Toilets and atmospheric pressure
Studying depressurization in airplane toilets
Author:
Title 4
Learning objectives :
This activity allows students to study a depressurization phenomenon by measuring pressure variations during the operation of airplane toilets. It allows you to concretely apply the gas laws during a flight.
Concepts covered
Atmospheric pressure; Depressurization; Boyle-Mariotte law; Vacuum drain systems; Pressure-volume relationship
What students will do :
The student uses the FizziQ barometer to measure variations in atmospheric pressure in an airplane toilet when the toilet is flushed. By recording the pressure before during and after using the toilet the student can observe a temporary drop in pressure and then approximately calculate the volume of air drawn in using the Boyle-Mariotte law.
What is required :
Smartphone with FizziQ application and barometric sensor; A plane flight; FizziQ experience notebook
Scientific background :
Modern aircraft toilets operate on a fundamentally different principle from conventional toilets. Instead of using gravity and a large volume of water, they operate a vacuum drain system. This choice is explained by several factors: water saving, weight reduction, and operation independent of the orientation of the device. When the flush is activated, a valve opens briefly between the bowl and a conduit maintained at negative pressure. This pressure difference creates a powerful suction that evacuates the waste to a central tank. This phenomenon causes a momentary drop in atmospheric pressure in the toilet cubicle, generally of the order of 5-15 hPa (0.5-1.5% of ambient pressure). A smartphone barometer, capable of measuring variations of around 0.1 hPa, is perfectly suited to detect this change. The cabin pressure of an airliner is maintained around 750-800 hPa (equivalent to an altitude of 1800-2400 meters), much lower than the pressure at sea level (1013 hPa). This partial pressurization represents a compromise between passenger comfort and the structural constraints of the aircraft. To estimate the volume of air sucked in when the flush is activated, the Boyle-Mariotte law can be applied (PV = constant for a gas at constant temperature). If V₁ is the initial volume of the toilet, P₁ the initial pressure, and P₂ the pressure after activation, then the volume of sucked air ΔV can be approximated by: ΔV = V₁×(P₁-P₂)/P₁. For a toilet cubicle of approximately 2 m³ and a pressure drop of 10 hPa from 800 hPa, this represents approximately 25 liters of air sucked in in a few seconds, explaining the characteristic noise and physical sensation when using these facilities.