# What is SI unit?

#1

What is SI unit?

International system of unit (SI) is world’s most used system of measurement.

International system of unit (abbreviated SI) is used officially in all countries except USA,Myanmar and Liberia.

It has seven basic units,from which all other units are derived.

Length - Meter
Mass - Kilogram
Time - Second
Electric current - Ampere
Thermodynamic temperature - Kelvin
Luminous intensity - Candela
Amount of substance - Mole

Unit symbols are normal vertical letters and Quantitative symbols are inclined italic letters.All formulas indicating quantities must be written with italic letters.

All other units are derived from base units. There are 22 derived units.

In instrumentation we use only 2 base units level and temperature and two derived units flow and pressure.

Measurement is comparison between the value of process quantity and its unit.

The main advantage of SI system compared to imperial system is that bigger units are obtained from base units by multiples of ten and smaller units by sub multiples of ten.

For example 1 ton = 1 kg * 1000 (Bigger unit)
1 gram = 1 kg / 1000 (Smaller unit)

What is an Imperial System of UNITS?
#2

SI unit in Instrumentation

2 base units are used in Instrumentation Level and Temperature. Flow and pressure are derived units.

Pressure = Force / Area.

Unit of pressure is Pascal and it is the force of 1 newton applied to the area of 1 square meter.

Pa = N / m2.

Pressure is the quantity very much used in instrumentation. Pascal is a very small unit of pressure. So practically a 100000 times greater unit is used and it is called Bar.

1 bar = 100000 Pa.

In instrumentation other than pressure three more quantities are used and they are

Temperature, Level, and Flow.

Temperature is one of the 7 base quantities of SI system of measuring units.

The unit of temperature is kelvin. For practical use another unit is used called Celsius.

Celsius is Kelvin minus 273.16 .

Flow rate is the volume or mass passing through a certain section in a unit of time. Flow is expressed in kilograms per second or liters per second or their multiples.

Level is the height of a liquid in container.It is measured in meters or percentage of a maximum level.