Partial Pressure Profiles Research Articles

Electrochemical concentration cell ozone soundings at two sites during the Stratospheric Ozone Intercomparison Campaign

The Stratospheric Ozone Intercomparison Campaign (STOIC) was designed to compare proposed instruments for the Network for the Detection of Stratospheric Change (NDSC) with established measurement techniques. Ground‐based measurements were conducted at Table Mountain, California (34.4°N, 117.7°W), and rocket profiles were made at San Nicolas Island, California (33.3°N, 119.5°W). In an effort to estimate site‐to‐site differences during the intercomparison, daily soundings were made with balloon‐borne electrochemical concentration cell (ECC) ozonesondes at the two sites. Comparisons have been made of the mean values for the soundings at each site. The soundings showed small temperature differences between the two sites with the nighttime Table Mountain measurements up to 2°K cooler than the daytime San Nicolas Island temperatures. Day/night temperature corrections based on the emissivity and absorptivity of the thermistors in the balloon instruments can account for these apparent temperature differences. Ozone partial pressure profiles from the ECCs have been used to estimate atmospheric ozone variability during STOIC. Above 20 km, the lowest altitude for the STOIC comparisons, the average data sets from both sites showed standard deviations that were at or below the estimated 5–6% profile‐to‐profile repeatability for the ozonesondes. This indicates that atmospheric ozone variability was only of the order of a few percent above 20 km during the 2 weeks of the STOIC campaign. Above 20 km the ozone partial pressures over the Table Mountain site averaged 2.0% lower than over San Nicolas Island.

Influence of storage time on parameters of colour stability of beef

Direct measurements of the oxygen partial pressure profiles at the surface of the longissimus dorsi muscle of six bulls were obtained by means of a solid-state-probe after 1 min and 5 h exposure to air at 5° C. Changes in muscle colour were examined at the same time. The results show the profiles of oxygen partial pressure at the muscle surface after differing storage times (1, 3, 5, 7, 11 and 13 days at 5° C). Up to 5 days post mortem, increases in oxygen partial pressure with lengthening storage periods also led to higher percentages of oxymyoglobin, following exposure to air 1 min to 5 h. Further storage for up to 13 days decreased oxygenation, despite higher oxygen partial pressures. There was no evidence of rapid autoxidation to metmyoglobin during a 5- h period of exposure to air. Obvious connections between oxygenation and measurements of a-value were not found. The Lab-values increased dependent on the length of storage time.

Dimensional Instability of Doped Lanthanum Chromites in an Oxygen Pressure Gradient

The volume expansion of samples of La1−yCayCrO3−δ in reducing atmosphere was measured for various degrees of doping. Combining the experimental data with calculated profiles of oxygen partial pressure, pO2, through plates exposed to pO2 differences relevant for SOFC interconnectors, the overall expansion of the interconnector–plate, as well as the resulting stresses in the stack are estimated. The behaviour of Ca– and Sr–doped material is compared, and the optimal degree of doping, as judged from a mechanical point of view, is discussed.

Radial mixing diffusion of hydrogen in a packed-bed type of palladium membrane reactor

A previous study has shown that a tubular packed-bed type of palladium membrane reactor can considerably promote the catalytic dehydrogenation reaction of cyclohexane by selectively separating hydrogen produced in the course of reaction through palladium membrane. Effects of radial diffusion on the hydrogen separation from a low-flow-rate gas mixture containing hydrogen was examined under atmospheric pressure at 473 K using an annular packed-bed type of palladium membrane reactor. The axial profiles of hydrogen concentration (partial pressure) in the packed bed were experimentally measured and mathematically analyzed using two flow models, ideal and radial diffusion models. The ideal flow model could not explain the experimental results very well. On the other hand, the radial diffusion model could successfully simulate the profiles of hydrogen partial pressure formed along the packed bed of palladium membrane reactor. It became clear that there exists a gradient of hydrogen partial pressure in the radial direction of the packed bed due to its permeation through palladium membrane. In addition, Peclet numbers at low Reynolds numbers were determined and were explainable by the extended Wen and Fan equation.

