Difference Between Atmospheric Pressure and Gauge Pressure with Examples
Atmospheric pressure and gauge pressure are foundational concepts in fluid mechanics and daily life applications. Atmospheric pressure is the force exerted by the air above the Earth's surface, while gauge pressure measures how much greater or lesser the pressure inside a system is compared to the surrounding atmosphere. Understanding these concepts is essential for solving problems involving fluids, weather, engineering systems, and even biological processes like blood flow.
To begin, atmospheric pressure results from the weight of the air column above a given point. At sea level, this value is typically about 1.013 × 105 N/m2 (Pascals). When a pressure gauge is used—such as a tire gauge or a blood pressure cuff—it usually indicates gauge pressure, which is defined as the difference between the internal pressure of the system and the atmospheric pressure outside. If the gauge reads zero, the pressure inside is equal to the atmospheric pressure.
Gauge pressure can be both positive (when internal pressure exceeds atmospheric) or negative (when it's below atmospheric, often called "vacuum" pressure). In contrast, absolute pressure measures pressure relative to an absolute vacuum, so it is always a positive value unless in extreme scenarios. The core relationship between these pressures is:
- Absolute Pressure (Pabs) = Gauge Pressure (Pg) + Atmospheric Pressure (Patm)
- Gauge Pressure (Pg) = Absolute Pressure (Pabs) − Atmospheric Pressure (Patm)
Mechanical devices such as aneroid gauges and manometers are commonly used to measure pressure. An aneroid gauge relies on the deformation of a flexible metal element, which moves a pointer on a scale. A manometer, often a U-shaped tube partially filled with liquid, measures pressure based on the height difference (h) between the two sides. This height directly relates to the pressure difference by the equation P = hρg, where ρ is the fluid density and g is the acceleration due to gravity.
For example, when using an open-tube manometer, if the heights of fluid on both sides are equal, the pressures must balance. This occurs when both sides are exposed to atmospheric pressure. However, connecting one side to a source of unknown pressure (like a balloon or a vacuum jar) causes the column heights to differ. The difference in heights, along with fluid density, indicates the gauge pressure.
| Type of Pressure | Description | Reference Point | Can be Negative? |
|---|---|---|---|
| Atmospheric Pressure | Pressure due to Earth's atmosphere on all surfaces | Perfect vacuum | No |
| Gauge Pressure | Measured above/below atmospheric pressure | Atmospheric pressure | Yes |
| Absolute Pressure | Total pressure relative to zero (vacuum) | Perfect vacuum | No |
To apply these concepts, consider a car tire. If the tire gauge reads 34 psi, this is the gauge pressure—34 psi above atmospheric. The actual pressure inside the tire (absolute) is this gauge pressure plus atmospheric pressure (about 14.7 psi at sea level): 34 + 14.7 = 48.7 psi.
The principle extends to biological systems. Blood pressure, for instance, is typically measured as gauge pressure, since what matters for blood flow is the pressure difference from the atmosphere. Manometers and barometers (which use mercury or water) further allow precise measurements by relating the height (h) of a liquid column to pressure, using the core formula:
- P = hρg, where P = pressure difference, h = height of fluid column, ρ = density of fluid, g = acceleration due to gravity.
| Device | Working Principle | Typical Application |
|---|---|---|
| Aneroid Gauge | Bellows expand with pressure, moving a pointer | Tire/blood pressure measurement |
| Open-tube Manometer | Height difference of fluid indicates pressure difference | Laboratory pressure measurement |
| Mercury Barometer | Atmospheric pressure supports mercury column | Weather and altitude sensing |
A quick example: To find the height to hang an IV bag (assuming fluid density = 1.00 g/mL) so that it just enters a vein with a blood pressure of 18 mm Hg above atmospheric, convert 18 mm Hg to Pascals (18 × 133 = 2400 Pa). Then use h = Pg/(ρg). With ρ = 1000 kg/m3 and g = 9.8 m/s2:
- h = 2400/(1000 × 9.8) ≈ 0.24 m
Therefore, 0.24 meters is the minimum height for the IV bag above the entry point. This calculation shows the direct role of pressure difference in medical and engineering contexts.
| Common Pressure Units | Relation to 1 atm | Value |
|---|---|---|
| Pascal (Pa) | 1 atm = 1.013 × 105 Pa | 101300 Pa |
| mm Hg (Torr) | 1 atm = 760 mm Hg | 760 mm Hg |
| bar | 1 atm = 1.013 bar | 1.013 bar |
| lb/in2 (psi) | 1 atm = 14.7 psi | 14.7 psi |
For more on pressure concepts, visit Pressure – Definitions, Units & Formula and Atmospheric Pressure. Mastering these principles prepares you for deeper topics like hydrostatics and gas laws.
