---
title: "Atmospheric, gauge, absolute pressure"
subject: "Fundamental of Thermodynamics & Heat Transfer"
chapter: 1
key: "semester_2/fundamental_of_thermodynamics_and_heat_transfer:atmospheric-gauge-absolute-pressure"
source: https://www.bctengineer.com/q/semester_2/fundamental_of_thermodynamics_and_heat_transfer/atmospheric-gauge-absolute-pressure
site: BCT Engineering
---

# Atmospheric, gauge, absolute pressure

Fundamental of Thermodynamics & Heat Transfer · Chapter 1: Introduction

- Asked in: 2071 Bhadra, 4 marks
- Video: [What is Definitions of Absolute Pressure, Guage Pressure, Atmospheric Pressure, Vaccum Pressure](https://www.youtube.com/watch?v=MJ_zjWyRHX8) — Ekeeda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/fundamental_of_thermodynamics_and_heat_transfer/atmospheric-gauge-absolute-pressure
- Whole chapter: https://www.bctengineer.com/md/semester_2/fundamental_of_thermodynamics_and_heat_transfer/chapter-1

## Question

Define atmospheric pressure, gauge pressure and absolute pressure. Also write down the relationship between them.

## Short answer

Atmospheric pressure is exerted by the air column; gauge pressure is relative to local atmospheric pressure; absolute pressure is measured from vacuum. Above atmosphere, P_abs = P_atm + P_gauge; below, P_abs = P_atm - P_vacuum.

## Answer

- **Atmospheric pressure (P_atm):** the pressure exerted by the weight of the air column above the earth's surface. At sea level it is 101.325 kPa = 760 mm Hg = 1 atm. It is measured by a barometer.
- **Gauge pressure (P_gauge):** the pressure of a fluid measured relative to the local atmospheric pressure, i.e. the reading of a pressure gauge or manometer. It is positive when the pressure is above atmospheric.
- **Absolute pressure (P_abs):** the pressure measured relative to a perfect vacuum (zero pressure). It is the true pressure used in thermodynamic equations.

#### Relationship
For pressure above atmosphere:  
**P_abs = P_atm + P_gauge**

For pressure below atmosphere (vacuum):  
**P_abs = P_atm − P_vacuum**

```text
 P_abs
  ^
  |   ___ pressure above atm: P_gauge
  | ----------------- P_atm (local)
  |   ___ pressure below atm: P_vacuum
  | ----------------- absolute zero (vacuum)
```

Example: a tyre gauge reads 200 kPa where P_atm = 101 kPa, so P_abs = 301 kPa.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Absolute pressure is measured relative to
   A. local atmospheric pressure
   B. a perfect vacuum
   C. sea-level gauge
   D. the boiling point of water
   Answer: B. Absolute pressure is measured from zero pressure (perfect vacuum); gauge pressure is relative to atmosphere. (Key point: Absolute pressure)

2. For pressure above atmosphere, the relation is
   A. P_abs = P_atm - P_gauge
   B. P_gauge = P_abs + P_atm
   C. P_abs = P_atm + P_gauge
   D. P_abs = P_gauge only
   Answer: C. Gauge pressure is the excess over atmospheric pressure, so P_abs = P_atm + P_gauge. (Key point: Relationship above atmosphere)

3. For pressure below atmosphere, P_abs equals
   A. P_atm + P_vacuum
   B. P_vacuum - P_atm
   C. P_vacuum
   D. P_atm - P_vacuum
   Answer: D. A vacuum reading is how far below atmosphere the pressure is, so P_abs = P_atm - P_vacuum. (Key point: Vacuum pressure)

4. A tyre gauge reads 200 kPa and P_atm = 101 kPa. The absolute pressure is
   A. 301 kPa
   B. 99 kPa
   C. 200 kPa
   D. 101 kPa
   Answer: A. P_abs = 101 + 200 = 301 kPa. (Key point: Tyre example)

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