Properties of fluids determine how fluid may behave in different engineering applications. Based on this engineers can decide which type of fluid they should use. So let’s find out different key properties of fluids in this article. At the end of this article, there will be a link for **fluid properties PDF** which you can download for your future reference.

## What is fluid?

The fluid is a substance that has no shape and can easily flow continuously with little or no external pressure. A typical example of fluids is liquid and gas. The fluid is also called zero memory substance.

## What is fluid flow?

Fluid flow is the relative motion of different particles of a fluid substance. The relative motion is continuous under shear force as those fluid particles can not resist shear force. Fluid can deform continuously under shear force without coming back to its original state.

You may like to read: Difference between screw and bolt

## Properties of fluid

When we talk about fluid, both liquids and gas come into the picture. But both behave in a different way. A liquid takes the shape of the surface where it is kept. But gases take the shape of the container where is stored. Due to this the properties of gas vary compare to the properties of a liquid.

- Density
- Viscosity
- Specific volume
- Specific weight
- Specific gravity

### Density

Density is the mass per unit volume of a fluid. So it is the ratio between mass and unit volume. The density of a fluid is denoted by the symbol “ρ ” and the unit is kg/m^{3} in the SI unit system. The density of fluid increases with the increase of pressure and temperature.

Density of fluid can be expressed as below

**ρ= Mass / Volume**

#### How to calculate Density of a fluid ( Gas)

Density of a fluid can be calculated easily by applying following formula if you know the pressure and temparature.

**PV= mRT**

P=(m/V) RT

P=ρRT

ρ=RT/P

Where

P = Pressure

V= Volume

m= Mass

R= Universal Gas Constant

T= Temperature

ρ= Density

#### Density of common fluids

Fluid | Density (kg/m3) |
---|---|

Water | 1000 |

Alcohol | 785 |

Brine | 1230 |

Kerosene | 820 |

Milk | 1020 |

Mercury | 13590 |

Petrol | 737 |

Diesel | 820 |

### Viscosity

Viscosity is the amount of internal resistance to flow deformation between particles. Viscosity is the property of a fluid that resists movement of particles from one layer to another layer. The opposite of viscosity is the fluidity. Mathematically viscosity is the amount of internal resistance between particles when they are in motion.

In the case of liquids, the viscosity decreases with the increase in temperature. But in the case of gas, viscosity increases with the increase in temperature. The unit of viscosity in the SI system is **Pascal-second or N s/m ^{2} **

#### Viscosity of common fluids

Fluid | Viscosity (Pascal-second or N s/m)^{2} |
---|---|

Water | .00089 |

Alcohol | .00109 |

Kerosene | .00164 |

Milk | .00300 |

Mercury | .00150 |

### Specific Volume

The specific volume of fluids can be defined as the ratio of the volume of fluid to the mass of fluid. In other words, a specific volume is a volume occupied by a unit mass of fluid. The unit of specific volume in the SI system is m^{3}/kg

**Specific Volume = Volume Of Fluid / Mass of Fluid = V/m**

If you see carefully, you will notice that the specific volume is nothing but the reciprocal of the density. So mathematically specific volume can also be defined as **1/ρ**

### Specific Weight

Specific weight can be defined as the ratio of the weight of the fluid to the volume of fluid. In other words, it is the weight of the unit volume of fluid. The unit of specific weight in the SI system is **N/m ^{3}**

Specific Weight= Weight / Volume

=W/V

=mg/V ( M is the mass & g is the gravity)

=(m/V)*g

= ρ*g (ρ is the density)

So as you can see that the specific weight depends on the density and gravity or acceleration. As we know that the density depends on pressure and temperature. So in other words, the specific weight depends on pressure, temperature, and gravity.

### Specific Gravity

The specific gravity of fluid can be defined as the ratio of the density of the fluid to the standard fluid. That is why it is also called relative density. It does not have any unit as this is the ratio of one density to the other.

In the case of liquid, water is the standard liquid and in case of gas, the air is the standard gas. So if we see the equation of specific gravity of liquid and gas, then it will look like this as shown below.

**Specific gravity of fluid = Density of fluid / Density of water**

**Specific gravity of Gas= Density of gas / Density of** **air**

## Conclusion

Here are a couple of properties of fluids that we should know. As an engineer, these are some of the basic properties that you should consider while choosing a fluid for your application. If you have any questions or queries on properties of fluids, please do write in the comment section and I will be happy to assist.