Short Answer:
Specific gravity is defined as the ratio of the density of a substance to the density of a standard substance, usually water for liquids and solids, and air for gases. It has no unit because it is a ratio of two similar quantities. Specific gravity tells how many times heavier or lighter a substance is compared to water.
In simple terms, specific gravity shows the relative heaviness of a material. For example, if the specific gravity of oil is 0.8, it means oil is 0.8 times as heavy as water. This property helps to compare materials and identify substances easily.
Detailed Explanation :
Specific Gravity
Specific gravity is an important physical property used to compare the density of a material with that of a reference substance. It helps determine how heavy or light a material is relative to water or air. In engineering, specific gravity is commonly used to identify materials and analyze their behavior in fluid systems such as pumps, tanks, and pipelines.
The specific gravity (SG) of a substance is mathematically expressed as:
where,
- ρ_substance = density of the material (kg/m³)
- ρ_reference = density of the reference material (kg/m³)
For most engineering purposes:
- The reference substance for liquids and solids is water at 4°C (density = 1000 kg/m³).
- The reference substance for gases is air at standard conditions (density = 1.225 kg/m³).
Since specific gravity is a ratio of two densities, it is dimensionless (has no unit).
Concept of Specific Gravity
The concept of specific gravity gives an easy way to compare materials without calculating their exact densities. If a material’s specific gravity is greater than 1, it is heavier than water and will sink in it. If it is less than 1, it is lighter and will float.
Example:
- Water has a specific gravity of 1.0 (as it is the reference).
- Oil has a specific gravity of 0.8 → Oil floats on water.
- Iron has a specific gravity of 7.8 → Iron sinks in water.
- Mercury has a specific gravity of 13.6 → Mercury is much heavier than water.
This comparison helps in designing systems where buoyancy, flow, or separation of fluids is important.
Derivation and Formula
Specific gravity can also be related to specific weight and mass. Since density (ρ) = mass/volume, we can write:
where,
- w = specific weight (weight per unit volume).
Thus, specific gravity can also be expressed in terms of specific weight, which makes it useful in fluid mechanics calculations involving pressure and buoyancy.
Measurement of Specific Gravity
The method to measure specific gravity depends on whether the substance is a solid, liquid, or gas.
- For Liquids:
- Hydrometer Method: A hydrometer is an instrument that floats in the liquid, and its scale directly gives the specific gravity.
- Specific Gravity Bottle (Pycnometer): A small glass bottle of known volume is used to measure the mass of the liquid and compare it with that of water.
- For Solids:
- If the solid is heavier than water, it is weighed in air and then in water. Using Archimedes’ principle, the specific gravity is calculated.
- If the solid is lighter than water, a sinker (heavier body) is used to hold it underwater for calculation.
- For Gases:
- The specific gravity of gases is determined by comparing the density of the gas with that of air under standard conditions.
Engineering Importance of Specific Gravity
Specific gravity is a key property in many engineering applications, especially in mechanical, civil, and chemical engineering fields. Some of its major uses are listed below:
- Fluid Mechanics:
Specific gravity helps in calculating pressure differences and buoyant forces when fluids of different densities interact. It also assists in identifying whether a fluid will float or sink in another fluid. - Hydraulic Machines:
Engineers use specific gravity to determine the performance of pumps, turbines, and other hydraulic equipment, since flow behavior depends on fluid density. - Material Identification:
In industries, specific gravity helps in identifying materials quickly. For example, different types of oils or fuels can be identified by their specific gravity values. - Design of Storage Tanks and Pipes:
Knowledge of specific gravity helps in determining the correct material strength and thickness for tanks and pipelines that store fluids of varying densities. - Buoyancy and Ship Design:
Specific gravity helps to design floating bodies such as ships, submarines, and buoys. A ship floats because its average specific gravity (including air spaces) is less than that of water.
Examples of Specific Gravity Values
Some typical values of specific gravity for common substances are:
- Water = 1.0
- Oil = 0.8 to 0.9
- Mercury = 13.6
- Iron = 7.8
- Air = 1.0 (reference for gases)
- Carbon dioxide = 1.53
These values clearly show that mercury is very dense compared to water, while oil and air are lighter.
Difference Between Density and Specific Gravity
While both properties are related, there is a key difference between them:
- Density has units (kg/m³) and gives the exact mass per unit volume.
- Specific Gravity has no unit and gives the ratio of densities (a relative measure).
Thus, specific gravity is more convenient for comparison, whereas density is used for exact calculations.
Effect of Temperature and Pressure
The value of specific gravity changes slightly with temperature and pressure, because the density of the material and the reference substance (water or air) changes. However, since both vary in a similar way, the effect on specific gravity is usually small and can be neglected for most engineering purposes.
Conclusion
In conclusion, specific gravity is the ratio of the density of a substance to the density of a standard reference, usually water or air. It is a dimensionless quantity used to compare how heavy or light a material is relative to water. Specific gravity plays a vital role in fluid mechanics, material selection, and hydraulic system design. Understanding this property helps engineers analyze fluid behavior, buoyancy, and flow characteristics effectively in real-world applications.