Absolute pressure is equal to:
Try it first: pick an option, then read the explanation.
Asked in SSC JE 06/06/2024 (09am-11am)
Detailed Explanation
Fluid pressure can be referenced either against a perfect vacuum (absolute zero pressure datum) or against the prevailing ambient atmospheric pressure. The fundamental relationship links absolute, gauge, and atmospheric pressure.
Definitions of pressure reference datums:
- Absolute pressure (\(p_{\text{abs}}\)): Pressure measured above absolute zero (a total void/perfect vacuum).
- Atmospheric pressure (\(p_{\text{atm}}\)): Ambient pressure exerted by the earth's atmosphere (standard sea-level value is \(101.325\text{ kPa}\)).
- Gauge pressure (\(p_{\text{gauge}}\)): Pressure measured relative to local atmospheric pressure by common pressure gauges.
The standard governing equation is:
\[ p_{\text{abs}} = p_{\text{atm}} + p_{\text{gauge}} \]
If the fluid pressure is below atmospheric, it is called vacuum pressure:
\[ p_{\text{abs}} = p_{\text{atm}} - p_{\text{vacuum}} \]
Why “Gauge pressure + Atmospheric pressure” Is Correct
Gauge pressure represents the excess pressure over and above the ambient atmospheric pressure. Adding local atmospheric pressure to the gauge reading restores the complete pressure measured from the absolute zero baseline.
Why the Other Options Are Wrong
- A. Gauge pressure − Atmospheric pressure: Gauge pressure minus atmospheric pressure is physically meaningless for positive pressures and would yield a negative value.
- B. Atmospheric pressure − Gauge pressure: Atmospheric pressure minus gauge pressure is incorrect; atmospheric pressure minus vacuum pressure gives absolute pressure.
- C. Atmospheric pressure × Gauge pressure: Multiplying pressures is dimensionally incorrect; pressure relationships are additive and linear.
Key Point
p_abs = p_atm + p_gauge (for p > p_atm) and p_abs = p_atm − p_vacuum (for p < p_atm).
Key Concepts for Students
- Absolute pressure cannot be negative; absolute zero represents complete molecular inactivity.
- Ordinary Bourdon gauges and manometers read zero when open to the atmosphere (gauge pressure = 0).
- Standard atmospheric pressure equals 101.325 kPa = 1.01325 bar = 760 mm of Hg = 10.33 m of water.
Practice more: this is question 2 in Hydrostatic Pressure MCQs – Part 1. Try the whole set and check every answer instantly.