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What Other Analytical Methods Are Available For Gas Monitoring?
Q. What is
gas dispersion? Q. What are the effects of temperature during a gas leak? In addition to density and air flow, temperature can also affect the dispersion of leaking gas. Most important, it can change the way a gas might normally behave. If the temperature of the air at the ceiling is much hotter than the rest of the room air, the ceiling air will have a lighter density because hot air rises. This "thermal barrier" may slow down the diffusion of the leaking gas enough to delay or prevent detection at the sensor. Also, many lighter-than-air gases are stored as compressed liquids. When these gases escape into the atmosphere, their density may at first be heavier than air until they are warmed by the ambient temperature and become lighter than air. Q. What are the effects of dilution during a gas leak? If the air volume moving through each of three rooms, for example, is roughly the same, a hazardous concentration in one room would be diluted to one-third of its true value because of the air movement from the other two rooms.
Q. What are the concerns when monitoring outdoors? When installing sensors in outdoor applications,
careful attention must be given to prevailing wind conditions. It may be
necessary to monitor a single hazard (such as a storage tank) using
several sensors so that an accidental leak can be detected regardless of
wind direction at the time of the leak. OPEN PATH monitoring
systems have been developed to address the issue of large area coverage. A molecule's IR spectrum is constant and dependent on
its chemical structure and to a lesser degree temperature and pressure.
Each compound possesses a characteristic IR or UV fingerprint that can
be identified by spectral analysis. The substances' IR and UV spectra
typically contain regions in which there are no absorbing species, and
these regions can provide a signal intensity reference.
Fourier Transform IR (FTIR) spectrometry, a unique
mathematical spectral analysis, is ideal for quantitative analysis of
complex gas mixtures, for concentrations from percent (%) to
sub-parts-per-million (ppm) levels. FTIR spectrometers have larger optical throughputs
than dispersive spectrometers and transmit more IR energy to the
detector. They also image all IR frequencies on the detector
simultaneously (the multiplex, or Fellgett, advantage), distributing
detector noise and limiting noise on the IR spectrometer across the
entire spectrum. FTIR spectrometers also maintain significantly better
frequency precision and accuracy. Q. What is Nondispersive IR (NDIR) Spectrometry? Filter-based nondispersive techniques allow for the manufacturing of low-cost, robust sensors that offer the advantages of IR absorption methods and are well suited for the production environment. Although single-wavelength detection can be done, dual-wavelength sensors allow for real-time correction of the sensor baseline. A low-cost, filter-based NDIR sensor can then be
implemented for production scale process monitoring and control.
Response times for this type of sensor are typically on the order of
tens of milliseconds. Noise levels are in the microtorr range, with
exact numbers depending on the specific design. Multiple filters can be
used, allowing simultaneous analysis of several gases by a single
sensor. Q. What is Differential Optical Absorption Spectroscopy (DOAS) DOAS is a method to determine concentrations of trace gases by measuring their specific narrow band absorption structures in the IR, UV and visible spectral regions. A typical DOAS instrument consists of a continuous light source, i.e., a Xe-arc lamp, and an optical setup to send and receive the light through the atmosphere. It is also possible to use the sun or scattered sun-light as light source. The typical length of the light path is the atmosphere ranges from several meters to many kilometers. After its path through the atmosphere, the light is spectrally analyzed and the gas concentrations are derived. In the atmosphere, the light of the lamp (I0(λ)), undergoes extinction processes by air molecules (σRay(λ)) and aerosols (σMie(λ)), turbulence (T(λ)), and absorption by many trace gases with the concentrations Ci and absorption cross sections σi (λ). The light intensity from a path of length L can be described by: I(l) = I0(l)´exp((Sσi(l)´Ci´L+ σRay(l)+ σMie(l))´T(l) (1) The task of any spectroscopic method in the
atmosphere is to separate these effects in order to derive the
concentrations of trace gases. DOAS overcomes this problem by separating
the trace gas absorption cross sections into low and high frequency
parts by specific numerical filtering methods.
Q. What dictates the selection of a sensor? Point sensors used in area monitoring applications are typically diffusion in design. This means that the sensor is not part of an active sampling system that draws the sample to the sensor, but instead relies on diffusion and convection to obtain the sample. That is, the gas will mix with ambient air and diffuse through the sensor's flame arrestor without requiring a pump or aspirator. Selecting the appropriate sensor for a given application is dictated by the gas or gases to be measured, the background gases present, and the conditions around the sensor's location. Flammable hazards are measured in the 0-100% lower flammable limit (LFL) or lower explosive limit (LEL) range. Toxic hazards are measured in the low parts per million (ppm) range. Several sensor technologies are available in diffusion designs: catalytic and IR sensors for LFL range monitoring of flammable gases, and electrochemical and solid-state sensors for ppm monitoring. Q. What are Catalytic Sensors? Catalytic sensors are appropriate for detecting flammable gases and vapors in the LFL range. When a flammable gas enters the sensor, it reacts with a catalyst-coated electrical coil. The resulting resistance change offsets the balance of a Wheatstone bridge circuit. The output signal is proportional to the concentration of flammable gas. Catalytic sensors have numerous strengths, including low cost, long life, and simplicity of design. But they can be affected by "catalytic poisons" such as silicones, plasticizers, and sulfur compounds that coat or corrode the sensor's catalyst. Infrared sensors are optical devices sensitive to IR radiation. The IR energy detected by the sensor is translated to an electrical signal that is further processed by the particular detection system. Point IR gas detectors analyze the presence of a gas by exposing it to IR radiation and measuring the radiation atenuation caused by the gas absorption . The detector comprises an IR light source, an IR sensor and several beam reflectors along the gas cell that provide the optical path for the light beam. The gas enters the cell by natural difussion. The point IR gas detector has proven useful in monitoring flammable gases in the LFL range. It's chief advantage over the catalytic type is that it is not subject to catalytic poisons. Because it is an optical device, however, care must be taken to prevent fouling of the optics.
