Directly measured

When direct measurement is not available, the molecular weight can be estimated by two different means ... 

This calculational method is nevertheless only of limited interest and concerns only certain pure organic products for which direct measurement of the NHVj is difficult owing to a variety of reasons such as cost and availability. 

Volatility can be characterized either indirectly, by measurement of the flash point (the temperature to which the oil must be heated for inflammation of its vapor to become possible) or by direct measurement, following the Noack method. 

The gradients may be caloulated from surface fluid densities, or may be directly measured by downhole pressure measurements using the repeat formation testing tool (RFT). The interfaces predicted can be used to confirm wireline measurements of fluid contact. 

The basic data gathering methods are direct methods which allow visual inspection or at least direct measurement of properties, and indirect methods whereby we infer reservoir parameters from a number of measurements taken in a borehole. The main techniques available within these categories are summarised in the following table ... 

To gain an understanding of the composition of the reservoir rock, inter-reservoir seals and the reservoir pore system it is desirable to obtain an undisturbed and continuous reservoir core sample. Cores are also used to establish physical rock properties by direct measurements in a laboratory. They allow description of the depositional environment, sedimentary features and the diagenetic history of the sequence. 

For direct measurement from core samples, the samples are mounted in a holder and gas is flowed through the core. The pressure drop across the core and the flowrate are measured. Providing the gas viscosity (ji) and sample dimensions are known the permeability can be calculated using the Darcy equation shown below. 

A further advantage is the Selenium halflife of 120 days, which is 60% more when compared to iridium and a factor of approx. 4 when compared to Ytterbium. These differences turn out to be an important economical aspect when comparing the different sources, as they are a direct measure of the useful life of sources. The short halflife and the very high costs for Ytterbium sources have been the main factors for the rather low importance of Ytterbium in the full range of gamma radiography. 

Projection radiography has long been used for pipe inspection and corrosion monitoring. In this traditional tangential wall thickness estimation the distance of border lines of the projected wall shadows of a pipe onto the film is a direct measure for the wall thickness. This method is not considered here, newer developments can be found in / /. 

Film pressure is often measured directly by means of a film balance. The principle of the method involves the direct measurement of the horizontal force on a float separating the film from clean solvent surface. The film balance has been considerably refined since the crude model used by Langmuir and in many... 

Fig. V-12. Variation of the integral capacity of the double layer with potential for 1 N sodium sulfate , from differential capacity measurements 0, from the electrocapillary curves O, from direct measurements. (From Ref. 113.)...

While evidence for hydration forces date back to early work on clays [1], the understanding of these solvent-induced forces was revolutionized by Horn and Israelachvili using the modem surface force apparatus. Here, for the first time, one had a direct measurement of the oscillatory forces between crossed mica cylinders immersed in a solvent, octamethylcyclotetrasiloxane (OMCTS) [67]. 

A direct measurement of surface tension is sometimes possible from the work of cleaving a crystal. Mica, in particular, has such a well-defined cleavage plane that it can be split into large sheets of fractional millimeter thickness. Orowan... 

Yaminsky and Yaminskaya [114] have used a Wilhelmy plate to directly measure the interfacial tension (and hence infer the contact angle) for a surfactant solution on... 

This chapter and the two that follow are introduced at this time to illustrate some of the many extensive areas in which there are important applications of surface chemistry. Friction and lubrication as topics properly deserve mention in a textbook on surface chemistiy, partly because these subjects do involve surfaces directly and partly because many aspects of lubrication depend on the properties of surface films. The subject of adhesion is treated briefly in this chapter mainly because it, too, depends greatly on the behavior of surface films at a solid interface and also because friction and adhesion have some interrelations. Studies of the interaction between two solid surfaces, with or without an intervening liquid phase, have been stimulated in recent years by the development of equipment capable of the direct measurement of the forces between macroscopic bodies. 

K. Kurihara, Direct Measurement of Surface Forces of Supramolecular Systems Structures and Interactions, in Microchemistry, H. Masuhara et al., Elsevier Science, 1994. 

