Introducing SI - Non-SI units accepted for use with SI

In the last 2 posts we talked about SI units and prefixes thoroughly. Although, in theory, SI can be used for any physical measurement, the CIPM has recognized that some non-SI units still appear in the scientific, technical, and commercial literature, and will continue to be used for many years to come. In addition, certain other units are so deeply embedded in the history and culture of the human race that they will continue to be used for the foreseeable future. The CIPM has catalogued several such units and published them in the SI Brochure so that their use may be consistent around the world. These units have been grouped as follows:

1. Non-SI units accepted for use with the SI (Table 6)

Certain units of time, angle, and legacy non-SI metric units have a long history of consistent use. Most of mankind has used the day and its non-decimal subdivisions as a basis of time and, unlike the foot or the pound, these were the same regardless of where they were being measured. The radian, being 1/2π of a revolution, has mathematical advantages but it is cumbersome for navigation, and, as with time, the units used in navigation have a large degree of consistency around the world. The tonne, litre, and hectare were adopted by the CGPM in 1879 and have been retained as units that may be used alongside SI units, having been given unique symbols. The catalogued units are:

minute (min) [1 min = 60 s]
hour (h) [1 h = 60 min = 3600 s]
day (d) [1 d = 24 h = 1440 min = 86400 s]
degree of arc (°) [1° = (π / 180) rad]
minute of arc (') [1' = (1 / 60)° = (π / 10800) rad]
second of arc ('') [1'' = (1 / 60)' = (1 / 3600)° = (π / 648000) rad]
hectare (ha) [1 ha = 100 a = 10000 m²= 1 hm²]
litre (l/L) [1 L = 1 dm³ = 0.001 m³ = 1000 cc = 1000 cm³]
tonne (t) [1 t = 10³ kg = 1 Mg]
astronomical unit (au) [1 au = 1.49597870691(6)E+11 m]

2. Non-SI units whose values in SI units must be obtained experimentally (Table 7)

Physicists often use units of measure that are based on natural phenomena, particularly when the quantities associated with these phenomena are many orders of magnitude greater than or less than the equivalent SI unit. The most common ones have been catalogued in the SI Brochure together with consistent symbols and accepted values, but with the caveat that their physical values need to be measured.

electronvolt (eV) [1 eV = 6981160217652999999♠1.60217653(14)E-19 J]
dalton/unified atomic mass unit (Da/u) [1 u = 1 Da = 6973166053886000000♠1.66053886(28)E-27 kg]

3. Other non-SI units (Table 8)

A number of non-SI units that had never been formally sanctioned by the CGPM have continued to be used across the globe in many spheres including health care and navigation. As with the units of measure in Tables 6 and 7, these have been catalogued by the CIPM in the SI Brochure to ensure consistent usage, but with the recommendation that authors who use them should define them wherever they are used.

bar (bar) [1 bar = 1E+5 Pa]
millimetre of mercury (mmHg) [1 mmHg = 7002133322387415000♠133.322387415 Pa at 0 °C = 7002133322387415000♠133.322387415 N/m² at 0 °C = 7002133322387415000♠133.322387415 kg/(cm·s²) at 0 °C]
torr (Torr) [1 Torr = 1/760 atm = 133.322368421... Pa = 133.322368421... N/m² = 133.322368421... kg/(cm·s²)]
atmosphere (atm) [1 atm = 1013.25 mbar = 1013.25 hPa = 7005101325000000000♠101325 Pa]
ångström (Å) [1 Å = 1E-10 m]
nautical mile (nmi) [1 nmi = 1852 m ]
are (a) [1 a = 1 dam² = 100 m²]
barn (b) [1 b = 1E-28 m²]
knot (kn) [1 kn = 1.852 km/h]
neper (Np) [1 Np = 1]
bel/decibel  (B/dB) [1 B = ln(10)/2 Np = ln(10)/2, 1 dB = 0.1 B]

Some of these units are very common in our life and can be seen almost everywhere, while some units are for some special areas. In short, SI units as well as these accepted non-SI units are commonly used both in life and in research. In the next post we will learn about the rules of using SI.