Some Non-SI Units

Some non-SI units are explicitly permitted. They include for crystallographic statements 1 A = 1CT10 m = 100 pm and 1° = tt/ 180 rad (plane angle). The liter is also permitted (both abbreviations are official, L or 1) 1 L = 1 dm3. 

A list of some non-SI units, together with their SI values, and a table containing the best values of some fundamental physical constants are given in appendix A. 

SI units are recommended for use throughout science and technology. However, some non-SI units are in use, and in a few cases they are likely to remain so for many years. Moreover, the published literature of science makes widespread use of non-SI units. It is thus often necessary to convert the values of physical quantities between SI and other units. This chapter is concerned with facilitating this process. 

TABLE VI The values in SI units of some non-SI units based on the 2006 CODATA adjustment of the values of the constants. 

These relations are listed in Table 4.1 together with the conversion factors for some non-SI units still commonly in use. 

Nonetheless, it is recognized that some non-SI units still appear widely in the scientific, technical, and commercial literature, and some will probably continue to be used for many years. Other non-SI units, such as the units of time, are so widely used in everyday fife and are so deeply embedded in the history and culture of human beings that they wiU continue to he used for the foreseeable future. For these reasons, some of the more irr5)ortant non-SI units are fisted. 

SI now has seven base units, and most other units are derived from these seven. Some non-SI units are still commonly used by chemists and are also used in this book. 

Some non-SI units continue to be used, such as the atmosphere (atm), which is a pressure defined to equal 101, 325Nm (101, 325 Pa), the liter (1), which is exactly 0.001 m, and the torr, which is a pressure such that exactly 760torr equals exactly 1 atm. The Celsius temperature scale is defined such that the degree Celsius (°C) is the same size as the kelvin, and 0°C is equivalent to 273.15 K. 

We will also use some non-SI units. The calorie (cal), which was originally defined as the amount of heat required to raise the temperature of 1 gram of water by 1°C, is now defined by ... 

We will quote a numerical constant in some of these equations to help with actual calculations. The units can be very confusing because it is conventional to use non-SI units for several quantities. The wavenumber value, i>, is usually taken to be in cm The extinction coefficient is conveniently taken in units of 1 moH ... 

A unit equality may link SI units and non-SI units (1 quart = 0.946353 L), decimally related units (10 c = 1 m ), or base units and derived units (1 L = 10 m ). Some of the more common unit equalities are given on the inside back cover of this text. Examples and treat unit conversions. 

Table 1.3 gives some commonly used non-SI units for certain quantities, together with conversion factors relating them to SI units. We use these in some examples and problems, except for the calorie unit of energy. This last, however, is frequently encountered. 

The units of e require some explanation here as they are generally expressed as non-SI units for historic reasons, having been used in spectroscopy for many years. 

It is generally acknowledged that the International System has brought order out of the previous multisystem chaos. The IUPAC recommendations regarding units will therefore be followed in the present book. In some countries, like the United States, units like the calorie, the torr, and the atmosphere, for example, are still common, but they have gradually been replaced by their SI equivalents [14], However, non-SI units, such as the electronvolt (eV) and the hartree ( h) are more convenient to use in many cases. These units, particularly the eV, are prevalent in a large number of recent publications on molecular energetics. 

Table A2 Some common non-SI units and their conversion factors...

Many non-SI units remain in common use some of these are given in the Table 1-2. 

Environmental Protection Agency policy is to express all measurements in SI units. When implementing this practice will result in undue costs or lack of clarity, conversion factors are provided for the non-SI units. This report uses British Engineering units of measure for some cases. For conversion to the SI system, use the following conversions ... 

These units are not part of the SI, but it is recognized that they will continue to be used in appropriate contexts. SI prefixes may be attached to some of these units, such as millilitre, ml millibar, mbar megaelectronvolt, MeV kilotonne, kt. A more extensive list of non-SI units, with conversion factors to the corresponding SI units, is given in chapter 7. 

The inclusion of non-SI units in this table should not be taken to imply that their use is to be encouraged. With some exceptions, SI units are always to be preferred to non-SI units. However, since many of the units below are to be found in the scientific literature, it is convenient to tabulate their relation to the SI. 

Many non-Sl units are now defined exactly in terms of SI some can only be related to SI units via fundamental constants, and the relationship is therefore restricted by the precision to which the constants are known. Factors for converting some non-Sl units into their SI equivalents are listed in ... 

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. 

In recognition of long-standing traditions, the various international commissions have acknowledged that a few non-SI units will remain in use in certain fields of science and in certain countries during a transitional period. Table 1-2 lists some units, both SI and non-SI, which will be used in this book, involving the quantities length, mass, and time. Other units will be introduced in subsequent chapters. 

Some of the common clinical chemistry units are provided here with factors to convert from the traditional non-SI units to SI unitage in plasma or serum. 

Older, non-SI units are in common use, and some of these are shown in Table 1-2. 

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