Enter the density of your material or substance to calculate it’s specific gravity. Specific gravity is a ratio of density of a material to the density of water.
Specific Gravity Formula
The following formula is used to calculate the specific gravity of a substance.
SG = ρ / ρW
- Where SG is specific gravity. (unitless)
- ρ is the density of your substance (kg/m^3)
- ρW is the density of water (kg/m^2)
A few things need to be kept in mind in order to accurately calculate specific gravity. First, the units of density need to be equal for both the substance and water or the result will not be accurate. The other important factor is that the density of water is measure at 4 degrees Celsius.
What is specific gravity?
As is mentions above, specific gravity is a ratio of density of a substance to the density of water. The temperature and pressures of the water and substance need to be the same. This is because as is described in the ideal gas laws, the density of a substance is directly proportional to both temperature and pressure.
There are many applications in which specific gravity can be used. It’s main use is providing a gauge of density that is easily comprehended. Since water is the most common substance on Earth, setting it as a standard makes a lot of sense. This is because water is an easily quantifiable and visualized substance. The point of having standard units is to simplify design and calculations. Choosing something people come in contact with daily achieves that goal.
Types of Specific Gravity Measurement
There are several ways that specific gravity can be measured instead of calculated using the formulas above. Some of the most common are listed below.
- Hydrostatic pressure-based instruments:
- These instruments rely on pascals principles of pressure between two points. They are mostly used in tank gauging applications.
- Vibrating Element Transducers
- This is a device that places a vibrating element into the substance it’s trying to measure. It then uses the physics of resonant frequency to calculate the result.
- Ultrasonic Transducer
- Just as it sounds, this instrument use ultrasonic waves to measure acoustic properties of a substance and uses that to calculate the density.
- Buoyant Force Transducers
- Since specific gravity is directly related to buoyancy, you can use the force of buoyancy to calculate specific gravity and that’s exactly what this type of device does.
There are over 50 other instruments that can be used to calculated specific gravity, but these are the main forms used in science.
How to calculate specific gravity
The following example will go over how to determine the specific gravity of any substance. You will need access to a calculator, scale, and measurement device. The scale and measurement device are used to calculate density.
- First, you must determine the temperature of this substance. The reason for this is that the density of water will be variable depending on the temperature. For this example we will assume a temperature of 4 C = 39.4 F.
- Next, you must find the density of water. At the temperature above the density of water is 997 kg/m^3. If the temperature is not at 39.4F in your example, you must use the ideal gas law to calculate the density.
- Next, find the density of your substance. Using the same process as before, you can calculate or measure the density. For this example we will assume it’s the same as water, 997 kg/m^3.
- Lastly, enter the information into the formula. SG = 997/997 = 1.
- Analyze you results and apply to other problems.
In this example we assume the density of the material was exactly the same as the water. In most cases this won’t be true. It was simply used to explain what specific gravity is more easily.
To analyze specific gravity a little deeper, lets take a look at what certain specific gravitates mean. If the SG of a substance is equal to that of water, it will have a neutral buoyancy. If the SG of a substance is less than one, it will have a less density then water and it will float. If the SG of a substance is greater than one, the substance or object will sink. These 3 situations are especially important when analyzing and designing sea vessels.