The purpose of a VOC detector on a confined space monitor
Answered by Werner R. Haag, senior applications chemist, RAE Systems Inc., Sunnyvale, CA.
The conventional, portable, handheld 4-gas monitor containing sensors for H2S, CO, O2 and combustible gas (LEL) was the standard for many years. These sensors protected the worker from the most common and immediate atmospheric hazards, namely asphyxiation from lack of oxygen; explosion by accidental ignition of such compounds as natural gas, propane or gasoline; and poisoning by CO or H2S. These hazards occur most commonly in wet, confined spaces where microbiological growth can consume the oxygen, and release H2S and methane or from leaks of natural gas, etc.
However, many other toxic vapors can be present in the workplace, such as ammonia, chlorine, sulfur dioxide, nitrogen oxides, acids, isocyanates, acrylonitrile, benzene, butadiene, etc. Some of these compounds can be detected using electrochemical sensors. These sensors are small, lightweight, relatively low-cost and sometimes interchangeable. A multigas meter typically can hold two to five of these sensors, and will contain LEL and O2 sensors with space for two to three interchangeable electrochemical sensors. EC sensors are fairly selective for the target compound and are available in about 30-40 types, most commonly CO, H2,S NO, NO2, SO2, NH3, PH3, Cl2, ClO2, O3, HCl, HF, HCN and H2. However, several thousand new chemicals are synthesized by industry each year, and thus EC sensors are not available for the vast majority of compounds. Most of these compounds are organic compounds. If they are volatile, there is a risk of exposure by inhalation of the vapors. Thus, detectors are needed for volatile organic compounds, or VOCs.
By "poisoning" or "toxic," we usually mean short-term toxicity in other words, a compound that can disable or kill a worker within a few minutes. However, many of the compounds present in the workplace are not immediately dangerous to life or health and instead exhibit long-term toxicity.
Low concentrations of organic vapors often cannot be detected by smell, and are well below the detection limit of common LEL sensors, which usually have a lower limit of a few hundred ppm. Therefore, newer VOC detectors are used to measure concentrations of a broad variety of organic vapors in the low ppm concentration range.
The most common VOC sensor is a photoionization detector because of its small size and weight (similar to EC sensors) and its sensitivity, typically down to 0.1 ppm. Other VOC sensors, such as infrared sensors and flame ionization detectors tend to have a larger size less suitable for a compact meter, and metal oxide semiconductors tend to be affected by humidity and temperature. Most instruments use isobutylene gas to calibrate the PID and have onboard libraries of correction factors that allow quantitation of a large number of compounds.
Another advantage of VOC sensors is their ability to measure explosibility of heavy compounds such as diesel and jet fuel, which give very weak response on a conventional LEL sensor. In addition, PIDs can measure VOCs directly in vessels that have been flooded with inert gas to prevent explosion. Most LEL sensors require dilution with air for the sensor to function properly. The broadband detectability of PID sensors is a double-edged sword: They can measure many different compounds, but to achieve accurate readings one must be sure the target compound is the only one present.