In gauge pressure, the atmospheric pressure on the scale is marked as :
Try it first: pick an option, then read the explanation.
Asked in SSC JE 23-09-2019 (Evening)
Detailed Explanation
Pressure measurement scales are defined by their reference datum. The gauge pressure scale sets the ambient atmospheric pressure as its zero baseline.
1. Definition of gauge pressure:
\[ p_{\text{gauge}} = p_{\text{abs}} - p_{\text{atm}} \]
2. When the measured pressure equals local atmospheric pressure (\(p_{\text{abs}} = p_{\text{atm}}\)):
\[ p_{\text{gauge}} = p_{\text{atm}} - p_{\text{atm}} = 0 \]
3. Consequently, all standard dial pressure gauges, tire gauges, and open manometers read zero when vented to the atmosphere.
Why “0” Is Correct
Because gauge pressure represents the differential pressure above or below atmospheric pressure, the ambient atmospheric pressure itself serves as the zero point and is marked as 0.
Why the Other Options Are Wrong
- A. 1: 1 represents 1 atmosphere or 1 bar on absolute scales, not the zero mark of gauge pressure.
- B. 10: 10 represents 10 metres of water head, not the zero reference mark.
- C. 100: 100 represents 100 kPa (approx 1 bar) in absolute SI units.
Key Point
Atmospheric pressure is marked as 0 on the gauge pressure scale (p_gauge = 0 at p_atm).
Key Concepts for Students
- Pressures above atmospheric have positive gauge pressure: p_gauge > 0.
- Pressures below atmospheric have negative gauge pressure (vacuum): p_gauge < 0.
- Absolute zero pressure corresponds to a gauge pressure of −p_atm (−101.325 kPa at sea level).
Practice more: this is question 13 in Measurement of Pressure MCQs – Part 1. Try the whole set and check every answer instantly.