Direct Reading Sensors for Toxic Killers

The mission for multi-sensor instruments is expanding rapidly. Customers have more options and better quality than ever before.

• By Bob Henderson
• May 01, 2004

THE North American multi-sensor gas detector market is very large. Most of these instruments are purchased, at least in part, for use in confined space entry programs.

Most estimates place the confined space market size between $120 million and $140 million USD. At an average purchase price of $700 to $1,000 USD per instrument, that's a lot of confined space gas detectors!

Because the market is so large, gas detection equipment manufacturers put a lot of emphasis on multi-sensor portables. The competition between manufacturers is fierce and has led to increasingly sophisticated designs, with significantly expanded capabilities.

For many instrument users, the "mission" for confined space gas detectors is expanding to include a variety of non-confined space activities. A fire department might use the same instrument both for confined space rescue and for general hazmat response. Compact, four-channel confined space gas detectors are increasingly used as personal monitors for workers at refineries, chemical plants, or oil platforms subject to the potential presence of dangerous atmospheric conditions. In the past, personnel at these facilities might have used colorimetric tubes or a single-sensor H₂S detector to verify atmospheric conditions were safe. Using multi-sensor detectors means workers are immediately aware of combustible as well as toxic gas releases or accumulations.

Carbon monoxide (CO) and hydrogen sulfide (H₂S) still are the most commonly encountered toxic gases. CO is a byproduct of incomplete combustion and will always be present where combustion occurs. H₂S is produced by bacterial action on materials that contain sulfur. It is especially associated with raw sewage, crude oil, animal products, and the pulp and paper industry, but it can be encountered occasionally in almost any confined space. The majority of multi-sensor instruments sold include the ability to monitor for one or both of these hazards. Increasingly, however, other hazards are being recognized as too important to omit.

The Organization of Economic Cooperation and Development (OECD) produces a list of High Production Volume (HPV) Chemicals. According to the OECD, more than 5,000 toxic chemicals are produced or imported in volumes in excess of 1,000 tons per year in the United States. On average, there are about 400 major accidents per year involving these chemicals. Fortunately, direct reading miniaturized sensors capable of being installed in portable multi-sensor instruments are available for many of the most common and dangerous of these hazards.

New Types of Sensors for Multi-Sensor Instruments
New sensor technologies--such as miniaturized photoionization detectors (PIDs) for volatile organic compound (VOC) measurement, non-dispersive infrared (NDIR) sensors for CO₂ or combustible gas, and additional types of substance-specific electrochemical sensors--are available for incorporation into multi-sensor portable instruments. As exposure limits continue to drop, atmospheric monitoring programs increasingly need to include direct quantifiable measurement for many additional toxic substances.

Chlorine
Chlorine (Cl₂) is a pervasively common industrial chemical. According to the OECD, 30.8 million pounds of chlorine are produced and used per year in the United States. Water treatment, the pulp and paper industry, chemical plants, and many other industries are all major users of this chemical. The federal OSHA Permissible Exposure Limit (PEL) for chlorine is a Ceiling of only 1.0 ppm. The NIOSH Recommended Exposure Limit (REL) is a Ceiling of only 0.5 ppm. The ACGIH® TLV® consists of an eight hour TWA limit of 0.5 ppm, and a 15-minute short term exposure limit of 1.0 ppm. Workers whose jobs put them at potential risk of exposure should consider equipping their instruments with sensors designed to directly measure this hazard.

The consequences of a major leak or release leak can be catastrophic. On May 6, 1991, a massive liquid chlorine leak from a chemical plant in Henderson, Nev., produced a cloud that drifted over the town, causing more than 200 injuries (at least 30 were very serious) and the evacuation of approximately 7,000 nearby residents. Thankfully, there were no fatalities. The number of injuries and likelihood of fatalities would have been much greater if not for the fact that plant employees were equipped with emergency escape respirators and direct reading chlorine detectors. In an emergency situation, planning, training, and on-hand personal protective equipment make all the difference.

Ammonia
Ammonia (NH₃) is even more common, with an estimated 40.6 million pounds produced and used per year. Ammonia is commonly used in many industries, including petrochemical, pulp and paper, fertilizer, and the oil industry, to name but a few.

Anhydrous ammonia (NH₃) is also very widely used as a coolant in large industrial refrigeration systems. While ammonia refrigeration has long been a standard in the food/beverage industry, it is also now found in pharmaceuticals production, in air-conditioning equipment for some public buildings, and in electric power generation plants.

Ammonia is a highly toxic gas, and proper safety monitoring procedures and equipment must be in place at all times to avoid serious accidental injury or death. The OSHA PEL for ammonia is an eight-hour TWA of 50 ppm. The NIOSH REL and ACGIH® TLV® both cite an eight-hour TWA of 25 ppm, with a 15-minute STEL of 35 ppm. Besides its toxic properties, ammonia is also an explosively flammable gas, with a lower explosion limit (LEL) concentration of 16 percent volume.

