Gas measurement serves as a technical aid and an assessment of the concentration is only possible with a gas measurement device. To determine the hazard potential of a gas it is necessary to measure its concentration and to consider the duration of exposure and other parameters such as the type of work being performed.
Natural, ambient air is chemically a gas mixture that consists of 78% nitrogen, 21% oxygen 0.03% carbon dioxide as well as argon, helium and other rare gases in trace centrations. In addition there is water vapor, e. g. humidity. If the concentrations of the components change, or a foreign gas is added, we no longer have natural air. When these changes occur, the potential for adverse health effects exist.
The spectrum of other so-called air components can be extremely broad. It can range from the pleasant fragrance of a good perfume to the over powering stench of hydrogen sulfide. Likewise, the hazard of each "air pollutant" varies considerably. The type of substance,its concentration and duration of occurrence, as well as probable synergistic effects with certain gas compounds must all be considered. In addition, there are many air pollutants which cannot be perceived by human senses because they are colorless and odorless (e. g. carbon monoxide).
If the composition of the natural air changes in any way, it should be tested, to determine the substance which caused this change. Even substances with distinctive odors cannot be reliably assessed with the aid of the olfactory nerve in the nose. The olfactory nerve can become desensitized after a certain period of time or repeated exposure, making it impossible to smell even immediately dangerous concentrations. After a few hours we do not even perceive the pleasant fragrance of our own perfume and high concentrations of hydrogen sulfide escape from the sense of smell even after a very short while.
Subjectively, one persons sense of smell may be more sensitive to certain air pollutants than others. In many cases substances are noticed in very low concentrations which, even after a long-term exposure do not necessarily cause adverse health effects. In general the sense of smell is sufficient in determining the presence of air pollutants, but the need exists for an objective gas analysis method. Gas measurement serves as a technical aid and an assessment of the concentration is only possible with a gas measurement device. To determine the hazard potential of a gas it is necessary to measure its concentration and to consider the duration of exposure and other parameters such as the type of work being performed.
An important prerequisite to determining the potential of any gaseous air pollutants is the determination of the concentration with a suitable gas measurement device. The kind of device to be used depends on which gases have to be measured and how often. Much to the dismay of both the user and the manufacturer, there is no universal instrument which measures all gases or vapors. The variety of substances is too wide for a single technique to measure all possible air pollutants. The more chemically complex a substance is, the more complex the gas measurement technique.
It may be that more than one measurement device or measurement method may be employed, each based on different operational principles. The instrumentation industry offers various devices for this purpose which can be used, individually or in combination on the measurement task:
- flame ionization detectors
- photo ionization detectors
- gas chromatographs
- infrared spectrometers
- UV-VIS photometers
- warning devices for explosion hazards
- Draeger-Tubes
- Draeger Chip-Measurement-System
- laboratory analysis in conjunction with sampling tubes or gas wash bottles (impinger)
- mass spectrometers
- substance selective instruments with e. g. electrochemical sensors
The choice of which monitor or measurement method to use depends upon the objective.
The user must evaluate the situation and determine which substances to measure, how often, etc. Each of the above mentioned devices and methods have advantages and limitations. There is no universal monitor for all possible scenarios. For the selection of a suitable measuring device and to support the user in solving measurement problems, Draeger Safety AG & Co. KGaA offers competent know-how and technical assistance. The customer / employer should carefully train the user / employee on the use of their measurement device. Any use of the measurement device without receiving prior comprehensive training can be permitted by the customer / employer to the user / employees, at the customer / employees own risk.
Photo and flame ionization detectors are distinguished by short response periods but they do not offer substance selectivity. Gas chromatographs, infrared and UV-VIS photometers are very versatile but on the other hand they are comparatively expensive and require a specialist to calibrate the devices and interpret the readings correctly. Warning devices for explosion hazards like the Drager X-am 5000 are equipped with catalytical sensors to determine explosion levels of combustible gases and vapors. They are not designed to measure lower concentrations and thus are not suitable for this task.
Draeger-Tubes with direct reading colorimetric indication have many applications. Approximately 500 different substances can be measured with Drager-Tubes. Limited selectivity and the fact that Drager-Tubes are usually capable of only being used a once may present a disadvantage. If repeated measurements of the same substance are to be performed daily, a measurement device like the Draeger Pac 7000 CO with its electrochemical sensor for the measurement of carbon monoxide is more economic than Draeger-Tubes.
When complex mixtures (e. g. solvent mixtures), are present, usually only a laboratory analysis will suffice. The prerequisite is that the contaminated air is trapped in a sorbent sampling tube like silica gel or activated charcoal.
After collecting the sample, analysis is performed in the laboratory with gas chromatographic methods, or sometimes by the combination of gas chromatography / mass spectroscopy. Laboratory procedures of this kind offer particularly high selectivity, but the analysis devices are very expensive, requiring high maintenance costs and operation by specialists.
Regardless of the gas measurement device or what analysis procedure is used, it is essential that the contaminant of interest be identifiable and measurable. Apart from a few exceptions in process monitoring, it is very unlikely that concentrations of other substances can be determined by subtracting the concentration of the gas which can be identified. For example, if the oxygen concentration is below the 17 or 19.5 Vol. % limit, it cannot be said which substance has displaced the oxygen without further investigation. In the case of very high carbon dioxide concentrations there is the danger of suffocation; likewise if there is a leak in a gas pipeline the presence of methane poses an explosion hazard. Other contaminants present in the ppm range would not influence the oxygen measurement enough to alert anyone to a potential hazard. Since many of the occupational exposure limits are in the range of 1 ppm, the measurement by difference technique is typically inadequate.
Before each measurement an assessment of the situation should be made as to what contaminants are in question, at what locations, at what times, and so forth, according to established safety procedures. Monitoring according to established safety guidelines will help ensure safety in the workplace and effective use of monitoring equipment.
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I'm Dan Hines, with Cross Instrumentation Company. Our focus is Industrial Sales of Process Measurement and Control equipment. We are specialists in hard to solve flow, level, analytical and safety applications.
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