Permeation tubes for

Typical materials available in permeation tubes for operation at 30°C are listed in Table 4.6 along with average rates per centimeter length of tube and the K factor. As the length of the tube increases the permeation rate increases in reasonable proportion. The data in Table 4.6 are for tubes 0.25 inches o.d. and a wall thickness of 0.062 inches. A typical example might be an SO2 tube 5 cm in length at a rate of 1350 ng/min. If the dilution gas flow across the tube is 1.35 /min the concentration would be... 

The gas analysis laboratory of the National Research Center for Certified Reference Materials (NRCCRM) is engaged in gas measurement at the state level. It is responsible for the uniformity of gas composition in China and for maintaining consistency and equivalence with international gas measurements. Our main tasks include research, development and maintenance of national primary reference materials research and establishment of precise reference methods for gas analysis to provide certified reference gas and working reference gas as well as permeation tubes for users to undertake the verification of gas analyzers and technical advisory services. The focus of our work is the development of research on gas analysis and measurement. 

Spiral-wound modules consist of several flat membranes separated by turbulence-promoting mesh separators and formed into a Swiss roll (Figure 16.18). The edges of the membranes are sealed to each other and to a central perforated tube. This produces a cylindrical module which can be installed within a pressure tube. The process feed enters at one end of the pressure tube and encounters a number of narrow, parallel feed channels formed between adjacent sheets of membrane. Permeate spirals roward the perforated central tube for collection. A standard size spiral-wound module has a diameter of about 0.1m, a length of about 0.9 m and contains about 5 m2 of membrane area. Up to six such modules may be installed in series in a single pressure tube. These modules make better use of space than tubular or flat sheet types, but they are rather prone to fouling and difficult to clean. 

Preservative availability may be appreciably reduced by interaction with packaging materials. Examples include the permeation of phenolic preservatives into the rubber wads and teats of multi-dose injection or eye-drop containers and by their interaction with flexible nylon tubes for creams. Quaternary ammonium preservative levels in formulations have been significantly reduced by adsorption onto the surfaces of plastic and glass containers. Volatile preservatives such as chloroform are so readily lost by the routine opening and closing of containers that their usefulness is somewhat restricted to preservation of medicines in sealed, impervious containers during storage, with quite short use lives once opened. 

Permeation tubes are very popular devices for generating stemdard vapor concentrations. The permeation tube contains a volatile liquid sealed in an inert permeable membrane, usually Teflon or a fluorinated copolymer of ethylene and propylene, through which it diffuses at a fixed and controlled rate. The driving force for the process is the dif ce in partial... 

Nitrogen dioxide permeation tubes were obtained from Metro-nics, Inc. These tubes were factory calibrated for use at 30.0°C. The permeation tubes were thermostated in a water bath and air passed over at a known flow rate (of at least 150 cc/min). Gas samples were drawn off at 80 cc/min. 

A clever means of dynamic generation of standards at the part-per-million level involves permeation through a polymer. In 1966 O Keeffe and Ortman (34) described this technique primarily for air pollution standards. A condensable gas or vapor is sealed as a liquid in a Teflon tube under its saturation vapor pressure as shown in Figure 4.14. After an initial equilibration period the vapor permeates through the tube wall at a constant rate. This rate is determined by weight loss over a period of time. Temperature must be controlled to within .0.1°C to maintain 1% accuracy. In use the tube is thermostatted in a chamber that permits a diluent gas to fully flush the chamber. The concentration is then determined by the same equation used for diffusion tubes. However, since the rate is generally much less in permeation tubes it is usually reported in ng/min. 

TABLE 4.6 PERMEATION RATES FOR CHEMICALS IN PERMEATION TUBES... 

Permeation tubes are not refilled, have a limited life and cannot be turned off. However their life can be prolonged during periods of non-use by storing them in a refrigerator to reduce the permeation rate. Not many solutes are practical for use in permeation tubes. However, when the technique can be used it is generally preferred as a means of standard preparation. 

SO2, NOx, and total hydrocarbons. The mass spectrometric gas analysis is on a wet basis, as water vapor is not condensed out of the gas, while the analyzers at the sample port measure a gas stream dried using a permeation tube and refrigeration-type dryers in series. In addition to the measurements described above, surface temperature measurements of the boiler skin are made to estimate radiation losses, using the skin temperature, the room temperature and tabulated heat loss factors based on the temperature difference. Particulate mass emission rate and carbon content are measured for heat and mass balance purposes. At present, material deposited within the boiler during a test is collected but not factored into the heat or mass balances, because this deposition is considered to be negligible. Data taken are used to examine the heat balance for the 20-hp system. 

Figure 4.15 shows the cross section of the spiral wound module.24 The spiral construction starts with two sheets of membrane placed back to back with a nylon tricot mesh spacer material in between. This tricot spacer provides the permeate channel for the membranes. These sheets of membrane and spacer are glued on 3 sides so that the permeate can only exit the spacer on one side. This set of membranes and spacer is called a "leaf." Leaves are then placed together with a low density polypropelene mesh spacer to provide the feed/reject channel for the membranes. The thickness of the mesh feed spacer can be adjusted from 28 mils to 34 mils to accommodate higher solids influent water (thicker feed spacers are more forgiving with respect to fouling with suspended solids than thinner spacers—see Chapter 4.4.2.3). The entire collection of leaves and mesh feed spacers are then wrapped around a perforated permeate collection tube so that the open side of the leaf is toward the perforated permeate tube (see Figure 4.16). Note that an 8-inch diameter membrane module has about 16 leaves, and each leaf is about 50 inches in length.

Figure 4.22 shows the flow characteristics for standard ATDs and the iLEC ATD. The reduced diameter of internal couplers and vessel adapters for the standard ATDs accounts for more than 70 percent of the permeate-tube pressure drop in some systems.26 The interlocking iLEC ATD design eliminates these restrictions, imposing less permeate backpressure, resulting in lower operating pressure requirements. 

Nitrogen Dioxide The line selected for NOa measurement lies in the 1600 cm-1 region. A commercial permeation tube is used for the calibration source for NOB. The permeation rate is determined by weight loss and also checked by N0/03 titration with NO then serving as the calibration standard. The response time is 3 sec and the MDC is 25 pptv. 

A permeation tube is a device for preparing gas standards having a known concentration of some trace constituent of interest. For example, one... 

For this reason reproducible permeation rates require the chamber with the permeation tube, as well as the entering gas, to be perfectly temperature-controlled. 

Of all the proportioning principles already known from the production of calibration gas from permanent gas, only the permeation method can be used because the diffusion process is not bound to the gaseous state and even for the dosage of permanent gases the permeation tubes contain two-phase mixtures. This method, as well as the mixed gas cylinders, are limited to the trace domain because the tubes only contain insignificant amounts. 

For monitoring the hydrazine gas, disposable hydrazine permeation tubes were used. Figure 3.15 shows the measurement system assembled to determine the LOD of the sensor for hydrazine in air. The disposable-permeation tube was heated to 80°C by a Kin Tek C0395 Certification Oven. At that temperature, the emission rate of the hydrazine is 2,750 ng min. That emission rate corresponds to a concentration of 3.84 ppm in a carrier gas flowing at 500 seem. Since the emission rate decreases by about 50% for every 10°C decrease of the oven temperature, a hydrazine concentration of OOppb is obtained when the oven is at room temperature. 

CI2 Permeation tube No suitable technique for continuous measurements... 

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