Ø    Metrology is the science of measurement. Metrology includes all theoretical and practical aspects of measurement. Standards are objects or ideas that are designated as being authoritative for some accepted reason. Whatever value they possess is useful for comparison to unknowns for the purpose of establishing or confirming an assigned value based on the standard. The design of this comparison process for measurements is metrology. The execution of measurement comparisons for the purpose of establishing the relationship between a standard and some other measuring device is calibration.

Ø    Accuracy is the degree of exactness which the final product corresponds to the measurement standard. Precision refers to the ability of a measurement to be consistently reproduced. Reliability refers to the consistency of accurate results over consecutive measurements over time. Traceability refers to the ongoing validations that the measurement of the final product conforms to the original standard of measurement.

Ø    A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of measurement.

Ø    Different systems of units are based on different choices of a set of fundamental units. The most widely used system of units is the International System of Units, or SI. There are seven SI base units. metre for length kilogram for mass (note: not the gram),second for time,ampere for electriccurrent,kelvin for temperature candela for intensity, mole for the substance. For example, length is a physical quantity. The metre is a unit of length that represents a definite predetermined length. When we say 10 metres (or 10 m), we actually mean 10 times the definite predetermined length called "metre".

Ø    All other SI units can be derived from these base units. For most quantities a unit is absolutely necessary to communicate values of that physical quantity. For example, conveying to someone a particular length without using some sort of unit is impossible, because a length cannot be described without a reference used to make sense of the value given.

Ø    According to Instrument Society of America (ISA) these standards, units and measurements are approved under ANSI (American National Standard Institute) conceptual structures, which are the basic standards for instrumentation.