Question

Consider the following statements regarding the pressure measured at a point in a fluid.
S1: Absolute pressure is gauge pressure plus atmospheric pressure.
S2: Gauge pressure is measured by considering absolute zero pressure as datum.
S3: Absolute pressure is measured by considering atmospheric pressure as datum.
S4: Vacuum pressure is negative pressure obtained by subtracting absolute pressure from atmospheric pressure.
Which ONE of the following options lists all the TRUE statements?

• A. S1 and S2
• B. S2 and S3
• C. S1 and S4
• D. S1, S2, and S4

Hint:

Write out P_abs = P_atm + P_gauge and P_vac = P_atm - P_abs, then check each statement's datum claim against these standard relations.

Answer 1

The Correct Option is C

This question checks the definitions used in fluid mechanics for absolute, gauge, and vacuum pressure, and asks which of four statements are correct.
Step 1: Check S1. By definition, absolute pressure at a point equals gauge pressure plus the local atmospheric pressure, written as \(P_{abs} = P_{gauge} + P_{atm}\). This is the standard relation used throughout fluid mechanics, so S1 is TRUE.
Step 2: Check S2. Gauge pressure is measured relative to the local atmospheric pressure, which is taken as the zero reference (datum), not relative to absolute zero pressure. A pressure gauge reads zero when exposed to the atmosphere, confirming that atmospheric pressure, not absolute zero, is its datum. So S2 is FALSE.
Step 3: Check S3. Absolute pressure is measured with a perfect vacuum (absolute zero pressure) as the datum, not atmospheric pressure. This is the opposite of what S3 claims, so S3 is FALSE.
Step 4: Check S4. Vacuum pressure applies when the absolute pressure at a point is below atmospheric pressure. It is defined as \(P_{vac} = P_{atm} - P_{abs}\), which is exactly subtracting absolute pressure from atmospheric pressure, and it is reported as a negative gauge pressure. So S4 is TRUE.
Step 5: Conclude. Only S1 and S4 are true, which corresponds to option (C).