A computer simulation of oxygen partial pressure and temperature profiles during hyperthermia.

Hyperthermia is a cancer therapy utilizing the difference in heat transfer characteristics between the tumor and normal tissue. In many cases, preferential heating of the tumor takes place because the perfusion rate is less in the tumor tissue relative to the normal tissue. This therapy has the advantage of fewer side effects during and after the treatment compared with other currently used cancer therapies. Hyperthermia can be used not only by itself and also in conjunction with other cancer therapies, such as chemo-therapy (Haranaka et al., 1987), x-irradiation (George et al., 1989; Fujiwara, et al., 1990), or both (Howard and Bleehen, 1988), to enhance tumor eradication or regression.

Embryonic Respiration and Oxygen Distribution in Foamy and Nonfoamy Egg Masses of the FrogLimnodynastes tasmaniensis

We measured embryonic oxygen consumption (V̇o2) and profiles of O2 partial pressure (Po2) in the egg masses of Limnodynastes tasmaniensis from two populations that produce either floating foam nests or submerged nonfoamy nests. The nests remained aerobic but became hypoxic during development in well-aerated water. As V̇o2 increased, anoxia in the central eggs was generally avoided by changes in nest structure. In the field, foamy nests floated at the surface in betteroxygenated water while nonfoamy nests were also supported by aquatic vegetation near the surface. Some embryos became anoxic and died when the egg masses were artificially placed on the bottom. Numerical models of O2, diffusion through egg masses confirmed the absence of convection in L. tasmaniensis nests but indicated that convection may be required in larger aquatic egg masses of other species. The models also demonstrated local depression of Po2 around individual eggs in the clutch and indicated that all O2 entering the perivitelline space is used by the embryo and none is released to eggs deeper in the mass.

A theoretical analysis of the effect of placental metabolism on fetal oxygenation under conditions of limited oxygen availability

The purpose of this theoretical paper is to examine the effects of placental metabolism on fetal oxygenation under conditions of limited oxygen availability. Features of the mathematical model used here include: (1) ordinary non-linear differential equations defining the oxygen partial pressure profiles in the maternal and fetal streams for a concurrent flow pattern; (2) the presence of maternal and fetal blood flow shunts; (3) consumption of oxygen by a metabolically active placenta; and (4) modification of the fetal input to the placenta by changing the rate of fetal oxygen consumption in response to changes in the rate of oxygen delivered to the fetus via the umbilical vein. Model parameters were chosen to be well within the range of values cited in the literature. Based on these calculations, we conclude that: (1) under normal conditions, approximately one-half of the fetal uterine-umbilical venous oxygen partial pressure difference can be attributed to placental oxygen consumption; (2) utilization of fetal oxygen to help maintain the metabolic activities of the placenta does not significantly impair fetal oxygenation under normal conditions; (3) consumption of oxygen by the placenta will have a significant detrimental effect on the rate of oxygen delivered to the fetus if oxygen availability is compromised; and (4) for the same rate of maternal oxygen delivered to the placenta, maternal hypoxemia has a significantly greater adverse effect on fetal oxygenation than does maternal anemia.

Cardiogenic oscillations in He and SF6 expirograms during airway and venous loading

Cardiogenic oscillations in the expired partial pressure profiles of two inert gases (He and SF6) were monitored in seven anesthetized paralyzed mechanically ventilated dogs. He and SF6 were administered either intravenously by a membrane oxygenator and partial arteriovenous bypass [venous loading (VL)] or by washin into lung gas [airway loading (AL)]. The single-breath expirograms obtained during constant-flow expiration after inspiration of test gas-free air displayed distinct and regular cardiogenic oscillations. The relative oscillation amplitude (ROA), calculated as oscillation amplitude divided by mixed expired-inspired partial pressure difference, was in the range of 1-8%. The ROA for both He and SF6 was approximately 4.2 times higher in VL than in AL, which indicated that among lung units that emptied sequentially in the cardiac cycle, the effects of alveolar ventilation-perfusion (VA/Q) inequality were more pronounced than those of alveolar ventilation-alveolar volume (VA/VA) inequality. In AL, He and SF6 oscillations were 180 degrees out of phase compared with CO2 and O2 oscillations and with He and SF6 oscillations in VL, which suggests that regions with low VA/VA had high VA/Q and very low Q/VA. The ROA was practically unaffected by breath holding in both AL and VL, which indicates that there was little diffusive or convective (cardiogenic) mixing between the lung units that were responsible for cardiogenic oscillations. The ROA was consistently higher for He than for SF6, and the He-to-SF6 ratio was independent of route of test gas loading, averaging 1.6 in both AL and VL. This result may be explained by laminar Taylor dispersion, whereby oscillations generated in peripheral lung regions are dissipated in inverse proportion to diffusion coefficient during transit through the proximal (larger) airways.

