Specific Gravity vs. Specific Weight

Attribute	Specific Gravity	Specific Weight
Definition	The ratio of the density of a substance to the density of a reference substance (usually water)	The weight per unit volume of a substance
Formula	Specific Gravity = Density of Substance / Density of Reference Substance	Specific Weight = Weight of Substance / Volume of Substance
Unit	No unit (dimensionless)	Newtons per cubic meter (N/m³) or pounds per cubic foot (lb/ft³)
Comparison	Compares the density of a substance to the density of water	Measures the weight of a substance per unit volume
Reference Substance	Water (at 4°C or 39.2°F)	No specific reference substance
Relative to	Water (SG = 1 for water)	No specific reference
Applications	Used in various fields like fluid mechanics, hydraulics, and geology	Used in engineering and physics to analyze forces and loads

Introduction

When studying the properties of fluids, two important concepts that often come up are specific gravity and specific weight. These terms are used to describe different characteristics of fluids and are crucial in various fields such as engineering, physics, and geology. While both specific gravity and specific weight provide valuable information about fluids, they differ in their definitions, units of measurement, and applications. In this article, we will explore the attributes of specific gravity and specific weight, highlighting their similarities and differences.

Definition and Calculation

Specific gravity is defined as the ratio of the density of a substance to the density of a reference substance, typically water. It is a dimensionless quantity and is often denoted by the symbol "SG." The specific gravity of water is always considered to be 1. Therefore, if the specific gravity of a substance is less than 1, it is less dense than water, and if it is greater than 1, it is denser than water.

On the other hand, specific weight refers to the weight per unit volume of a substance. It is calculated by multiplying the density of the substance by the acceleration due to gravity. The specific weight is typically expressed in units of force per unit volume, such as pounds per cubic foot (lb/ft³) or newtons per cubic meter (N/m³).

Units of Measurement

As mentioned earlier, specific gravity is a dimensionless quantity and does not have any units. It is simply a ratio of densities. On the other hand, specific weight is expressed in units of force per unit volume. The choice of units depends on the system of measurement being used. In the International System of Units (SI), specific weight is measured in newtons per cubic meter (N/m³), while in the United States Customary Units, it is commonly measured in pounds per cubic foot (lb/ft³).

Applications

Specific gravity is widely used in various industries and fields. In the construction industry, it is used to determine the quality and strength of materials such as concrete and aggregates. It is also used in the petroleum industry to measure the density of crude oil and other petroleum products. Additionally, specific gravity plays a crucial role in the brewing and winemaking industry, where it is used to measure the sugar content in liquids during the fermentation process.

Specific weight, on the other hand, finds applications in engineering and physics. It is used to calculate the buoyant force acting on submerged objects, which is essential in designing ships, submarines, and other floating structures. Specific weight is also used in geotechnical engineering to analyze soil properties and stability. Furthermore, it is utilized in fluid mechanics to study fluid flow and pressure distribution in pipes and channels.

Relationship between Specific Gravity and Specific Weight

Although specific gravity and specific weight are distinct concepts, they are related to each other through the density of the substance. Density is defined as the mass per unit volume of a substance and is calculated by dividing the weight of the substance by its volume. Since specific weight is the weight per unit volume, it can be obtained by multiplying the density by the acceleration due to gravity. Therefore, the specific weight of a substance can be determined if its specific gravity and the density of the reference substance (usually water) are known.

Conclusion

In conclusion, specific gravity and specific weight are both important attributes used to describe fluids. While specific gravity is a dimensionless quantity that compares the density of a substance to the density of a reference substance, specific weight represents the weight per unit volume of a substance. Specific gravity is widely used in various industries such as construction, petroleum, and brewing, while specific weight finds applications in engineering, physics, and geotechnical engineering. Although they have different units of measurement, specific gravity and specific weight are related through the density of the substance. Understanding these concepts is crucial for professionals working with fluids and can provide valuable insights into their properties and behavior.