Units

• Units of measurement
• The Greek alphabet
• Commonly used SI Units, physical quantities and units
• Frequently used physical constants in SI units
• Non-SI units
• Metric prefixes

Units of measurement

The international nature of science demands standardisation of nomenclature and units of measurement to simplify communication. Mathematical formulae and the labeling of elements in chemicals, nutritional compounds, solutions, etc. frequently use letters from the Greek alphabet , which should be memorised, since they are so common in a scientist's vocabulary. The French Système International d'Unités (the SI-system) is the accepted convention for units of measurement. Some commonly used SI units and a list of physical constants expressed in SI units , common prefixes to ease expressions for the extremes found in biological systems are listed. Volumes of liquids used are frequently very small. Despite recommendations that they be abandoned in exact scientific wink, litres (l), millilitres(ml), microlitres(µl) and nanolitres(nl) not only remain in common usage but are accepted terms for most scientific journals probably because they are easier both to pronounce and to write down than their SI equivalents. Next table gives some non-SI units of volume and their SI equivalents. Non SI terms are also to be found in older published work. As a consequence it is important to have some appreciation of the interconversion of units for interpretation.

The Greek alphabet

Commonly used SI Units, physical quantities and units

The SI is founded on seven SI base units for seven base quantities assumed to be mutually independent, as given in the table below.

From these 7 base units a list of commonly used units is derived.

Frequently used physical constants in SI units

Non-SI units

Certain units are not part of the International System of Units, that is, they are outside the SI, but are important and widely used. Consistent with the recommendations of the International Committee for Weights and Measures (CIPM, Comité International des Poids et Mesures), the units in this category that are accepted for use with the SI.

Metric prefixes

When the metric system was devised in the late 1700's there was no particular need for very large or very small numbers. In the two centuries since that time we have learned to measure objects and distances, both large and small, to the limits of nuclear particles and astronomical bodies. The metric measurements are all in decimal form, and are used very consistently from one parameter to another.
The mass of the earth is 5983 Yg (yottagrams), and it gains another 40 Gg (gigagrams) every year from captured meteorites and cosmic dust. The average distance to the moon is 384.4 Mm (megameters). The average distance to the sun is 149.5 Gm (gigameters). The wavelength of yellow light is 590 nm (nanometers). The diameter of a hydrogen atom is about 70 pm (picometers). The mass of a proton is about 1.67 yg (yoctograms), and that of an electron about 0.000 91 yg (yoctograms).

The scientific notation used in the factors column helps to reduce long numbers to a manageable width. By convention, the number is always shown as a unit [ 1 to 9 ], with decimal places chosen to suit accuracy, and the size of the number is adjusted by changing the magnitude [E+?]. E+01 means moving the decimal point one space to the right so 1.00E+01 is shorthand for 10, then 1.33E+00 stays at 1.33 and 1.33E-01 becomes 0.133. This format tends to be used when the figure gets longer so E+09 or E-09 cuts out a lot of noughts.