Measurement fundamental units

Measurements usually consist of a unit and a number expressing the quantity of that unit. Unfortunately, many different units may be used to express the same physical measurement. For example, the mass of a sample weighing 1.5 g also may be expressed as 0.0033 lb or 0.053 oz. For consistency, and to avoid confusion, scientists use a common set of fundamental units, several of which are listed in Table 2.1. These units are called SI units after the Systeme International d Unites. Other measurements are defined using these fundamental SI units. For example, we measure the quantity of heat produced during a chemical reaction in joules, (J), where... 

For scientific purposes the convenient unit to employ for measuring reasonably large volumes of liquids is the cubic decimetre (dm3), or, for smaller volumes, the cubic centimetre (cm3). For many years the fundamental unit employed was the litre, based upon the volume occupied by one kilogram of water at 4 °C (the temperature of maximum density of water) the relationship between the litre... 

Fryer, P.J. 210,229 Fuller, E. N. 584,655 Fully developed flow 61.681 Fundamental units, choice of 12 Further reading, flow and pressure measurement (Chapter 6) 272... 

The fundamental quantity for interferometry is the source s visibility function. The spatial coherence properties of the source is connected with the two-dimensional Fourier transform of the spatial intensity distribution on the ce-setial sphere by virtue of the van Cittert - Zemike theorem. The measured fringe contrast is given by the source s visibility at a spatial frequency B/X, measured in units line pairs per radian. The temporal coherence properties is determined by the spectral distribution of the detected radiation. The measured fringe contrast therefore also depends on the spectral properties of the source and the instrument. 

The fundamental unit in chemical measurement is the mole - amount of substance. A mole is the amount of a substance that contains as many atoms, molecules, ions or other elementary units as the number of atoms in 0.012 kg of carbon 12 (12C). It is the only dimensionless SI unit. In practical terms, it is almost impossible to isolate a mole of pure substance. Substances with a purity of better than 99.9% are rare one exception is silver, which can be obtained with a purity of 99.9995% which is referred to as five nines silver . Another problem is that it is not always possible to isolate all of the analyte from the sample matrix, and the performance of the chemical measurement may be matrix-dependent - a given response to a certain amount of a chemical in isolation may be different from the response to the same amount of the chemical when other chemicals are present. If it is possible to isolate quantitatively all of the analyte of interest from the accompanying sample matrix, then a pure chemical substance may be used for calibration. The extent to which the analyte can be recovered from the sample matrix will have been determined as part of the method validation process (see Chapter 4, Section 4.6.3). 

According to the modern convention, measurable quantities are expressed in SI (System Internationale) units and replace the centimetre-gram-second (cgs) system. In this system, the unit of length is a metre (m, the unit of mass is kilogram (kg) and the unit of time is second (s). All the other units are derived from these fundamental units. The unit of thermal energy, calorie, is replaced by joule (1 J = 107 erg) to rationalize the definition of thermal energy. Thus, Planck s constant... 

Unit of energy. The fundamental unit of energy in modern thermochemical measurements is the electrical joule, which is derived from standards of resistance and electromotive force maintained at the various national standardizing laboratories. 

In the section on Thermochemistry in the International Critical Tables (see Bichowsky1), the values were recorded in joules, in the hope that thermochemists might come to use this fundamental unit in their calculations and writings. But the attempt to break away from the calorie as a unit in thermochemical and thermodynamical calculations proved to be unpopular and apparently hopeless of accomplishment. In order to satisfy the popular demand for the calorie as a unit in calculations and tabulations, and at the same time depart as little as possible from the fundamental unit of energy, the joule, in terms of which all accurate thermochemical measurements are actually made, we have used in this book a defined calorie, that is, one which has no actual relation whatever, except incidentally and historically, to the heat capacity of water. 

SI units of measurement, used by scientists around the world, derive their name from the French Systeme International d Unites. Fundamental units (base units) from which all others are derived are defined in Table 1-1. Standards of length, mass, and time are the meter (m). kilogram (kg), and second (s), respectively. Temperature is measured in kelvins (K), amount of substance in moles (mol), and electric current in amperes (A). 

Table 1-2 lists some quantities that are defined in terms of the fundamental quantities. For example, force is measured in newtons (N), pressure is measured in pascals (Pa), and energy is measured in joules (J), each of which can be expressed in terms of the more fundamental units of length, time, and mass. 

