Question

The pressure measured above or below the atmospheric pressure is known as

• A. suction pressure
• B. vacuum pressure
• C. negative pressure
• D. gauge pressure

Hint:

Keep this key relation in mind:
\[ P_{\text{absolute}} = P_{\text{atmospheric}} \pm P_{\text{gauge}} \] Use the positive sign for pressures above atmospheric, and the negative sign for pressures below atmospheric.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks for the terminology used to describe a pressure measurement that uses local atmospheric pressure as its reference datum.

Step 2: Key Formula or Approach:

• Pressure scales are defined based on the reference point:
1. Absolute Pressure ($p_{abs}$): Measured relative to a perfect vacuum ($p = 0$).
2. Atmospheric Pressure ($p_{atm}$): Pressure exerted by the Earth's atmosphere.
3. Gauge Pressure ($p_{gauge}$): Measured relative to atmospheric pressure: \[ p_{gauge} = p_{abs} - p_{atm} \]

Step 3: Detailed Explanation:

• When the pressure of a system is greater than atmospheric pressure, the gauge pressure is positive: \[ p_{gauge} = p_{abs} - p_{atm} > 0 \]
• When the pressure of a system is less than atmospheric pressure, the pressure is below atmospheric pressure. This is called negative gauge pressure, which is also referred to as vacuum or suction pressure: \[ p_{vacuum} = p_{atm} - p_{abs} \]
• The broad term that encompasses measurements both above and below atmospheric pressure is gauge pressure, where positive gauge represents pressures above atmospheric and negative gauge represents pressures below atmospheric.

• Hence, gauge pressure is the correct general term.

Step 4: Final Answer:
The pressure measured above or below the atmospheric pressure is known as gauge pressure.