Toilets and atmospheric pressure

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.

Few passengers give a second thought to the powerful whoosh of an airplane toilet, but it conceals a fascinating application of gas physics. Unlike household toilets that rely on gravity and several liters of water, aircraft toilets use a vacuum drainage system that operates on the pressure difference between the cabin and a waste tank maintained at lower pressure. When the flush valve opens, air rushes from the cabin into the waste pipe at near-sonic speeds, carrying waste efficiently with minimal water. This brief opening causes a measurable drop in the local atmospheric pressure, a transient depressurization that a smartphone barometer can detect. The event provides a real-world application of the Boyle-Mariotte law (PV = constant at constant temperature): as air is removed from the small lavatory volume, the pressure decreases proportionally. By measuring this pressure drop with FizziQ, students can estimate the volume of air evacuated and understand the engineering principles behind one of aviation's most taken-for-granted technologies.

Learning objectives:

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.

Level:

High school

FizziQ

Author:

Duration (minutes) :

20

What students will do :

- Measure the atmospheric pressure variation in an aircraft lavatory during a flush using the smartphone barometer
- Observe the characteristic pressure drop and recovery caused by the vacuum drainage system
- Apply the Boyle-Mariotte law (PV = constant) to estimate the volume of air evacuated
- Understand the engineering principles of vacuum drainage systems used in aviation
- Connect pressure measurement to practical applications of gas physics

Scientific concepts:

- Atmospheric pressure
- Depressurization
- Boyle-Mariotte law
- Vacuum drain systems
- Pressure-volume relationship

Sensors:

- Barometer (atmospheric pressure sensor)

What is required:

- Smartphone with FizziQ application and barometric sensor
- A plane flight
- FizziQ experience notebook

Experimental procedure:

Before the flight, open FizziQ and verify that the Barometer sensor is available and reading the local atmospheric pressure (approximately 1013 hPa at sea level).
During the flight, note the cabin pressure reading (typically 750-800 hPa at cruising altitude, equivalent to ~2000 m altitude).
Enter the aircraft lavatory with your smartphone. Close the door and wait 10 seconds for the pressure to stabilize.
Start recording the atmospheric pressure at the highest available sampling rate.
After recording a 5-second baseline, activate the toilet flush.
Continue recording for at least 15 seconds after the flush, capturing the full pressure drop and recovery.
Stop recording and exit the lavatory.
Examine the pressure-time graph. You should see a sharp pressure drop of approximately 0.5-2 hPa at the moment of the flush, followed by a gradual recovery.
Measure the magnitude of the pressure drop (ΔP) and the recovery time.
Estimate the volume of the lavatory (typically about 1.5-2.5 m³ for a standard aircraft lavatory).
Apply the Boyle-Mariotte law: ΔV = V × ΔP / P, where V is the lavatory volume, ΔP is the pressure drop, and P is the ambient cabin pressure.
Calculate the estimated volume of air evacuated through the drainage system during the flush. Compare with typical values (about 1-5 liters).

Expected results:

The pressure drop during a flush typically ranges from 0.5 to 2.0 hPa, lasting 1-3 seconds, followed by a recovery over 5-15 seconds as cabin air seeps back through gaps around the door. For a lavatory volume of 2 m³ at a cabin pressure of 770 hPa, a 1 hPa drop corresponds to an evacuated volume of approximately 2.6 liters using the Boyle-Mariotte law. The barometer's precision (typically ±0.1 hPa) is sufficient to clearly detect this event. Students may also observe smaller pressure fluctuations from the aircraft ventilation system as background. The recovery time depends on how well sealed the lavatory door is.

Scientific questions:

- Why do aircraft use vacuum drainage instead of gravity-based toilet systems?
- How does the Boyle-Mariotte law relate the pressure drop to the volume of air evacuated?
- What maintains the low pressure in the aircraft waste tank?
- Why is the cabin pressure lower than sea-level pressure during flight?
- What would happen if the flush valve remained open? Would the cabin depressurize?
- How does temperature affect the Boyle-Mariotte calculation?

Scientific explanations:

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.

Extension activities:

Frequently asked questions:

Q: I do not see any pressure change during the flush. Is my barometer working?
R: Some aircraft lavatories are well-ventilated, which may minimize the pressure drop. Also ensure you are recording at the highest sampling rate. Try standing as close to the toilet as possible.

Q: The pressure seems to fluctuate even without flushing. Is this normal?
R: Yes, aircraft ventilation systems create continuous small pressure variations (±0.1-0.3 hPa). The flush event should be clearly distinguishable as a sudden, larger drop.

Q: Why does the pressure recover gradually rather than instantly?
R: After the flush valve closes, air slowly seeps back into the lavatory through ventilation ducts and gaps around the door. The recovery time depends on the air exchange rate.

Q: Can I do a similar experiment at home with a regular toilet?
R: Home toilets do not create significant air pressure changes because they use gravity and water displacement rather than vacuum suction. The aircraft vacuum system is unique in this regard.