IEEE 10:2016 pdf free download – American National Standard for Metric Practice
2.2.2 Limited character sets
When the metric system developed in the 19th century, all educated persons were familiar with the Greekalphabet, and the Greek letters lowercase mu (for micro) and uppercase omega (for ohm) were standardizedand have been included in the official Sl Brochure. This presents a problem now with limited character sets,especially in embedded and applied computer operating systems used in manufacturing and labware.IEEEStd 260.1 TM-2004 [B9] addresses this problem and prescribes lowercase u as a substitute prefix symbol formicro and Ohm (note the uppercase O) as a substitute unit symbol for ohm when Greek letters are notavailable. It also gives recommendations where only uppercase or only lowercase letters are available.
3.Use of the sl
3.1 General
Sl is the form of the metric system that shall be used for all applications. It is important that this modernform of the metric system be thoroughly understood and properly applied. The remainder of this standardgives guidance concerning the use of the system,including the limited number of cases in which unitsoutside Sl are appropriately used, and makes recommendations concerning usage and style.
3.2 Application of Sl prefixes
3.2.1 General
In general，use the Sl prefixes (see 2.2) to indicate orders of magnitude.Thus，one can eliminatenonsignificant digits (for example,12 300 m becomes 12.3 km, or 12.30 km, or 12.300 km depending onthe appropriate number of significant digits) and leading zeros in decimal fractions (for example,0.001 23 um becomes 1.23 nm). Sl prefixes provide a convenient alternative to the powers-of-ten notation(for example,12.3 × 103m becomes 12.3 km). Never use a prefix alone.
3.2.2 Selection
When expressing a quantity by a numerical value and a unit, give preference to a prefix that yields anumerical value between 0.1 and 1000. For simplicity, give preference to prefixes representing 1000 raisedto a positive or negative integral power. However, the following factors may justify deviation from theseprefixes:
a) ln expressing area and volume, the prefixes hecto, deka, deci, and centi may be convenient, for
example, cubic decimeter, square hectometer, or cubic centimeter.
b)In tables of values of the same quantity, or in a discussion of such values within a given context, it
is preferable to use the same unit multiple or submultiple throughout.
c)For certain quantities in particular applications, one particular multiple or submultiple is often used.
For example,the millimeter is used for linear dimensions in engineering drawings even when thevalues lie far outside the range of 0.1 mm to 1000 mm; the centimeter is usually used for bodymeasurements，clothing sizes，household products，and other everyday purposes for whichmillimeters are inconveniently precise.
3.3.2.2 Time
The SI unit of time is the second (s), which should be used in technical calculations.However, where timerelates to life customs or calendar cycles, the minute(min), hour (h), day (d), and other calendar units maybe used.For example, vehicle speed is often expressed in the non-Sl unit kilometer per hour (km/h) insteadof the Sl unit meter per second (m/s). There are no values or symbols given in the Sl for week, month, andyear. The latter two especially have varying values depending on the actuarial or astronomical unit intended.However, industry often uses the units month and year for statistical and financial purposes and so Table 6of this standard includes those two units and their symbols.