Units used in Engineering Calculations

Units use in Engineering

Units used in Engineering:

Proper Handling of Units is an essential and skill that every Chemical Engineer should have. There are a lot of processes occurring in the Chemical Industries and while handling those processes it is necessary that Chemical Engineer should be able to deal with every unit related to that process. Also while solving different Equation with minimum errors consistency in units is the best helping tool. An Engineer has to deal with different calculations and therefore Engineer should be able to deal with Mathematical description not only with words. So before starting with Unit Conversion let us discuss first:
1. What are the Units?
2. What are Dimensions?
3. Difference between Units and Dimensions?

Dimensions:

Dimensions are the general expression of a characteristic of measurement such as length, mass, time, temperature, and so on.

Units:

Units are the means of expressing these dimensions, such as feet or meter for expressing length, second or hour for the time, kilogram or pound for mass.

Two types of units are majorly used while dealing with Engineering problems:

1. SI (System International) Units.

2. AE (American Engineering) Units.

SI Units:

SI Units are formally known as Le Systeme Internationale d'Unites and informally called SI or, more often, the SI systems of Units.

AE Units:

AE or American Engineering system of Units, not to be confused with what is called US Conventional System (USCS) or the English system of Units.

SI Units have certain advantages on AE Units and conversion of SI Units is easier as compared to AE Units but AE has a deep root in the US and therefore it is mostly used in the United States.

Most software available which are used for Engineering purposes like MathCAD or ChemCAD have both type of Units to be used.

Dimensions and their respective units are classified into two categories:

1. Basic or Fundamental Dimensions/Units: 

Basic Units/Dimensions are those units which are independently measured for physical quantities such as length, mass, time, and temperature.

2. Derived Dimensions/Units:

Derived Dimensions/Units are those which are measured in terms of Fundamental Dimensions/Units such as Force, Acceleration, Velocity.

SI Units of some Physical Quantities:

Here are SI Units of some Physical Quantities with symbols

Basic SI Units

Length = Meter (m)

Mass = Kilogram (kg)

Time = Second (s)

Temperature = Kelvin (K)

Amount of Substance = mole (mol)

Derived SI Units (Units made up of Basic SI Units)

Force = Newton (N) (kgms^-2)

Energy = Joule (J) (kgm^2s^-2)

Power = Watt (W) (kgm^2s^-3)

Density = no unit name (kg/m^3)

Velocity = no unit name (m/s)

Acceleration = no unit name (m/s^2)

Pressure = Pascal (N/m^2)

Heat Capacity = no unit name (J/kg.K)

AE Units of some Physical Quantities:

Here are some AE units of some Physical quantities

Basic AE Units:

Length =  foot or inch (ft or in)

Mass = pound (lbm)

Time = second or hour (s or hr)

Temperature = degree Rankine or degree Fahrenheit (R or F with degree sign)

Amount of Substance = pound mole (lbmol)

Derived AE Units:

Force = pound (force) (lbf)

Energy = British thermal unit or foot pound (force) (Btu or ft.lbf )

Power = horsepower (hp)

Density = no unit name (lbm/ft^3)

Velocity = no unit name (ft/s)

Accelaration = no unit name (ft/s^2)

Pressure = no unit name (lbf/in^2)

Heat Capacity = no unit name (Btu/lbm.deg F)

Volume = no unit name (ft^3)