Methane steam reforming: II. Diffusional limitations and reactor simulation

AbstractGiven the intrinsic kinetics, the tortuosity factor of a Ni/MgAl2O4 catalyst was determined under reaction conditions by minimizing the sum of squares of residuals of the experimental and the simulated conversions. The parallel cross‐linked pore model with uncorrelated pore size distribution and orientation was used in the calculation of the effective diffusivities. A modified collocation method was used to obtain the partial pressure profiles of the reacting components in the catalyst pellet. The simulation of the experimental reactor during the optimization of the tortuosity factor also yielded the effectiveness factors of the reactions. The results of the simulation of an industrial steam reformer are also discussed.

Proximal HCO3- reabsorption and the determinants of tubular and capillary PCO2 in the rat.

Studies were carried out in Munich-Wistar rats to define the CO2 partial pressure (PCO2) profile in the surface tubules and capillaries of the kidney and to relate these measurements to proximal tubular HCO3- reabsorption, renal blood flow, and O2 consumption. In euvolemic rats, PCO2 in Bowman's space (BS) was 12.5 mmHg higher than in arterial blood, indicating CO2 addition to the arterial tree as it traverses the cortex. PCO2 further rose by 3.9 mmHg between the efferent arteriole (EA) and the peritubular capillaries (PC) (P less than 0.01) and by 4.9 mmHg between BS and the early proximal tubule (EP) (P less than 0.01). In studies with paired measurements, PCO2 in EP was 1.8 mmHg higher than in the adjacent PC (P less than 0.05). HCO3- reabsorption in EP (first 0.4-1.25 mm) was 579 pmol X min-1 X mm-1 (34.3 +/- 4.6% of the filtered load). By use of a model of facilitated diffusion of CO2 across the cell, the trans-epithelial PCO2 gradient in EP can be accounted for by the CO2 generated from HCO3- reabsorption, assuming an intracellular pH of 7.3. In the vascular compartment, roughly half the rise in PCO2 between the afferent arteriole (estimated to equal BS PCO2) and PC can be accounted for by metabolic CO2 production and half by titration of blood buffers by reabsorbed HCO3-.

Uncertainties propagation for combustion diagnostics using infrared band models

The development of matrix equations for an uncertainty propagations analysis of an emission-absorption (E/A) band-model technique is described. The E/A technique is used for the determination of local values of temperature and species concentration from measurements of radiance and transmittance on axisymmetric gas flows. The approach used depends on linearization of the original nonlinear system of equations comprising the solution to the radiative transfer problem for an emitting-absorbing source such as a turbojet or rocket engine exhaust plume. The resultant linear transformation matrix provides a means for determination of the variance-covariance matrix for inverted temperature and partial pressure, based on the experimentally determined variance-covariance matrix of the measured radiance and transmittances. The diagonal elements of the propagated variance-covariance matrix yield the square of the standard deviation of the temperature and partial pressure. Although the equations are developed for uncertainty propagations analysis of the E/A band-model technique, the results are a general property of linear transformations. They are thus applicable to any situation in which the dependence of analysis on measurements can be approximated by a linear relationship. Inversion of radiance and transmittance lateral profiles obtained from a liquid-propellant rocket engine to radial profiles of temperature and partial pressure with the associated uncertainties propagations is included as an illustrative example.

Activation energy in the mammalian brain slice as determined by oxygen micro-electrode measurements.

