Note that if nothing is connected to a pressure gauge, it shows zero.
Or absolute pressure equals gauge pressure plus atmospheric pressure: That means gauge pressure is equal to absolute pressure minus atmospheric pressure: At the same time, we usually measure gauge pressure, which is zero-referenced pressure against ambient air pressure (101.325 kPa or 760 mm Hg). Absolute pressure is zero referenced against a perfect vacuum. Here the subscripts 1 and 2 refer to the initial and final states of a gas in a system.Īll the above formulas are valid only for absolute pressure. It is convenient to write this law for comparing the same gas in a closed system the volume of which can be freely changed under the two different sets of conditions in the following form: Where P is the gas pressure, V is the gas volume and k is the constant. Mathematically, Boyle’s law is expressed as: If we double the gas pressure, its volume will be halved. The absolute pressure exerted by a given mass of an ideal gas is inversely proportional to the volume it occupies if its temperature and amount remain unchanged.
In English-speaking countries, they prefer referring to it as Boyle’s law or first gas law because Boyle formulated the law 17 years earlier than Mariotte. In many countries, this law is called Boyle-Mariotte law. Alternatively, you can enter the atmospheric pressure P atm = 100 kPa, calculate the initial gauge pressure P g1 = P 1 – P atm = 100 – 100 = 0 kPa (if a pressure gauge is not connected, it shows zero!) and the final gauge pressure P g2 = P 2 – P atm = 80 – 100 = –20 kPa and enter these values to the calculator.Īt the end of the 17th century, an Anglo-Irish natural philosopher, chemist, physicist, and inventor Robert Boyle and a French physicist Edme Mariotte conducted experiments to determine the relationship between air pressure and volume and independently discovered that the pressure of a gas is inversely proportional to the volume occupied by it if its temperature and mass remain constant. Solution: Because this calculator accepts only gauge pressure, enter the initial pressure P 1 = 100 kPa, final pressure P 2 = 80 kPa and set the atmospheric pressure to zero. What volume will this gas have at cruising altitude where cabin drops to P 2 = 80 kPa (absolute pressure) if the atmospheric pressure at the airport is 100 kPa? What is the reading P g2 of a pressure gauge attached to the second vessel if the temperature remains constant and the atmospheric pressure is P atm = 100 kPa?Įxample 2: An airplane passenger has V 1 = 150 mL of air in his stomach before takeoff. The gas was transferred to another vessel with a volume of V 2 = 3 m³. A pressure gauge of the vessel shows P g1 = 200 atm. If you want to determine the absolute pressure instead, just set the atmospheric pressure to zero.Įxample 1: Methane gas is stored in a pressure vessel with a volume of V 1 = 2.5 m³.
This calculator determines the initial and final volumes and gauge pressures of a fixed amount of an ideal gas if its temperature remains constant as described by Boyle’s law.