SI units and their use

Dimensi nless measu rements - some quantities can be expressed. as diitiensip flesajatiosjDr-logarithmV(e.g.  

When describing a measurement, you normally state both a number and a unit (e.g. the length is 1.85 metres ). The number expresses the ratio of the measured quantity to a fixed standard, while the unit identifies that standard measure or dimension. Clearly, a single unified system of units is essential for efficient communication of such data within the scientific community. The Systeme International d Unites (SI) is the internationally ratified form of the metre-kilogram-second system of measurement and represents the accepted scientific convention for measurements of physical quantities. 

Another important reason for adopting consistent units is to simplify complex calculations where you may be dealing with several measured quantities (see p. 260). Although the rules of the SI are complex and the scale of the base units is sometimes inconvenient, to gain the full benefits of the system you should observe its conventions strictly. 

Multiple-1)refix Syrhb TVIultiple-Prefix Symbol 

Measij H Name of unit Symbofi r Definition in Bf base units - Ss - in derivedi - - 

Dimensionless measurements - some quantities can be expressed as dimensionless ratios or logarithms (e.g. absorbance and pH), and in these cases you do not need to use a qualifying unit. 

Multiple Prefix Symbol Multiple Prefix Symbol 

Measured quantity Name of unit Symbol Definition in base units Alternative in derived units 

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Making and recording measurements SI units and, their use Scientific method and design of experiments ... 

SI units and their symbols have distinctive type styles. Items given in italic type are variables, quantity symbols, superscripts and subscripts if they represent variables, quantities, or running numbers. Items given in roman type are unit symbols, superscripts, and subscripts that are descriptive. The typeface used in the surrounding text of the document does not change these rules. 

SI Units and Recommendations for the Use of Their Multiples and of Certain Other Units (ISO 1000-1981), American National Standards Institute, 1430 Broadway, New York, 10018, 1981. 

ISO 1000 1992, SI units and recommendations for the use of their multiples and certain other units relative to the air. 

The following table is an alphabetical listing of some common laboratory tests and their reference ranges for adults as measured in plasma or serum (unless otherwise indicated). Reference values differ among laboratories, so readers should refer to the published reference ranges used in each institution. For some tests, both SI units and conventional units are reported. 

Science cannot be performed without an accurate system of measurement, which is globally standardized and compulsory. Units and standards of measurement are agreed upon and harmonized on an international basis by the Bureau International des Poids et Mesures in Sevres, France, and by the International Organization for Standardization in Geneva, Switzerland. The units and standards are then laid down in national laws. Nearly all countries have accepted the Systeme International d Unites (SI units) as their system of measurement. This also applies to countries that had been accustomed to use British units like Australia, Canada, South Africa and the United States. In Britain, SI units are official from January 2010. The valid standards are available from the competent bureaus, for example Bureau International des Poids et Mesures, www.bip.fr National Measurement Institute (Australia), www.measurement.gov.au National Institute of Standards and Technology (NIST, USA), www.physics.nist.gov/ Pubs/SP811/... 

The earlier literature references and some of the older text books, naturally use the older units. Even now many practicing scientists have not adopted the SI unit as their working unit. It is therefore necessary to know of the older units and be able to interconvert with SI units. 

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). 

National standards are usually kept at the national institutes responsible for their development and maintenance and are used only for linking up secondary standards to them, which then are made available as transfer standards. In most countries this responsibility has been entrusted by law to the national metrology institutes (NMIs). In the field of chemical measurements for which traceability to the SI units and the development of national measurement standards have been the focal point of interest for only a few years, decentralized national responsibilities are now developing in such a way that high-level national chemistry institutes are entrusted in part with the task of maintaining national standards by agreements with NMIs. In this way the national reference level can be established on a broad basis, and this is underway now in many countries. 

However, general acceptance of SI units makes the use of atomic units in publications prohibitive. This does not entail a problem if the results are to be presented for observable quantities. A more complicated situation arises with the presentation of data such as e.g. total energies and correlation energies which should serve as a comparative material for further calculations, A convenient way of preserving atomic units for practical needs but to avoid their meaning... 

SI units and equations wBll be used throughout Most of the original work, however, was done in the c.g.s.-e.s.u. system. Table 1 lists the SI units for the electromagnetic properties referred to in this artide, as well as the conversion factors from the SI to the probably more-familiar e.s.u. stem. To convert all equations to their c.g.s.-e.s.u. equivalents replace Co> the permittivity of free space (8.8S x 10 C m by (47t)-. ... 

The preferred units of viscosity are Pa s or mPa s. Some of the older units and their relation to the SI unit are given in Table 1-1. It is clear that the shear rate employed in the calculation must be specified when the magnitude of apparent viscosity is discussed. Apparent viscosity has many useful applications in characterizing a fluid food in particular, in the characterization of shear-thinning fluids, the apparent viscosity at low shear rates, called the zero-shear rate viscosity ( jo), is a useful parameter. 

One of the best features of the SI system is that (except for time) units and their multiples and submultiples are related by standard factors designated by the prefixes indicated in Table 1.4. Prefixes are not preferred for use in denominators (except for... 

Become familiar with the steam tables and their use both in SI and American engineering units. 

The minute, hour, and day have had such long-standing use in everyday life that it is unlikely that new SI units derived from the second could supplant them. Some other non-SI units are still accepted, although they are rarely used by most individuals in their daily lives but have been very important in some specialized fields. Examples of such units are the nautical mile, knot, and hectare. Examples of non-SI units retained for use with the SI are illustrated in Table 1-4. 

The SI system is convenient because unit sizes vary by powers of 10. Prefixes are used to name units. Look at Table 3 for some common SI prefixes and their meanings. 

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