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: Set up a simple pressure number line with perfect vacuum at 0 and local atmospheric pressure marked at some positive value \(P_{atm}\).
Step 2: Absolute pressure is read directly off this line starting at 0, and nothing on this line sits to the left of 0, so absolute pressure is always positive, matching (A).
Step 3: Gauge pressure is just the same line but re-zeroed at \(P_{atm}\) instead of at 0. Moving the zero point does not change the actual pressures, it only changes what number we assign to them, and the new reading equals the old absolute value minus \(P_{atm}\). That is the definition in (C).
Step 4: Anything that reads negative on this shifted scale is below atmospheric pressure, and this negative reading is exactly what engineers call vacuum, so (D) is just restating the shifted scale in different words.
Step 5: Since the line never goes below 0 on the absolute scale, the most negative the shifted gauge reading can become is \(0 - P_{atm}\), that is, minus the atmospheric value. So the vacuum reading can equal atmospheric pressure at best, at perfect vacuum, but never go beyond it, which is what (B) is saying.
Step 6: Every statement lines up with this single number line picture.
\[\boxed{\text{(A), (B), (C) and (D)}}\]