On Sept. 21, 2001, an ammonia/ammonium nitrate explosion at a fertilizer plant in Toulouse, France, killed 30 and injured 2,500 other workers and nearby residents. According to the government's investigation, as horrific as the accident was, it could have been much worse if intervening buildings had not broken the force of the explosion, preventing the potential detonation of 20 more railroad tank cars full of anhydrous ammonia.

Carbon dioxide
Carbon dioxide is a toxic gas with a PEL time weighted average of 5,000 ppm. As awareness of CO₂ as a toxic hazard increases, regulations are beginning to require direct measurement as part of confined space monitoring programs. In Germany, confined space regulations now include direct measurement of CO₂ as a mandatory requirement.

Carbon dioxide usually is measured by means of a non-dispersive infrared sensor. NDIR sensors detect gas by measuring the absorbance of infrared light. Specific molecules (such as CO₂) absorb infrared radiation at precise frequencies. NDIR sensors include a source of infrared light that is filtered to provide a narrow range of frequencies. As the infrared radiation passes through the sensing chamber, only those frequencies that match the contaminant being measured are absorbed. The rest of the light is transmitted through the chamber without hindrance.

Dedicated substance-specific detectors exist for carbon dioxide, halogenated hydrocarbons such as Freons®, and other gases with good absorbence characteristics. NDIR sensors also can be used to measure methane and other hydrocarbon contaminants in oxygen deficient or hostile environments that would disable traditional combustible sensors. NDIR detectors are not affected by high concentrations of sulfides, silicones, or other substances that would quickly destroy a hot-bead catalytic LEL sensor.

Volatile Organic Compounds
VOCs are organic compounds characterized by their tendency to evaporate easily at room temperature. Familiar substances containing VOCs include solvents, paint thinner, nail polish remover, as well as the vapors associated with fuels such as gasoline, diesel, heating oil, kerosene, and jet fuel. The category also includes many specific toxic substances, such as benzene, butadiene, hexane, toluene, xylene, and many others.

In the past, because most VOCs are combustible at higher concentrations, the tendency has been to monitor them by means of the percent LEL combustible sensor included in most multi-sensor instruments. Unfortunately, we know today that many VOCs present a toxic hazard when present at much lower concentrations.

The 2002 edition of the American Conference of Governmental Industrial Hygienists' Threshold Limit Values® for Chemical Substances and Physical Agents includes a new exposure limit for diesel fuel. The new TLV® specifies an eight-hour time weighted average for total diesel hydrocarbons (vapor and aerosol) of 100 mg/m³. This is equivalent to approximately 15-ppm diesel vapor. This concentration limit is far too low for detection by means of a standard LEL range combustible sensor. Photoionization detectors capable of ppm range measurement and ppm range-capable combustible gas sensors are the most widely used techniques for TLV® range measurement of this hazard.

Photoionization detectors (PIDs) use high-energy ultraviolet (UV) light from a lamp housed within the detector to remove an electron from neutrally charged VOC molecules. This produces a flow of electrical current proportional to the concentration of contaminant. PIDs are non-specific, that is, they provide a "broad range" indication of all detectable molecules present in the atmosphere being monitored. PIDs can detect only certain gases and vapors. Nonvolatile liquids and solids, particulates, and many toxic gases and vapors cannot be detected at all.

Other types of direct reading sensors
There is an increasing number of direct reading miniaturized sensors capable of being installed in multi-sensor portable instruments. A partial list of the available sensors includes sulfur dioxide (SO₂), chlorine dioxide (ClO₂), hydrogen cyanide (HCN), phosphine (PH₃), phosgene (COCl₂), ozone (O₃), ethylene oxide (EtO), nitrogen dioxide (NO₂), nitric oxide (NO), and a number of other toxic hazards.

Choosing the Best Multi-Sensor Instrument
Make sure the instrument chosen for a specific application can accommodate the needed number and type of sensors. Most multi-sensor instruments include an oxygen sensor, a catalytic (hot bead) flammable/combustible gas sensor, and one or two electrochemical sensors for detecting specific toxic gases. Some gas detectors additionally include either an MOS sensor or a photoionization detector for broad range monitoring purposes. Some instruments also incorporate an infrared sensor for carbon dioxide or methane. The types of sensors selected should reflect the known and potential atmospheric hazards associated with environments to be monitored.

Calibration should be simple and straightforward. Given the requirement for documentation, the capability of instruments to log or automatically retain calibration information is highly desirable. Most datalogging multi-sensor instruments automatically update and store dates and other calibration information. This is one more reason to select a datalogging design.

Several manufacturers offer automatic calibration or "docking" stations that can automatically calibrate, recharge, and store instrument calibration records. The availability--and price point--of automatic calibration stations can have a significant effect on both the usability as well as cost of ownership of the instrument over the life of the product. Make sure to find out about the availability and cost of calibration and docking stations before rather than after you purchase the equipment.

The bottom line is that the features, capabilities, and dependability of multi-sensor gas detectors continue to increase, while the prices continue to drop. Customers have more options and better quality than ever before. There is an instrument with the features and pricing to fit every atmospheric monitoring program and budget. It's a great time to be a gas detecting instrument customer.

This article originally appeared in the May 2004 issue of Occupational Health & Safety.