Membrane deployment

Sampling rates for the case of total boundary layer-control can be expected to be nearly independent of temperature, since both the diffusion coefficients in air, and the kinematic viscosity of air are only weak functions of temperature (Shoeib and Harner, 2002). This leaves the air-flow velocity as the major factor that can be responsible for the seasonal differences among sampling rates observed by Ockenden et al. (1998). The absence of large R differences between indoor and outdoor exposures may be indicative of membrane-control, but it may also reflect the efficient damping of high flow velocities by the deployment devices used for SPMD air exposures (Ockenden et al., 2001). 

Louch, J. Allen, G. Erickson, C. Wilson, G. Schmedding, D. 2003, Interpreting results from field deployments of semipermeable membrane devices. Environ. Sci. Technol. 37 1202-1207. 

Diagrams of benthic sampiing equipment, (a) Pore water peepers used for collecting pore water in situ via moiecuiar diffusion across a 0.4- j,m polycarbonate membrane. These probes were 36 inches iong. Each of the six ports on the peeper held 6mL of equilibration fluid. The probes were deployed for 1 to 2.5 y to ensure equilibrium had been reached with the pore waters, (b) Benthic flux chamber. This chamber covered a bottom area of SSScm. Source From Aller, R. C., et al. (1998). Deep-Sea Research, 45, 133-165. 

In order to improve volume efficiency and reduce payload weight for earth-orbital remote-sensing applications, low-mass membrane-based synthetic aperture radar array concepts are being developed. One such system is an inflatable deployable SAR consisting of thin fabrics or membranes that are deployed for L-band operation with dual polarisation. The entire assembly is flexible before employment and is rolled up onto the spacecraft bus. The antenna comprises three membranes positioned vertically over one another the ground plane, the radiation patch, and the microstrip transmission line membranes74. 

A suggested means of correcting for the positive artifact is to deploy two samplers, the primary one containing a quartz filter and a gas trap and a second one containing a quartz filter behind a Teflon membrane filter (operated at the same face velocity as the primary hi-vol). Analytes found on the backup quartz filter are used as BF quantities in equations (1) and (2 )(Hart and Pankow, 1994 Turpin et al., 1994). However, this method is inconvenient and involves extra analytical work, and the double glass- or quartz-fiber technique (Equation 1-2) is judged to be satisfactory for most purposes (Hart and Pankow, 1994). 

The preceding discussions illustrate that membranes have shown great potential as an alternative for olefin/paraffin separation, yet the performance of current membranes is insufficient for commercial deployment of this technology. Advanced material development is highly desired to improve the membrane properties and reduce cost. Another possible approach involves hybrid membranes with zeolites or CMS incorporated in a continuous polymer phase. More discussion in this regard will be covered later in this chapter. 

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