Pressure modulation

Pneumatic controllers, which may include part of the sensing instrument, are supplied with compressed air at 1 bar gauge which is allowed to escape from an orifice controlled by a detector. The resulting pressure modulates about 0.4 bar and is used in a servo... 

Figure 4. Pressure Modulation Spectroscopy System (Reproduced from Goody, 1968).

The pressure module needs to be tested for accuracy at both low and high pressure. Typical values would be 0.5 and 7.5 psi with an acceptance criterion of 0.2psi for the low pressure and 20 and 80 psi with an acceptance criterion of 1.0 psi for the high pressure. 

Starting with the selective chopper radiometers (SCR) on Nimbus 4 and 5, the Department of Physics at the University of Oxford developed a series of instruments observing infrared emission from the atmosphere. A pressure modulated instrument flew on Nimbus 6 and the Stratospheric and Mesospheric Sounder (SAMS) flew on Nimbus 7 (Table 1). SAMS measured in limb viewing geometry the gases C02, H20, CO, N20, CR, and NO in the stratosphere and mesosphere. An improved version of ISAMS flew aboard the UARS and added channels for N02, N205, HN03, 03, and H20 (e.g. Barnet et al., 1992) (Table 1). 

Fig. 16.4. Three methods of obtaining Raman-based estimates of biofluid concentrations in vivo, a Confocal isolation of a subsurface volume occupied by a blood vessel, enabling direct measurement of a blood spectrum, b Difference measurement between tissue in two states, one with more blood in the sampling volume (in this case, due to pressure modulation by the subject [6]). Computing the difference removes the bulk tissue contributions to the spectral measurement and emphasizes the contribution from blood, c Statistical correlation approach of measuring many volunteers tissue in a region where sufficient blood is present (e.g., the forearm as shown here) and obtaining a correlated reference value from a blood sample drawn at the same time. Multivariate calibration is then used to find correlations between the reference value and the spectral data vector. Unlike the previous two methods, this does not intrinsically isolate the blood chemicals Raman signatures from those of the surrounding tissue volume...

Cost, always important, is difficult to quantify because the actual selling price of the same module design varies widely, depending on the application. Generally, high-pressure modules are more expensive than low-pressure or vacuum... 

A very rich scenario was, on the other hand, observed with Pt(110) if perturbed under conditions for which regular autonomous oscillations with well-defined frequencies existed (91-93). Typically, these autonomous oscillations were established at fixed external parameters and then one of the partial pressures was periodically modulated by use of a feedback-regulated gas inlet system with frequencies up to 0.5 s l and relative amplitudes around 1% (31, 33). Following the pioneering mathematical treatment of forced nonlinear oscillations by Kai and Tomita (SO), the results can be rationalized in terms of a dynamic phase diagram characterizing the response of the system as a function of the amplitude A and of the period of the pressure modulation Tcx with respect to that of the... 

Development of satellite-based pressure-modulated radiometer (PMR) techniques by the Oxford University [ 9 ] research team has now dramatically demonstrated the power of global maps of CHa and H2O for the interpretation and testing of two dimensional models. The first results of this work were reported by Jones and Pyle [ 9 ] in a comparison between satellite and in situ results using the Oxford two-dimensional model to interpret the results. A critically important extension of this work to include the relationships between methane and water vapor has been reported by Jones. The key concept in the work is that the sum [CH4] + [H2O] is a conserved quantity in the stratosphere so that global maps of methane can be used to deduce the water vapor composition of the atmosphere. This has critical implications for deducing free radical concentrations from satellite measurements. 

Fig. 5.4. (A) Elements of a high-pressure vessel. (B) Arrangement of the vessels in a high-pressure module.

In the temperature range of 373-573 K no transformation of the C1-C5 alkanes occurred on the H-ZSM 5 samples. The adsorption of the small methane molecules was very weak, while the large neopentane molecules could not enter the narrow zeolitic channels. Thus, the response to the applied pressure modulation was too small to record meaningful FR spectra with these molecules. For propane and isobutane the FR results suggest that diffusion in the micropores is the rate limiting process of transport over the entire temperature range. 

With respect to gas separation hollow fiber modules, the chemical compatibility and the mechanical stability of the fibers and potting are paramount. High pressure modules use shell-side feed while low pressure modules... 

In general the results in Figure 9 mean that the reaetion rates can he extracted from the coverage oscillations hy conscious tuning of the external influence, namely the frequency of the pressure modulation. The measurements at different frequencies will define the solution for the rates. 

The frequency of the pressure modulation was chosen to be optimal for the detection of gas 1. It means, that the oscillations of the coverage response to the modulated pressure are easily detected and analysed (curve 1 in Figure 11). An injection of another gas instead of gas 1 in the atmosphere is... 

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