commonly known as relative density, specific gravity is the ratio of density between a particular object and a standardized object such as water and air, acting as a basic comparison.
Specific Gravity Details
To understand how specific gravity works, we must understand object density since specific gravity is a derivative measurement of object density. Object density is how many molecules exist within an object, or in mathematical terms: mass divided by volume. We use the Greek letter ρ (pronounced as "rho") to define the formula of object density.
While density has an SI (Système International) unit of kilograms per cubic meter (kg/m3) or grams per cubic centimeter (gr/m3), specific gravity does not have any units because it's just a ratio. We can make sense of the scale by using particular objects as the standard comparison of the specific gravity, such as water. To be exact, the standard comparison for specific gravity is 1 kilogram of pure water (H2O) with the temperature of 4°C, which, at its densest state, has a density of precisely 1000 kg/ m3. This specific comparison gives the measurement specific gravity a sense of solidity and uniformity, even though it has no unit.
Water is the primary comparison for specific gravity, but that doesn't mean we can't use anything else. You can't bend reality to fit your singular formula; you have to shape your formula according to nature. Another standard measurement comparison we use extensively in aerospace engineering is air, which has a typical density of 1.29 kg/m3. Another measurement comparison often used in the manufacturing industry is oil, which has a density of around 950 kg/m3.
Example of Specific Gravity
Let's start with something simple: A block of wood has a density of 500 kg/m3; what's the block's specific gravity? The default comparison for specific gravity measurement is water, which has 1000 kg/m3 density, so let's use it.
The specific gravity of the block (compared to water) is 0.5. Notice how we use the density of wood (its mass divided by volume) to determine the density of the block. In a real-world application, the water we use for comparison often has a different mass and temperature than the standard (1 kg, 4°C). Keep in mind that the many components we used. We can rearrange the formula to find the value of other components as well.
Archimedes’ Eureka Moment
A Greek philosopher and inventor named Archimedes was bathing inside his private estate in Syracuse, Italy (the ancient-day equivalent of modern-day Sicily). He was in his bathtub pondering his task—which his king, Hieron, had given him—to find out whether his newly-gifted golden crown was pure gold (Aurum) or fool's gold (Pyrite). He shifted and noticed a little bit of water spilled out of his bathtub. That's when he got his eureka moment and proceeded to run out to the streets celebrating his newfound idea.
Archimedes found out that he could calculate the specific gravity and ultimately the crown's density by observing how much water the crown displaced when put in a fully-loaded water container. Pure gold has a density of 19000 kg/m3, while fool's gold (pyrite) has a density of just 5000 kg/m3—almost four times lower than gold! He further proves the specific gravity disparity by sinking the crown in a bathtub full of water, which means that gold will sink lower faster and displaces more water than fool's gold.
Significance of Specific Gravity
Specific gravity is a ratio between the density of two objects, with water as the primary comparison. The specific gravity ratio tells us how objects will behave when we place the object in a pool of water. Will it float? Or will it sink?
In the movie Titanic, a tiny iceberg destroys the 'invincible' cruise ship Titanic. The captain fails to spot the iceberg because only the tip of the iceberg is above the waterline. But the actual iceberg is even bigger than the Titanic itself, only hidden beneath the ocean's rolling waves. So, what makes the iceberg behave that way?
The answer is because of buoyancy. The iceberg has a specific gravity of around 920 kg/m3. The water's density is 1000 kg/m3, which in Titanic's case is very close to the standard 4°C temperature due to the cold rainstorm in the North Atlantic Ocean. We take those two data and calculate it:
The specific gravity of the iceberg is 0.92. The specific gravity ratio also tells us how much volume of the iceberg is below the water. So, the 0.92 ratio means 92% of the iceberg is submerged under the water, hiding under the ocean's rolling waves. With only 8% of the iceberg visible, combined with the dark of night and the massive rainstorm the ship was caught in, it's no wonder the Titanic captain failed to spot it.
We also use the concept of specific gravity to measure air pressure. If you partly fill a U-shaped tube with water, you can connect one end to a bike tire and leave the other end open. Because the specific gravity of air is lower than water, this will create a pocket of air just above the connected tube.
As the pressure of the bike tire goes up, the water is pushed down on one side and up on the other. Then we measure the height difference between the level of water on both tubes.