Surrounding atmospheric pressure is taken as datum to find _______.
Try it first: pick an option, then read the explanation.
Asked in SSC JE 05/06/2024 (09am-11am)
Detailed Explanation
Pressure measurements require a reference datum. Absolute zero pressure represents complete vacuum. When surrounding atmospheric pressure is chosen as the reference level (datum = 0), pressures above it are measured as gauge pressures.
Basic pressure relationships with respect to atmospheric datum:
\[ p_{\text{abs}} = p_{\text{atm}} + p_{\text{gauge}} \]
\[ p_{\text{vacuum}} = p_{\text{atm}} - p_{\text{abs}} \]
Commission note: Both gauge pressure (above atmospheric) and vacuum pressure (below atmospheric) use atmospheric pressure as their datum. Because both (b) and (d) are conceptually referenced to atmospheric pressure, this question was subsequently cancelled by the SSC JE examination authority.
Why “gauge pressure” Is Correct
Gauge pressure is the standard engineering term for pressure referenced to atmospheric datum. Standard pressure gauges read zero at atmospheric pressure.
Why the Other Options Are Wrong
- A. absolute pressure: Absolute pressure uses absolute zero (a complete vacuum) as its datum, not atmospheric pressure.
- C. Null pressure: Null pressure is not a standard thermodynamic pressure term.
- D. vacuum pressure: Vacuum pressure also uses atmospheric pressure as its datum (measuring pressure deficit below atmospheric); this ambiguity led to cancellation of the question.
Key Point
Gauge pressure is measured above atmospheric pressure datum; p_abs = p_atm + p_gauge.
Key Concepts for Students
- Atmospheric datum gives gauge pressure: positive above atm, negative (vacuum) below atm.
- Absolute pressure datum is absolute zero (perfect vacuum) and can never be negative.
- 1 standard atmosphere = 101.325 kPa = 760 mm of Hg = 10.3 m of water.
Practice more: this is question 7 in Properties of Fluid MCQs – Part 1. Try the whole set and check every answer instantly.