Question

Consider the following statements:
(A) Absolute pressure is always positive.
(B) Vacuum can not exceed local atmospheric pressure.
(C) Gage pressure is the difference between absolute pressure and atmospheric pressure.
(D) Negative gauge pressure is same as vacuum.

Choose the correct answer from the options given below:

• A. (A), (B) and (C) only.
• B. (B), (C) and (D) only.
• C. (A), (C) and (D) only.
• D. (A), (B), (C) and (D).

Hint:

Draw a simple absolute-pressure number line starting at zero and see where gauge pressure and vacuum sit on it.

Answer 1

The Correct Option is D

Step 1: Recall the pressure scale used in fluid mechanics. Absolute pressure is measured starting from a perfect vacuum, taken as zero. Since nothing can be emptier than a perfect vacuum, absolute pressure can never go below zero, so it is always positive. This matches statement (A).
Step 2: Gauge pressure is measured relative to the local atmospheric pressure instead of a perfect vacuum, using the relation \(P_{gauge} = P_{abs} - P_{atm}\). This is exactly what statement (C) describes, so (C) is correct.
Step 3: When a location has pressure below atmospheric, gauge pressure comes out negative, and this negative gauge pressure is what we call vacuum pressure. So vacuum is just another name for negative gauge pressure, confirming statement (D).
Step 4: Now look at how far vacuum can go. Using \(P_{gauge} = P_{abs} - P_{atm}\) and knowing absolute pressure cannot drop below zero, the smallest gauge pressure possible is \(0 - P_{atm} = -P_{atm}\). So the magnitude of vacuum can reach at most the value of local atmospheric pressure, corresponding to a perfect vacuum, and cannot exceed it. This confirms statement (B).
Step 5: All four statements (A), (B), (C) and (D) check out as correct.