What is the specific gravity of SAE 30 oil, which has a specific weight of 782 times 103N/m3
The correct answer and explanation is :
Answer:
The specific gravity (SG) of SAE 30 oil is calculated using the formula:
[
SG = \frac{\text{Specific Weight of the Fluid}}{\text{Specific Weight of Water}}
]
Given:
- Specific weight of SAE 30 oil = ( 782 \times 10^3 ) N/m³
- Specific weight of water at 4°C = ( 9.81 \times 10^3 ) N/m³
[
SG = \frac{782 \times 10^3}{9.81 \times 10^3}
]
[
SG = 79.8
]
This value is incorrect because the given specific weight is unusually high. Typically, SAE 30 oil has a specific gravity of around 0.87 to 0.89 at room temperature.
Explanation:
1. Understanding Specific Gravity
Specific gravity (SG) is a dimensionless number that compares the density (or specific weight) of a substance to that of water. It helps engineers and scientists determine how a fluid behaves in different applications.
2. Why the Given Specific Weight Seems Incorrect
A typical SAE 30 oil has a density of around 870 kg/m³. If we use the equation:
[
\text{Specific Weight} = \text{Density} \times \text{Gravity}
]
[
\gamma = 870 \times 9.81 = 8.54 \times 10^3 \text{ N/m}^3
]
Using this corrected value:
[
SG = \frac{8.54 \times 10^3}{9.81 \times 10^3} = 0.87
]
3. Importance of Specific Gravity in Lubricants
- It helps determine whether the oil will float or sink when mixed with water.
- It affects lubrication performance, viscosity, and fuel efficiency.
- It is essential in hydraulic and automotive applications where oil must be compatible with other materials.
Thus, SAE 30 oil typically has a specific gravity of about 0.87 to 0.89, not 79.8. The given data seems to have an error in magnitude.