Large variety
Measuring instruments are tools to measure a quantity, such as the length of a rod, an angle, a time interval, a temperature, an electrical current, an amount of gas or fluid, the composition of a sample. For these and many other purposes a wide variety of instruments has been developed during the centuries. The development of ever more precise instruments goes on in line with the changing and increasing demands in society.

Demands on measuring instruments
The demands with respect to measuring instruments are closely related to the intended purpose. For navigation of satellites and radio astronomy more precise time measurement is needed than for household purposes. Length measurement in the production of computer chips needs more precise length standards than the building or construction industry. The measurement characteristics of an instrument under certain conditions can be determined by a calibration, which is by comparison with a standard.

Legal requirements
For certain purposes public authorities have introduced strict regulations on the specifications and the use of measuring instruments, for instance if they are used for trade purposes or for official determinations. Examples are the weighing scales in shops, gas meters, petrol pumps and other fuel dispensers. This field is covered by legal metrology.