Dead space of the apparatus

Improved alveolar ventilation may be partly compromised by an increase in the dynamic dead space (VDdyn), derived from the physiologic dead space (VDphys) plus the dead space of the apparatus (VDap). Whereas the physiologic dead space is influenced by the tidal volume, the dead space of the apparatus is a fixed consequence of the internal volume of the interface. Differences in flow pattern and pressure waveform associated with the machine and mode of ventilation, also affect the dead space of the apparatus. Saatci et al. (36) noted that during spontaneous breathing, a face mask increased VDdyn from 32% to 42% of tidal volume (VT) above VDp ys. Positive pressure during the expiratory phase reduced VDdyn close to VDphys, while inspiratory pressure support without positive end-expiratory pressure decreased VDdyn from 42% to 39% of VT, i.e., VDdyn remained higher than VDphys. When the exhalation port was placed close to the nasal bridge, VDdyn was lower than VDp ys as a consequence of a beneficial flow path that decreased VDdyn (from 42% to 28% of VT), in the presence of an expiratory positive pressure. 

Figure 1 shows a plot of pressure increase against time for an experiment at 189.2° with 14.9 mm. of formic acid decomposing on a 31.3-mg. high-vacuum film, and values calculated from Equations (1), (2), and (3) with appropriate constants. The exact dependence of pressure increase on the time is somewhat obscured at the end of the reaction by diffusion of formic acid vapor, partly present as dimer in the cooler portion of the apparatus, into the reaction flask from the dead space of the spoon gauge. 

The volume calibration is generally obtained by gas expansions from a volume Vj, calibrated before the building of the apparatus and taken as the primary standart, to the unknown volume which is the introduction volume or the dead space of the sample cell. The ratio Vj/V2 has to be settled at an optimal value, as will be shown here. 

Based on the above relation, for most practical operations, this reactor behaves much hke the plug-flow reactor, where ReF = pdtL/pOT, ReM = pdiNjp. Here, 0T is the average residence time. The relationship between power consumption and mixing time reveals the similarity of this vessel to the double helical-ribbon mixer. The fraction of dead space in this apparatus appears to be small. The relationships described above are valid for the fluids in the viscosity range of 50-5000 poises. 

Prior to gas adsorption, it is common practice to pretreat or condition the catalyst surface. Frequently high temperatures, about 500°C, are employed. Therefore, a sample furnace is an essential part of the apparatus. After pretreatment, evacuation at pretreatment temperature, and cooling to adsorption temperature, it is necessary to determine the dead space volume (the volume in the sample tube which the adsorbate would occupy provided no adsorption occurred). Helium is most often used for this purpose. After evacuation, the adsorbate is added to the manifold and its pressure noted. Subsequently, it is expanded into the sample chamber, and adsorption, if any, commences. The pressure is monitored until no further variation with time is noted. The pressure over the sample can then be increased via a gas burette and readings again taken until equilibrium is established. When there is no longer gas uptake by the sample with increasing pressure, the desirable portion of the isotherm is complete, and the total volume adsorbed, expressed at S.T.P. per gram, can be determined. This procedure must be repeated for the support. The volume adsorbed at any pressure is subtracted from the volume adsorbed on the supported catalyst at the same pressure. Further details can be found in the books by Hayward and Trapnell (1964) and by Anderson (1968). 

Since the amount adsorbed represents the differerKe between the amount admitted to the dead space and the amount remaining at equilibrium, it can only be evaluated with confidence when the quantities are of unlike magnitude. To achieve this, the apparatus is designed so as to minimize dead space volume. 

A schematic of the apparatus is shown in Figure 2.12. A nitrogen/helium mixture is used for a single point determination and multiple points can be obtained using several such mixtures or premixing two streams of gas. Calculation is essentially the same as for the static method but no dead space correction is required. 

The resulting chart record contains a point by point representation of each dose cycle. The apparatus has usually been programmed to deliver four or five consecutive doses, and then wait for equilibration. Only the equilibration points are read from the chart. The points on the chart record represent a pressure-volume relation which has not been corrected for the amount of vapor not adsorbed by the sample, the so-called dead space correction/ ... 

The procedure followed in studying such reactions is to fill a flask with known amounts of the reactants (or with a reaction mixture of known composition) and then follow the total pressure of the system as a function of time. Figure IV. 1 illustrates such an apparatus. There is always the difficulty with such equipment of making corrections for the dead space, ... 

Fig. IV. 1. Typical apparatus for studying gas reactions. Note A stirring device operated magnetically may be used inside the reaction vessel. Also, the connecting tubing may sometimes be wound with nichrome heating elements to prevent condensation of less volatile vapors and reduce the effective volume of the dead space. The use of too small or too long capillary tubing is restricted by the difficulty of evacuating the reaction vessel, which increases with longer or smaller capillary leads.

The apparatus used to determine separation factors for an adsorbed monolayer was similar to that used by Cunningham, Chapin, and Johnston 4,5) but was modified to minimize the dead space and ensure good thermal equilibrium between the feed gas and the adsorbent. The inner copper cylindrical chamber was filled with 67 grams of the same y-alumina used in the isosteric heat of adsorption experiments for the establishment of the distribution function. The y-alumina was, however, free of any paramagnetic material to permit ortho-para separation factor measurements. 

Mixed-exhaled air. This technique involves the collection of the entire volume of exhaled air. It corresponds to a mixture of the alveolar air with air from the dead space. The collection apparatus may also contribute to the dead space. Total dead space should be considered and the concentration adjusted, either by subtraction, or by regression against some other technique unaffected by the dead space. Timing of the brea collection is important here since the concentration of the air in the dead space may equal that of the air in the workroom if the sample is taken during exposure, or it may equal zero if taken after the end of exposure. 

---