Oxygen total flux measurements

The applicability of oxygen membrane materials depends essentially on their oxygen permeation fluxes. The dependence of the flux density j02 on the oxygen pressure difference across the membrane of thickness L can be calculated by the using of the measured total conductivity o and calculated values Oi for the ion conductivity. 

The multitude of transport coefficients collected can thus be divided into self-diffusion types (total or partial conductivities and mobilities obtained from equilibrium electrical measurements, ambipolar or self-diffusion data from steady state flux measurements through membranes), tracer-diffusivities, and chemical diffusivities from transient measurements. All but the last are fairly easily interrelated through definitions, the Nemst-Einstein relation, and the correlation factor. However, we need to look more closely at the chemical diffusion coefficient. We will do this next by a specific example, namely within the framework of oxygen ion and electron transport that we have restricted ourselves to at this stage. 

By measuring rates of ventilation and the degree of oxygen undersaturation in deeper waters, an estimate of the rates of oxygen utilization (OUR) may be made and integrated to yield the total amount of oxygen consumed beneath the euphotic zone each year (Jenkins, 1982). From this number a flux of POC and PON may be calculated. Estimates of export production based on OURs are reasonably in line with the amount of new production that could be supported from measured fluxes of N03 into surface waters (Jenkins, 1988). 

Since there are no standards for flux determinations, as there are for chemical concentration measurements in water or sediments, evaluating their accuracy is difficult. One is forced to rely on comparison of different anal5dical approaches and modeling of nutrient and oxygen distributions to achieve a consensus. Of the different methods used to determine the organic carbon export presented in the following section, the first two (sediment traps and thorium isotope disequilibria) are used to evaluate the POC flux only. They must be combined with estimates of the DOC flux to achieve the total. The second two methods, molecular oxygen and carbon isotope mass balance, determine the sum of DOC and POC export. 

The determination of in-situ fluxes of nitrate is comparatively limited. Although there are electrodes for the determination of microconcentration profiles in form of biosensors (Larsen et al. 1996), they are not yet snitable to be nsed on lander systems in the deep sea. The measurement of total nitrate flnxes by benthic chambers in continental slope sediments off California is shown in Figure 3.24 (Jahnke and Christiansen 1989). At this site, nitrate flnxes are directed into the sediment indicating strong denitrification snpported by very low oxygen concentrations in the overlying bottom water. 

The SiC content of silicon carbide products is now usually determined by measuring the carbon contents. The total carbon content is determined by combustion of the sample in a stream of oxygen at 1050°C in the presence of lead borate. The CO2 produced is absorbed in Ba(C104)2 solution and determined by coulometry [236,237]. An alternative technique is to oxidize the SiC with oxygen in a high-frequency induction furnace containing a flux metal, and to detect the CO2 produced by IR absorption. 

A hood attached to a sampling duct is used to capture heat and chemical compounds that are released during the test. In the sampling duct measurements are made for gas temperature, concentrations of chemical compounds released in the fire and oxygen, light obscuration by smoke, total flow of the mixture of air and chemical compounds and heat flux values at various locations in the room. Two parameters are used for ranking the products [47,48,58] ... 

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