making it possible for employers to protect more workers against exposure to gas hazards in the workplace. Gas detector manufacturers have invested considerable research and resources in the development of small, personal multi-gas monitors that can be worn comfortably as part of a worker's personal protective equipment. Wider acceptance by workers of smaller, more comfortable multi-gas detectors translates into a higher level of protection against the potential dangers presented by toxic gas poisoning, oxygen-deficient and -enriched environments, and the potential for explosive atmospheres in the presence of combustible gases.

Design improvements have also mitigated the effects of humidity on a PID sensor by placing a third "fence" electrode between the sensing and reference electrodes.

A key contributing factor to the development of smaller personal gas monitors is new sensor technologies. At the root of every gas detection measurement is a sensor. In life safety gas detectors, a catalytic bead sensor is most commonly used for combustible gas detection and electrochemical sensors are used for oxygen and toxic gases. Photoionization (PID) sensors are also becoming more widely used as the hazards presented by a wide variety of volatile organic compounds (VOCs) are being recognized globally. Small non-dispersive infrared (NDIR) sensors are now found in personal detectors for the detection of carbon dioxide and combustible gases. Sensors available today are a fraction of the size and weight of those available less than a decade ago; however, the same proven detection principles are used.

The most common gas sensor configuration for personal safety monitors used in routine confined space entry applications continue to be oxygen, combustible, hydrogen sulfide, and carbon monoxide. Detection of these standard four gases provides the most accepted level of industrial protection in confined spaces where the potential gas hazards are unknown.

In a 2006 gas detection market overview by Frost & Sullivan, trends in gas detection technology were discussed. "Major market leaders such as City Technology Ltd developed and introduced a range of very small gas sensor, including the catalytic bead (MICROceL), which is now quite revolutionary in the gas sensors marketplace." The new MICROcel™ OX oxygen sensor uses the industry-standard capillary diffusion barrier technology. This new sensor is 38 percent smaller than its immediate predecessor, the 4-Series, and incorporates the innovative IMES (Injection Molded Electrolyte Seal) process that ensures sensor seal integrity for the entire product life. MICROcel™ electrochemical sensors for hydrogen sulfide and carbon monoxide are a remarkable 17 mm2 x 4 mm deep and weigh only 1.2 g. The MICROpel™ catalytic bead combustible gas sensor features new advancements in intrinsically safe design. The sensor consumes 40 percent less power than conventional catalytic bead sensors and is the same size as the MICROcel™. When the MICROpel™ was introduced by City Technology in spring 2003, it was hailed as the biggest development in the portable gas detection industry in 25 years. Combined, these four new-generation gas detection sensors occupy 70 percent less volume than their predecessor, the 4-Series sensor.

Developments in PID sensor technology have also resulted in dramatic reductions in sensor size, allowing the use of PID sensors in small, hand-held, portable industrial multi-gas instruments. The smaller PID sensor is the result of innovations in PID lamp design and a shorter UV path length, resulting in considerably reduced cell volume. Decreasing the size of the PID sensor does not result in a loss of sensitivity. With brighter lamp technology and improved signal amplification, increased linearity and faster response times are achieved.

PID Developments
Design improvements have also mitigated the effects of humidity on a PID sensor. Previously, PID performance was hampered by interference from humidity, which caused false readings. In conventional PID sensors, moisture could create a current path between the sensor's two electrodes that the detector interpreted as a gas concentration. A revolutionary new PID sensor incorporates a three-electrode design. The innovation of a third "fence" electrode placed between the sensing and reference electrodes mitigates the effects of humidity. Background current created by moisture is absorbed by the fence electrode. With more efficient control of background interference, the PID readings are more reliable.
A short number of years ago, PID instruments were used almost exclusively by industrial hygienists and environmental consultants, but this technology is no longer limited to use for hygiene studies and soil remediation screening. PID technology is becoming more widely used in a multitude of industrial applications, including confined space entry, indoor air quality monitoring, and industrial safety. Increased awareness of the toxicity hazard presented by many VOCs has increased the need for a PID, which can provide real-time measurement of these hazards.

Many familiar substances containing VOCs can be encountered every day, including solvents, paint thinners, and nail polish remover. Vapors associated with fuels such as gasoline, diesel, heating oil, kerosene, and jet fuel also contain VOCs. These vapors can contain specific toxic substances, such as benzene, butadiene, hexane, toluene, xylene, and many others.

The health effects of VOCs can manifest themselves either acutely or chronically. Some are known to be human carcinogens, while others may have more subtle yet very debilitating effects. Solvents, fuels, and many other VOCs are pervasively common in many workplace atmospheres, and many have surprisingly low occupational exposure levels. The PID provides non-specific, broad-range detection with parts per million (ppm) sensitivity for hundreds of potentially hazardous compounds.

