To change from psig to psia, you must add _____ to psig.
The correct answer and explanation is:
To change from psig (pounds per square inch gauge) to psia (pounds per square inch absolute), you must add 14.7 to the psig value.
Explanation:
- Understanding Psig and Psia:
- Psig (Gauge Pressure): This measures pressure relative to atmospheric pressure. A reading of 0 psig means the pressure is equal to atmospheric pressure.
- Psia (Absolute Pressure): This measures pressure relative to a perfect vacuum. It accounts for atmospheric pressure, which is approximately 14.7 psi at sea level.
- Relationship Between Psig and Psia: The formula to convert psig to psia is: PSIA=PSIG+Atmospheric Pressure\text{PSIA} = \text{PSIG} + \text{Atmospheric Pressure} Atmospheric pressure is 14.7 psi at sea level. Therefore: PSIA=PSIG+14.7\text{PSIA} = \text{PSIG} + 14.7
- Why Add 14.7? Atmospheric pressure at sea level exerts approximately 14.7 psi on everything. When using psig, this atmospheric pressure is ignored because psig measures pressure above atmospheric pressure. However, when calculating absolute pressure (psia), you need to include atmospheric pressure to account for the total pressure exerted, including the force from the air around us.
- Practical Example:
- If a gauge reads 50 psig, the absolute pressure is: PSIA=50+14.7=64.7 psia.\text{PSIA} = 50 + 14.7 = 64.7 \text{ psia.}
- If the gauge reads 0 psig, the absolute pressure is: PSIA=0+14.7=14.7 psia.\text{PSIA} = 0 + 14.7 = 14.7 \text{ psia.} This shows the atmospheric pressure at sea level.
- Importance of Conversion: This conversion is crucial in engineering, thermodynamics, and other fields where accurate pressure measurements are necessary for processes that depend on absolute pressure, such as gas laws or fluid dynamics.