The activation energies were calculated from oxygen consumption rates obtained from oxygen partial pressure (PO2) profiles in olfactory cortical slices of guinea pig in vitro, assuming that the Krogh coefficient varied linearly over the range of investigated temperatures (19-39 degrees C).

Observations of a ‘notch’ in the stratospheric ozone layer

A persistent minimum was observed in the ozone partial pressure profile at 38 mb over Yorkton, Saskatchewan (51°N, 102°W), on each of 27 ozonesonde flights between August 7 and 31, 1976. This notch in the profile has an average vertical extent of 3 km and an average decrease in the partial pressure or mixing ratio of 35% below the adjacent maxima. The persistence and spatial extent of this phenomenon were investigated. An ozone‐aerosol flight at Laramie, Wyoming (41°N, 106°W), on August 6 also showed a minimum in both the ozone and aerosol profiles. It is demonstrated that this is a frequent phenomenon of the stratosphere which occurs at mid‐latitudes in late summer. The phenomenon occurs within a broader layer of very light winds situated between the westerlies below and easterlies above. From the evidence available it is not possible to choose between differential quasi‐horizontal advection and a photochemical sink as the mechanism responsible for producing the layer.

Theory of gas exchange in the avian parabronchus

To understand the distribution of oxygen and carbon dioxide in the avian lung, a theoretical treatment of gas exchange in the parabronchus of the avian lung is described. The model is modified after Zeuthen (1942). In addition to bulk flor through the parabronchial lumen, diffusion through the air spaces of both the parabronchial lumen and air capillaries is treated. The relationship of P O 2 and P CO 2 within the blood capillaries, air capillaries, and parabronchial lumen to parabronchial blood flow and ventilation is graphically shown. The results indicate that the variations of P O 2 and P CO 2 along an air capillary are less than one torr under resting conditions. Removal of diffusion resistance within the air space of the air capillaries increases calculated parabronchial gas exchange by less than 0.1% at rest. At high or resting ventilation rates the partial pressure profile along the parabronchial lumen calculated considering bulk flow only agrees well with the profile calculated considering bulk flow and axial diffusion, but as the ventilation rate decreases there is increasingly large disagreement. Forward diffusion of O 2 toward the parabronchus reduces pre-parabronchial P O 2 and backward diffusion of CO 2 from the parabronchus increases P CO 2 . Neglecting diffusion within the air spaces of both the lumen and the air capillaries increases calculated parabronchial gas exchange by less than 2% (CO 2) or 6% (O 2) at rest.

A stochastic model for the transport of oxygen to brain tissue.

Material balances around a small, but finite volume element have formed the basis for previous mathematical models describing the transport of oxygen in the brain microcirculation. Seeking a model which would be both simple and versatile, a stochastic model was proposed based on the assumption that oxygenation of the brain can be described quantitatively by simulating the activity of only one erythrocyte and the oxygen molecules surrounding it. Compared with existing deterministic models, the capillary space-average oxygen partial pressure profiles were in close agreement. Tissue tensions were decidedly different.

Significance of the Bohr and Haldane effects in the pulmonary capillary

A mathematical model was developed to assess the significance of the Bohr and Haldane shifts on determination of partial pressure profiles in the pulmonary capillary. Digital computer calculations showed that these shifts have a marked effect on the partial pressure profiles for both oxygen and carbon dioxide during both hypoxia and normoxia. An effective red cell diffusion capacity for CO 2 (θ 2) was calculated as 1.0 ml·ml −1·min −mm Hg −1. The half-time (t 1 2 ) of the “intracellular Bohr-shift” was 25 msec. These values are used to calculate carbon dioxide profiles. Carbon dioxide is not completely equilibrated by the time that blood has traveled through the pulmonary capillary. There is a marked indirect interaction between CO 2 and O 2 diffusion mediated through the Bohr and Haldane effects.