Photoelectron spectroscopy provides a direct measure of the filled density of states of a solid. The kinetic energy distribution of the electrons that are emitted via the photoelectric effect when a sample is exposed to a monocluomatic ultraviolet (UV) or x-ray beam yields a photoelectron spectrum. Photoelectron spectroscopy not only provides the atomic composition, but also infonnation conceming the chemical enviromnent of the atoms in the near-surface region. Thus, it is probably the most popular and usefiil surface analysis teclmique. There are a number of fonus of photoelectron spectroscopy in conuuon use. 

Femtosecond lasers represent the state-of-the-art in laser teclmology. These lasers can have pulse widths of the order of 100 fm s. This is the same time scale as many processes that occur on surfaces, such as desorption or diffusion. Thus, femtosecond lasers can be used to directly measure surface dynamics tlirough teclmiques such as two-photon photoemission [85]. Femtochemistry occurs when the laser imparts energy over an extremely short time period so as to directly induce a surface chemical reaction [86]. 

Note that in this special case, the heat absorbed directly measures a state fiinction. One still has to consider how this constant-volume heat is measured, perhaps by an electric heater , but then is this not really work Conventionally, however, if work is restricted to pressure-volume work, any remaining contribution to the energy transfers can be called heat . 

In general, the phonon density of states g(cn), doi is a complicated fimction which can be directly measured from experiments, or can be computed from the results from computer simulations of a crystal. The explicit analytic expression of g(oi) for the Debye model is a consequence of the two assumptions that were made above for the frequency and velocity of the elastic waves. An even simpler assumption about g(oi) leads to the Einstein model, which first showed how quantum effects lead to deviations from the classical equipartition result as seen experimentally. In the Einstein model, one assumes that only one level at frequency oig is appreciably populated by phonons so that g(oi) = 5(oi-cog) and, for each of the Einstein modes. is... 

This internal potential, cj), is not directly measurable it is temied the Galvani potential, and is the target of most of tire modellmg discussed below. Clearly we have = A j x +... 

Barker J R 1984 Direct measurement of energy transfer in rotating large molecules in the electronic ground state J. Chem. Phys. 88 11... 

The temi action spectroscopy refers to those teclmiques that do not directly measure die absorption, but rather the consequence of photoabsorption. That is, there is some measurable change associated with the absorption process. There are several well known examples, such as photoionization spectroscopy [47], multi-photon ionization spectroscopy [48], photoacoustic spectroscopy [49], photoelectron spectroscopy [, 51], vibrational predissociation spectroscopy [ ] and optothemial spectroscopy [53, M]. These teclmiques have all been applied to vibrational spectroscopy, but only the last one will be discussed here. 

Paul J B, Collier C P, Saykally R J, Scherer J J and O Keefe A 1997 Direct measurement of water cluster concentrations by infrared cavity ringdown laser absorption spectroscopy J. Phys. Chem. A 101 5211-14... 

One of the principal experimental advantages of this method of detemiining relaxation times is that it may be carried out with standard EPR spectrometers using CW-detected EPR lines [9, 10]. A discussion of more direct measurements of and T2 using time-resolved EPR techniques is deferred to a later point (see sections bl. 15.4 and Bl. 15.6.3(b)). 

Closs G L and Redwine O D 1985 Direct measurements of rate differences among nuclear spin sublevels in reactions of biradicals J. Am. Chem. Soc. 107 6131-3... 

Another specialized application of EM image contrast is mass measurement. Using the elastic dark-field image in the STEM or the inelastic image in the EETEM, a direct measurement of the scattering mass can be performed. Eor reviews on this teclmique see [60.61]. 

Ducker W A, Senden T J and Pashley R M 1991 Direct measurement of colloidal forces using an atomic force microscope Nature 353 239... 

Jarvis S P, Durig U, Lantz M A, Yamada H and Tokumoto H 1998 Feedback stabilized force-sensors a gateway to the direct measurement of interaction potentials Appl. Phys. A 66 S211... 

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