To practice, try solving: If a manometer contains water and shows a height difference of 0.050 m, what is the gauge pressure? Use P = hρg. If ρ = 1000 kg/m3, g = 9.8 m/s2:
- P = 0.05 × 1000 × 9.8 = 490 Pa (gauge pressure)
If you want to reinforce these concepts, review Hydrostatic Pressure & Fluid Pressure and Pascal’s Law for practical applications and advanced questions.
| Key Steps for Pressure Problem Solving | Example |
|---|---|
| Identify if pressure given is gauge or absolute | Tire gauge reads 34 psi (gauge); actual = 34 + 14.7 (atm) = 48.7 psi |
| Use correct conversion factors and units | 1 mm Hg = 133 Pa; 1 atm = 101300 Pa |
| Apply P = hρg for fluids | h = P/ρg (find height for given pressure difference) |
To continue your learning journey, explore these next steps:
FAQs on Understanding Atmospheric Pressure and Gauge Pressure
1. What is the difference between atmospheric pressure and gauge pressure?
Atmospheric pressure is the pressure exerted by the Earth's atmosphere at a given point, while gauge pressure is the difference between the measured pressure and atmospheric pressure.
• Gauge Pressure = Absolute Pressure − Atmospheric Pressure
• Atmospheric pressure is always positive and standard at sea level (~1.013 × 105 Pa).
• Gauge pressure can be positive (above atmospheric) or negative (below atmospheric).
2. What is absolute pressure in fluids?
Absolute pressure is the total pressure exerted on a system, measured from absolute vacuum.
• It is the sum of atmospheric pressure and gauge pressure.
• Formula: Absolute Pressure (Pabs) = Atmospheric Pressure (Patm) + Gauge Pressure (Pg)
3. How do you calculate gauge pressure at a given depth in a fluid?
Gauge pressure in a fluid can be calculated using hydrostatic principle:
Formula: Pg = hρg
Where: h = depth of the fluid, ρ = density of the fluid, g = acceleration due to gravity.
4. Is atmospheric pressure always 14.7 psi?
No, atmospheric pressure is approximately 14.7 psi (pounds per square inch) only at sea level under standard conditions.
• It decreases with altitude (higher places have lower atmospheric pressure).
• Atmospheric pressure can also change with weather conditions.
5. What units are used to measure atmospheric and gauge pressure?
Common units for atmospheric and gauge pressure include:
• Pascal (Pa) - SI unit
• Atmosphere (atm)
• Bar
• Millimeter of mercury (mmHg)
• Pounds per square inch (psi)
Atmospheric pressure at sea level ≈ 1 atm ≈ 1.013 × 105 Pa ≈ 760 mmHg ≈ 14.7 psi.
6. How does atmospheric pressure affect gauge pressure readings?
Gauge pressure is measured relative to atmospheric pressure.
• If atmospheric pressure changes, the gauge pressure reading for the same absolute pressure will change.
• Most gauges are calibrated to read zero at current atmospheric pressure, so an increase or decrease in atmospheric pressure affects what is displayed on the gauge.
7. Can gauge pressure be negative? If yes, what does it mean?
Yes, gauge pressure can be negative.
• A negative gauge pressure means the measured pressure is below atmospheric pressure (common in vacuum systems or suction situations).
• Absolute pressure, however, cannot be negative, as the lowest possible value is zero (total vacuum).
8. What is the formula for converting gauge pressure to absolute pressure?
To find absolute pressure from gauge pressure, add atmospheric pressure:
• Absolute Pressure (Pabs) = Gauge Pressure (Pg) + Atmospheric Pressure (Patm)
Use correct units throughout the calculation.
9. Explain with an example: How do you use a manometer to measure gauge pressure?
A manometer is a U-shaped tube filled with liquid.
• One side is open to the atmosphere, the other connects to the vessel containing the gas or liquid to be measured.
• The difference in liquid levels (h) gives gauge pressure: Pg = hρg
• For example, if the liquid in the manometer rises by 10 cm (0.1 m) on one side, using ρ = 1000 kg/m3 (for water) and g = 9.8 m/s2: Pg = 0.1 × 1000 × 9.8 = 980 Pa.
10. Why is gauge pressure used in everyday measurements instead of absolute pressure?
Gauge pressure is commonly used because:
• It measures pressure relative to the current ambient atmospheric pressure (what we actually feel and interact with).
• Most practical devices (like tire gauges, blood pressure cuffs) are designed to show pressure above or below atmospheric air, making readings simpler and more relevant for daily use.
11. What happens to atmospheric pressure as you go up a mountain?
As altitude increases (e.g., climbing a mountain), atmospheric pressure decreases because there is less air above exerting downward force.
• For every ~12 meters of elevation, atmospheric pressure drops by about 1 mmHg.
12. What is the significance of absolute, gauge and atmospheric pressure in competitive exams?
Understanding the difference and relations between absolute, gauge, and atmospheric pressures is essential for correctly solving Physics numericals in exams like JEE, NEET, and CBSE.
• Accurate application of formulas ensures correct answers in pressure-based numericals, fluid mechanics, and problem-solving scenarios.
• These concepts are frequently tested in one-mark formula questions and reasoning-based MCQs.