Q. What are Electrochemical Sensors? Electrochemical sensors are excellent for detecting low parts-per-million concentrations of a select gas. They contain an electrolyte that reacts with a specific gas, producing an output signal that is proportional to the amount of gas present. Electrochemical sensors exist for gases such as chlorine, carbon monoxide, hydrogen sulfide, and hydrogen, but cannot be used to measure hydrocarbons. The number of gases that can be detected using this technology is relatively small, but is increasing from year to year. Q. What are Solid-State Sensors? Solid-state sensors, typically based on a tin oxide semiconductor, respond to gases by changing resistance. They are used to measure numerous gases in the parts per million range. The devices are relatively inexpensive and have a long operating life. They have a low selectivity, however, and background gases can create inaccurate readings. In addition, the sensor's nonlinear output signal makes calibration more complicated. Q. What are the requirements for all point detection methods? All point detection methods require that gas be drawn from the monitored area, sampled over a period of time, introduced into the detection system for gas analysis by chemical, physical and electro-optical methods.
Q. What are the advantages of point-source detectors?
Q. What are the disadvantages of point-source detectors?
Q. What is an open path gas detection system? Open Path Gas Detection System is an optical device that analyzes a gas concentration along a line-of -sight between a light source and a detector over a distance of up to several hundred meters. The system comprises two units that are located at a predetermined distance or collocated when a retro-reflector is used. The radiation intensity emitted by the light source is detected by the detector and is related to the concentration of the chemical substance in the optical path measured by the system. Such systems are using either IR or UV spectral sensors in the detection unit and detect gases at very low concentrations in air, PPM (Part Per Million) or LEL levels. Q. What is 'spectral fingerprint'? Spectral fingerprint is the specific absorption/emission spectrum of each chemical substance, and is typical to the molecular structure of the substance. Chemical vapors with similar molecular structure (like the hydrocarbons) will exhibit similar (but not identical !) spectral fingerprints, thus enabling their spectral identification as a family.
Q. What are the advantages of Open Path Detectors?
Q. What are the disadvantages of Open Path Detectors?
Q. What is the philosophy of gas detection on Offshore installations? Generally, the philosophy of gas detection on offshore installations is that a gas cloud at stoichiometric concentration of 2 LEL with a diameter of 5 meters poses a real explosion threat; hence, it is critically important to detect this cloud as early as possible, and before it grows in size or migrates to hot (ignition source) areas. The present Oil and Gas companies's philosophy is that all leaks result in a plume (cloud), which is concentrated at source and which becomes more diluted as it disperses in the air-flows in the area. Open Path (beam) detects without regard to the beam's proximity to the source of leak, provided the whole width of the plume is within the beam, the gas monitoring system will see a similar response of average gas concentration in the cloud.
Q. What is the Gas Concentration Definition? Gas concentration is determined as an average value over the entire optical path, either in PPM x m or LEL x m. The gas concentration is obtained by dividing the above value by the distance L(m) between the light source and the detector.
Q. What are typical open path system applications?
What Other Analytical Methods Are Available for Gas Monitoring? Gas chromatography (GC) and mass spectrometry (MS) are often used for gas analysis. But these methods fall short when used for real-time process monitoring and control. Historically, gas chromatographs produce useful data only a small fraction of their working time because of the sample collection requirements and in some cases the sample preparation. Once a sample has been passed to the chromatograph, separation times have been relatively long (minutes to hours), depending on sample complexity. Recent advances in high-speed GC have reduced separation times from hours to minutes, and in some cases from minutes to seconds. But even with these improvements, you'll still encounter limitations when working with this method. In most applications, the component interacts with a detector, such as flame ionization (FID), electrolytic conductivity (ELCD), electron capture (ECD), thermal conductivity, mass spectrometry (MS) and numerous others. Because molecular interaction is required, the instruments need frequent calibration and/or internal standards to maintain quantitative accuracy. Mass spectrometers are often used after gas chromatographs, which adds the aforementioned problems-slow response time, varying calibration, and sample incompatibility. When used alone in the semiconductor industry, the spectrometers are known as residual gas analyzers (RGAs). In stand-alone applications, RGAs have quick response times and are sensitive detectors for most gas species. However, they require low pressures and lack quantitative accuracy because of constant instrument variations and pressure fluctuations. Also, in semiconductor processing, these sensors are in contact with corrosive and reactive materials, which affect performance and shorten the instrument's operating life. |
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