Scientific measurements range from fantastically large to incredibly small numbers, and units that are appropriate for one measurement may be entirely inappropriate for another. To avoid the creation of many different sets of units, it is common practice to vary the size of a fundamental unit by attaching a suitable prefix to it. Table 4-1 shows common metric prefixes and the multiples they indicate for any given unit of measurement. Thus a l g gs ater is 1000 meters, a microgram is 10-6 ram ana a nanosecond is Q-9... 

Under an international agreement concluded in 1960, scientists throughout the world now use the International System of Units for measurement, abbreviated SI for the French Systeme Internationale d Unites. Based on the metric system, which is used in all industrialized countries of the world except the United States, the SI system has seven fundamental units (Table 1.3). These seven fundamental units, along with others derived from them, suffice for all scientific measurements. We ll look at three of the most common units in this chapter—those for mass, length, and temperature—and will discuss others as the need arises in later chapters. 

Epoxy resins and curing agents must have a relatively low viscosity so that formulation compounding can be accomplished easily and without a great deal of energy or degradation of the components. Viscosity is defined as the resistance of a liquid material to flow. It is usually measured in fundamental units of poise (P) or centipoise (cP). Table 3.2 shows a relationship between various common fluids and their viscosity as measured in centipoise. 

The second, symbol s, is the SI unit of time, defined as the duration of 9,192,631,770 cycles of radiation associated with a specified transition of the cesium atom. The meter, symbol m, is the fundamental unit of length, defined as the distance light travels in a vacuum during 1/299,792,458 of a second. The kilogram, symbol kg, is the mass of a platinum/ iridium cylinder kept at the International Bureau of Weights and Measures at Sevres, France. The unit of temperature is the kelvin, symbol K, equal to 1/273.16 of the thermodynamic temperature of the triple point of water. A more detailed discussion of tern-perature, the characteristic dimension of thermodynamics, is given in Sec. 1.4. The measure of the amount of substance is the mole, symbol mol, defined as the amount of substance represented by as many elementary entities (e.g., molecules)... 

We have already dealt with a number of expressions in which frequency appears, and we shall encounter many more such expressions. In some instances it is preferable to use the more fundamental unit of radians/second, for which we employ the symbol co or ft, while in other cases it is more convenient to use the measured unit of cycles/second, or hertz, designated v (or occasionally F). Also,... 

The Metric System originated in France during the French Revolution, and its use has since been required or legalized in most civilized countries. The fundamental unit cf the system is the meter, which is approximately equal to the ten-millionth part of the distance from the equator to the north pole. This distance was ascertained by actual measurement of an arc of a meridian passing through Barcelona in Spain and Dunkirk in France. The legal equivalent of the meter in the United States is 39.37 inches. 

Absolute Viscosity A term used to indicate viscosity measured by a standard method, with the results traceable to fundamental units. Absolute viscosities are distinguished from relative measurements made with instruments that measure viscous drag in a fluid, without known and/or uniform applied shear rates. See also Viscosity. 

The units for an absorbed dose are discussed in refs. 167 and 168. The fundamental unit, in terms of the energy absorbed per gram, is the rad, which is 100 erg. g and is equivalent to 6.24. eV g or 6.24x 10 xp eV. cm , where p is the density of the material. This unit is required when making dosimetric measurements. For X- or y-radiation, the exposure dose is used. The unit is one roentgen... 

G values have been predominantly used during the last two decades. Recently, however, authors have returned to M/N since it is the more fundamental unit . Also, G depends upon the measurement of the total energy absorbed which is difficult or impossible (with y-radiation) to determine in a gaseous system. When the ionisation energy of the compound investigated is lower than the energy of the photon absorbed, the product yields can be expressed unambiguously in terms of the number of molecules formed per ion pair (M/N). 

The gaseous potassium chloride molecule has a measured dipole moment of 10.3 D, which indicates that it is a very polar molecule. The separation between the nuclei in this molecule is 2.67 A. What would the dipole moment of a KCl molecule be if there were opposite charges of one fundamental unit (1.60 X 10 C) at the nuclei ... 

Only when a laboratory has implemented a valid quality system is it reasonable to assume that the data generated in that laboratory are acceptable and fit for purpose. But this, in turn, implies that measurements be traceable, i.e., that an unbroken chain of calibrations can be set up to link the actual measurement process to the relevant fundamental units, so as to unequivocally demonstrate that no unexpected factors have impaired the final results. 

---