Conventional catalytic bead combustible gas detection sensors are effective for the measurement of most combustible gases, but they lack the resolution to monitor low ppm levels. This low resolution limitation is a concern because the catalytic bead sensor is currently the most widely used technology for detection of VOCs. The use of PID sensors as the most effective means of detection will increase as awareness of the health hazards presented by exposure to VOCs increases. Concern over the toxicity of VOCs is leading to the lowering of exposure levels for many compounds including diesel vapor, kerosene and gasoline. The catalytic bead combustible sensor and PID sensor are complimentary measurement techniques that now can be included in your personal safety gas detector. The catalytic bead sensor is exceptional for the measurement of common combustible gases such as methane and propane, which are not detectable by the PID, while a PID sensor can detect large VOC and hydrocarbon molecules not easily detected by the catalytic bead sensor. Thus, the introduction of the PID sensor in a portable multi-gas detector is providing a new level of protection for the industrial workforce.

NDIR Technology
Non-dispersive infrared (NDIR) sensor technology is most often used for real-time measurement of ambient carbon dioxide levels. NDIR sensors also are used for the detection of combustible gases and are especially useful for inerting applications because of their ability to detect combustible gases in the absence of oxygen. The more conventional catalytic bead combustible sensor requires the presence of oxygen to catalytically oxidize combustible gas molecules. Another advantage for the detection of combustible gases with NDIR technology is an inherent immunity to ambient poisons such as silicone vapors that can permanently damage catalytic bead sensors.

There are, however, limitations to NDIR technology that prevent it from completely replacing the catalytic bead sensor in safety gas detection instruments. NDIR sensors are more expensive, and the range of detectable combustible gases is more limited than catalytic bead technology.

The NDIR sensor monitors the presence of gases by their molecular infrared spectral signature. This spectral absorbance "fingerprint" makes it possible to identify the gas and concentration. Many gases absorb radiation in the 2 to 14 micron region of the infrared spectrum. The compact sensor housing contains an infrared source, detectors, and an optical cell. The sensor uses wavelength-specific filters, which tune it to a specific gas(es). NDIR gas sensors are very robust devices that exhibit immunity to harsh chemical environments. The gas detector electronics compensate for changes in ambient conditions, such as temperature.

In the past, NDIR-based instruments have been bulky, expensive, and required a high level of operator expertise. The new generation of miniaturized NDIR sensors now found in personal safety gas detectors and indoor air quality monitors provide ease of use and more reliable carbon dioxide (CO₂) measurement. CO₂ is the fourth most common gas in our atmosphere with an ambient concentration of approximately 350 ppm. The most widely accepted workplace TWA for CO₂ exposure is 5,000 ppm. According to NIOSH, concentrations of 40,000 ppm (4 percent v/v) should be regarded as immediately dangerous to life and health. Exposure to concentrations of 6 percent by volume CO₂ for long periods, or to 30 percent by volume for approximately half an hour, can cause permanent heart damage. Concentrations of 40 percent by volume or higher may cause brain damage, coma, and death as a result of oxygen deprivation.

A heightened awareness of the hazards presented by high concentrations of CO₂ in the workplace has been precipitated by fatalities in the wine and beer industry. In Europe, both Germany and Austria have adopted regulations that require monitoring of CO₂ levels in most confined space entry procedures. And, of course, CO2 levels are extremely important when monitoring indoor air quality. Current NDIR sensors are as small as 20 mm diameter x 16.6 mm deep. The availability of direct measurement CO₂ personal safety gas detectors is attributable to advancements in NDIR sensor technology.

Unrecognized atmospheric hazards are the number one cause of fatalities in confined spaces. The potential for these hazards exists every time a confined space is entered, and changes in atmospheric conditions can occur at any time. Regretfully, complacency is all too common. A worker may have entered confined spaces many times without incident and believes he is never in jeopardy, but it takes only one instance where an atmospheric hazard exists to create another fatality statistic.

Worker safety continues to improve globally. An employer's failure to provide a safe workplace is an indictable offence in some countries. Fines for safety violations are increasing, and in some jurisdictions, individuals in managerial positions in a company can be personally prosecuted. Proper training in safety procedures and in the proper use of the equipment is essential.

The evolution of electronic components, plastic materials, and sensor technologies contributes to the trend of producing smaller, more efficient, less expensive safety gas detectors. Emerging world markets in China, India, and other developing nations will fuel continuing research and development. New sensor technologies such as LED, fiber optic, and laser detection will play an important role. Although the United States continues to use a large number of single-gas detectors, as the price and size of multi-gas detectors decreases, manufacturers have seen a definite trend in sales shifting from single-gas to multi-gas monitors. Equipping all workers at risk of encountering gas hazards in the workplace with multi-gas industrial safety detectors is becoming more affordable with each new generation of product development. Providing additional protection so that a person can be safe on the job and go home following every work shift is never a bad thing.

This article originally appeared in the May 2007 issue of Occupational Health & Safety.