Instrumentation concepts accuracy

Basic spectroscopic measurements involve the instrumental concepts of bandpass and resolution, signal-to-noise ratio, dynamic range, stray light, wavelength accuracy and precision, and photometric accuracy and precision. These concepts were described in Chapter 1. 

In earlier experiments the effect of branching on the second virial coefficient was not seriously considered because the accuracy of measurements were not sufficient at that time. With the refinements of modern instruments a much higher precision has now been achieved. Thus A2 can also now be measured with good accuracy and compared with theoretical expectations. The second virial coefficient results from the total volume exclusion of two macromolecules in contact [3,81]. Furthermore, this total excluded volume of a macromolecule can be expressed in terms of the excluded volume of the individual monomeric units. In the limit of good solvent behavior this concept leads to the expression [6,27] as shown in Eq. (24) ... 

The operating principle of the CSIRO (Australian Commonwealth Scientific and Industrial Research Organization) King probe (Particle Measuring Systems Inc., Boulder, Colorado) is similar in concept to that of the Johnson-Williams probe. The King probe measures the amount of power necessary to maintain a heated wire at a constant temperature, whereas the Johnson-Williams probe measures the change in resistance due to cooling of the wire by water evaporation. The probe consists of a heated coil of wire that is maintained at a constant temperature. The amount of excess power required to maintain the wire at this temperature when it is impacted by water droplets is measured and is proportional to the cloud liquid water content. The nominal response time of the instrument is 0.05 s, and it has an accuracy of 20%. This instrument uses less power than a Johnson-Williams probe, an important consideration in aircraft applications. 

Phase Dynamics utilizes a unique, patented microwave concept to diagnose and measure molecular transformation process parameters with high sensitivity and accuracy (Phase Dynamics 1992). While originally developed for fluid measurements, the instrumentation is adaptable to most pumpable process lines and to some batch applications. The technique has been utilized for compositional analyses of true solutions as well as complex solid-liquid systems such as colloids and emulsions. Monitoring of molecular transitions which occur in cooking processes, hydrogenation, gelatinization and hydrolysis can also be monitored. 

Particle size and distributions can be determined by a number of different methods. The technique described here is light scattering. Different measurement methods produce different results which can be correlated experimentally. The absence of distribution standards for light scattering particle sizing instruments precludes any determination of size accuracy. This is further complicated by particles of non-spherical shape which makes the concept of size very difficult to define. However, for particulate materials encountered in most industrial processes, the assumption that particles are spheres produces quite useful results that are repeatable and relate to important parameters of many processes. 

Accuracy — The closeness of a measurement regarding its accepted or - true value. Accuracy is also a concept to characterize a measuring instrument that gives responses very close to a true value. 

In many countries undertaking the most technically demanding analyses, it is expected that the laboratories will be registered under quality assurance schemes. Accuracy and repeatability should be covered by quality assurance requirements, but resolution and detection limits are critically dependent on technique and instrumentation. Details on the general subjects of quality assurance/quality control, quantification of uncertainty in analytical measurements, accreditation of laboratories, and on the general concepts and strategy for ensuring that analytical chemical measurements are comparable - in one word traceability - can be found elsewhere in the chemical literature (26,32-36). 

An understanding of some of the basic vocabulary and principles employed in archaeological chemistry is essential to understanding this field of study. In the following paragraphs a brief discussion of matter and energy includes these relevant concepts. This is followed by a consideration of measurement issues and the very small quantities of elements, isotopes, and molecules we often have to measure in the lab. Finally the meaning of accuracy, precision, and sensitivity provide perspective on the results obtained from scientific instruments. 

The distinctions established by IUPAC are clear-cut. Thus, the speed of titrant addition is always constant in an automatic titrator, whereas it Is adjusted by a feedback system according to the nearnesa of the equivalence point in an automated titrator. However, some workers [11,12] acknowledge the accuracy of these definitions but consider them too stringent. Very often, the term automatic Is used to refer to systems with and without feedback Indistinctly. In any case, whenever the concept automatic process Is referred to In this book, It will be meant In its widest connotation, namely that Involving partial or complete elimination of human Intervention not related to Instrumentation. 

In Chapter 1, the rales of nomenclature are reviewed— units of physical quantities, abbreviations, conversion between SI and British Units— and the various national and international standards bureaus are mentioned. Chapter 2 introduces significant figures and concepts of accuracy, precision and error analysis. Experimental planning is discussed in some detail in Chapter 3. This subject is enormous and we try to distil the essential elements to be able to use the techniques. Chapters 4 and 5 cover many aspects of measuring pressure and temperature. The industrial context is often cited to provide the student with a picture of the importance of these measurements and some of the issues with making adequate measurements. Flow measurement instrumentation is the subject of Chapter 6. A detailed list of the pros and cons of most commercial... 

The manufacture of primitive stone tools and fire required a qualitative appreciation for the most common measures of mass, time, number, and length. The concept of time has been appreciated for millennia. In comparative terms it is qualified by longer and shorter, sooner and later, more or less. Quantitatively, it has been measured in seconds, hours, days, lunar months, and years. Calendars have existed for well over 6000 yr and clocks—instruments to measure time intervals of less than a day—were common as long as 6000 yr ago. Chronometers are devices that have higher accuracy and laboratory models have a precision of 0.01 s. 

Both the concepts of repeatability and reproducibility are related to accuracy repeatability is defined as the closeness of agreement between the results of successive measurements of the same measurand carried out subject to all of the following conditions the same measurement procedure, the same observer, the measuring instrument used under the same conditions, the same location, and repetition over a short period of time. The definition of reproducibility is the closeness of agreement between the results of measurements of the same measurand, where the measurements are carried out under changed conditions such as different principles or methods of measurement, different observers, different measuring instruments, different locations, different conditions of use, or different periods of time. ... 

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