Units Used with the SI

In 1996 the CIPM agreed upon a categorization of the units used with the SI into three groups units accepted for use with the SI, units accepted for use with the SI whose values are obtained experimentally, and other units currently accepted for use with the SI to satisfy the needs of special interests. The three groups are listed in Tables 1.2-8-1.2-10. 

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. 

There are non-Sl units which are accepted for use with the SI min (minute), h (hour), d (day), eV (electronvolt). Da (Dalton this is equivalent to u, 1 Da = 1 u), u (unified atomic mass unit) and some more (see section 3.7 in [1]). The minute (60 s), the hour (3600 s) and the year (86400 s) are defined here as exact multiples of the second. The year (symbol a) is not among those units, because there are different kinds of year , the Julian year being defined in terms of days as 365.25 d, the Gregorian year as 365.2425 d, and the Mayan year as 365.2420... 

The following sections discuss in detail the units that are acceptable for use with the SI. 

Certain units that are not part of the SI are essential and used so widely that they are accepted by the CIPM for use with the SI. These units are given in Table 6. The combination of units of this table with SI units to form derived units should be restricted to special cases in order not to lose... 

There are a few highly specialized units not listed in Table 6 that are given by the International Organization for Standardization (ISO) or the International Electrotechnical Commission (lEC) and which are also acceptable for use with the SI. They include the neper (Np), bel (B), octave, phon, and sone, and units used in information technology, including the baud (Bd), bit (bit), erlang (E), hartley (Hart), and shannon (Sh) It is the position of NIST that the only such additional units that may be used with the SI are those given in either the International Standards on quantities and units of ISO or of lEC. ... 

The CIPM also finds it necessary to accept for use with the SI the two units given in Table 7. These units are used in specialized fields their values in SI units must be obtained from experiment and, therefore, are not known exactly. 

Note In some fields the unified atomic mass unit is called the dalton, symbol Da however, this name and symbol are not accepted by the CGPM, CIPM, ISO, or lEC for use with the SI. Similarly, AMU is not an acceptable unit symbol for the unified atomic mass unit. The only allowed name is "unified atomic mass unit and the only allowed symbol is u. 

Table 7. Units accepted for use with the SI whose values in SI units are obtained experimentally...

SI Unit The basic unit used in the SI system is not the unit of entropy, but the temperature unit called Kelvin, abbreviated to K. This was done by giving the melting temperature of pure airless water in a sealed container with pure water vapor (no air) above it a value, namely... 

The variables of state for thermomechanical analysis are deformation (strain) and stress. The SI units of deformation are based on length (meter, m), volume, (cubic meter, m ) and angle (radian, rad, or degree) as listed in Fig. 4.143 (see also Fig. 2.3). Stress is defined as force per unit area with the SI unit newton m", also called by its own name, pascal. Pa. Since these units are not quite as frequently used, some conversion factors are listed below. The stress is always defined as force per area. 

There are several traditional units, which are so important for everyday life or for a certain field that they have been accepted for use with the SI. The accepted units are listed in O Table 11.5, together with their value in SI units. It is noteworthy that two of them - the unified atomic mass unit and the electronvolt energy unit- do not have an exact equivalent in SI units. Their values are fundamental constants, which are obtained experimentally (Mohr 1999, 2000). There are some other traditional units, which have been accepted only temporarily, as shown in O Table 11.6. 

Physical quantities are tools which allow us to specify and quantify the properties of physical objects and to model the events, phenomena, and patterns of behavior of objects in nature and in technology. The system of physical quantities used with the SI units is dealt by Technical Committee 12 of the International Organization for Standardization (ISO/TC 12). Since 1955, ISO/TC 12 has published a series of international standards on quantities and their units, in which the use of SI units is strongly recommended. 

Table 1.2-9 lists three non-SI units accepted for use with the SI, whose values expressed in SI units must be obtained by experiment and are therefore not known exactly. Their values are given with their combined standard uncertainties, which apply to the last two digits, shown in parentheses. These units are in common use in certain specialized fields. 

Table 1.2-10 lists some other non-SI units which are currently accepted for use with the SI to satisfy the needs... 

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