Units Used with SI

As stated earlier, SI units can be used to measure any physical quantity, and the use of other units with them must be carefully and sensibly limited. If this principle is not respected, the new system will soon be as uncontrolled and incoherent as the older ones. 

A very limited list of non-SI units has been accepted for use with SI for varying reasons. 

Time—Since we live our lives by the sun, it has been widely agreed that the hour and the calendar units will continue to be used with SI, particularly in such quantities as vehicle velocity. 

Angle— Due to the much simpler use in geometry, the plane angle unit degree has been accepted for use along with SI unit, radian. The units minute and second, however, are unnecessary, and subdivisions of degree should he produced by decimalizing. 

Temperature—Wide acceptance is established of the degree Celsius (formerly called centigrade) in addition to the kelvin, particularly in everyday use. Temperature intervals are identical in the two scales. The kelvin will, of course, he used in thermodynamics. 

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Units Used with SI. A number of non-SI units are used in SI (Table 2). 

Quantity Sl-unit of the SI-unit used with SI temporafily that should not be used... 

The angstrom and bar are approved for temporary use with SI units however, they should not be introduced if not used at present. 

Units Used Temporarily with SI. Additional non-SI units are used with SI units until the CIPM considers their use no longer necessary (Table 3). 

Dimensionless Quantities. Certain quantities, eg, refractive index and relative density (formerly specific gravity), are expressed by pure numbers. In these cases, the corresponding SI unit is the ratio of the same two SI units, which cancel each other, leaving a dimensionless unit. The SI unit of dimensionless quantities may be expressed as 1. Units for dimensionless quantities such as percent and parts per million (ppm) may also be used with SI in the latter case, it is important to indicate whether the parts per million are by volume or by mass. 

Still other units encountered in the literature and workplace come from various other systems (absolute and otherwise). These include metric systems (c.g.s. and MKS), some of whose units overlap with SI units, and those (FPS) based on English units. The Fahrenheit and Rankine temperature scales correspond to the Celsius and Kelvin, respectively. We do not use these other units, but some conversion factors are given in Appendix A. Regardless of the units specified initially, our approach is to convert the input to SI units where necessary, to do the calculations in SI units, and to convert the output to whatever units are desired. 

UNITS, NOT PART OF COHERENT SYSTEM, BUT GENERALLY ACCEPTED FOR USE WITH SI UNITS... 

TABLE 2. STANDARD prefixes USED WITH SI units ... 

Several important and widely used units are not officially part of the SI, but the CIPM has accepted them for use with SI units (see Table 4). They include the liter, day, hour, minute, electronvolt, and degree. Although the CIPM adopted the liter in 1879, it is not a current SI unit. Its symbol (L in the United States, 1 everywhere else) causes some confusion, but the CIPM has approved neither. 

Table 1 lists common derived SI units with special names. The table also gives the approved SI symbol and the expression for the term in base units and in terms of other units. Table 2 gives examples of other derived units which are commonly used in chemical engineering including a description and SI units. Table 3 shows units which are not officially recognized as usable with SI but which are authorized for use to a certain extent, while Table 4 gives units which are not acceptable for use with SI. 

The following common units have been authorized for use with SI to a certain extent and continue in use on an unofficially accepted basis. 

A mass unit frequently used for convenience in the United States and elsewhere is the so-called "metric ton." For commercial use with SI this unit is called the ton (t) and is equal to 1000 kg, or 1 Mg. 

In the SI system, the meter (m) and the centimeter (cm) are common units of length. An English unit, the foot (ft), still is used to some extent in fields such as hydrology and meteorology. Table A-2 gives common units of length and interconversion factors. Table A-3 presents multiplier prefixes for use with SI units. 

Common Decimal Prefixes Used with SI Units... 

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