A model of fluctuating alveolar gas exchange during the respiratory cycle

The effect of fluctuating alveolar gas concentrations and pulsatile blood flow on gas exchange between pulmonary capillary blood and alveolar gas is analyzed by means of a digital computer model. The model makes provision especially for: 1) variable partial pressure profiles in pulmonary capillary blood, 2) solubility of gases in pulmonary tissue, 3) pulsatile blood flow, and 4) the complex interrelationships of CO 2, O 2 and H + in the blood. It is shown that fluctuating alveolar concentrations pulsatile blood flow and presence of lung tissue between alveolar gas and capillary blood all play an important role in the determination of instantaneous values of: 1) alveolar gas tensions, 2) oxygen uptake and carbon dioxide output, 3) gas exchange ratio of alveolar gas, 4) gas exchange ratio of pulmonary capillary blood and 5) gas tension profiles in the pulmonary blood. All of these values show large fluctuations during the normal respiratory cycle. These fluctuations are heavily dependent upon the pulmonary blood flow characteristics. The calculation of gas tension profiles in the pulmonary capillary is also discussed. The inclusion of the Bohr effect is shown to be very important. The variables are assessed for both normoxia and hypoxia.

Simulating oxygen transport in the microcirculation by Monte Carlo methods

Oxygen transport in the human cerebral cortex has been simulated previously by considering a single straight capillary and a concentric tissue cylinder as a representa tive sample (the Krogh tissue cylinder). The same geo metric model has been assumed for this study because of its probable accuracy and to allow comparison with previous calculations. Because of the nonlinear equilibrium relationship be tween dissolved and hemoglobin-bound oxygen (the oxy gen dissociation curve), a differential mass balance yields a nonlinear partial differential equation describing oxy gen transport in the capillary. In the tissue a constant consumption rate is assumed and a linear partial differ ential equation results. The tissue equation and the capillary equation are solved simultaneously, to account for capillary-tissue interaction, thus giving axial and radial oxygen partial pressure profiles for the total system. The Monte Carlo method has been employed to solve the resulting set of equations on a digital computer. This technique involves a numerical scheme wherein a process is simulated directly by a random walk phenomenon (Markov process). A particular advantage of this method is that solutions may be obtained for one point in space independently of all others. Therefore, an analysis of the transient response of the "lethal corner" is possible with out solving for the entire mesh. Computation time with the Monte Carlo method was considerably less than that with standard deterministic numerical calculation tech niques. Two models were considered. In the first model the oxygen dissociation curve was linearized, thus giving a linear partial differential equation describing oxygen transport in the capillary. The second model considered the nonlinear nature of the oxygen dissociation curve, and a nonlinear partial differential equation was ob tained. However, because of difficulties with the Monte Carlo method, it was necessary to solve the equations in a quasi-nonlinear manner. For both the linear and the quasi-nonlinear models, solutions were obtained for the steady state with normal conditions and for dynamic cases. Perturbations of ar terial-oxygen partial pressure and blood flow rate were forced, and the behavior of the system was determined. The pure diffusional model checked with previous de terministic calculations demonstrating very rapid re sponse with time constants in the order of 2 seconds. However, the shapes of the intracapillary and tissue oxy gen tension profiles differed somewhat and no allow ances were made for autoregulatory phenomena such as varying flow rates.

Intraparticle diffusion and conduction in porous catalysts—II: Complex reactions

The problem of intraparticle diffusion and conduction is considered for complex reactions taking place in porous catalytic spheres. It is assumed that Knudsen diffusion prevails and that heat generated in the reaction is conducted through the porous structure to the catalyst surface. Equations are written for an arbitrary number of reactions and criteria are developed to determine the number of conservation equations for complex systems. A particular numerical example is worked both approximately and exactly by the same technique as used in the previous paper; the term exactly meaning that partial pressure and temperature profiles may be calculated numerically as accurately as desired. The approximate method should be satisfactory for almost all industrial work.

Intraparticle diffusion and conduction in porous catalysts—I: Single reactions

Calculations are made for a single porous catalytic sphere in which a single chemical reaction is carried out. Transport within the sphere is by Knudsen diffusion. Heat is carried by molecular flow and by conduction through the solid structure. Approximate and rigorous calculations are made for partial pressure and temperature profiles and an approximate solution based on a linearization of the heat generation function is shown to be adequate for a number of cases. Effectiveness factors are computed based on the external surface conditions.