Aircraft Pressure Gauges: Aneroid, Bellows, and Bourdon Applications

Pressure gauges provide critical information about the state of aircraft systems, from engine health to flight data. The choice of pressure sensor depends heavily on the magnitude of the pressure being measured, ranging from delicate atmospheric variations to high-pressure hydraulic lines.

Pressure Sensing Mechanisms

Pressure sensors are generally classified by the range of pressure they are designed to measure:

Low Pressure: Aneroid Capsules

Aneroid capsules are sealed diaphragms that have been evacuated to a near-vacuum state. This internal vacuum makes them sensitive to absolute pressure (pressure measured relative to a full vacuum). Due to their thin and delicate structure, they are used for precision sensing in low-pressure systems. A classic example is the altimeter, which uses an aneroid capsule to measure static pressure; as the aircraft climbs and atmospheric pressure decreases, the capsule expands. The machmeter also utilizes an aneroid capsule to determine differential pressure.

Medium Pressure: Bellows

Bellows consist of several diaphragms joined together in a series. They are typically employed for pressure measurements in the medium range. Applications include monitoring the Low Pressure (LP) Booster pump or the gas turbine intake pressure. A common use is in the Manifold Absolute Pressure (MAP) gauge, which displays the absolute pressure of the fuel/air mixture in the inlet manifold. The MAP gauge measuring element often pairs two bellows: one aneroid (sealed) and one connected to the manifold pressure, with the movement reflecting the absolute pressure.

High Pressure: Bourdon Tubes

For high-pressure systems, the Bourdon tube is the standard instrument. It consists of a rigid, hollow metal tube shaped like a 'C'. Fluid enters the tube through a damping choke. As internal pressure builds, the tube tends to straighten or "unroll," and this movement is transferred through a mechanical linkage to a pointer on the gauge.

• Stiffness: The pressure range of a Bourdon tube is determined by the material's stiffness—stiffer metals are used for higher pressures.
• Temperature Compensation: A bi-metal compensator is often fitted to correct for stiffness changes caused by temperature variations in the fluid.
• Applications: Engine oil pressure, oxygen bottle pressure, and direct reading hydraulic gauges.

Remote Sensing in High-Pressure Systems

While Bourdon tubes can directly measure high pressures (such as in hydraulic systems operating at 3000 psi), running high-pressure lines directly to the cockpit indicators is often impractical due to weight, complexity, and safety concerns.

Modern aircraft address this by using pressure transducers. Located remotely near the pressure source, transducers convert the physical pressure into an electrical signal. This signal is then transmitted to the cockpit for display, eliminating the need for long, heavy, and potentially hazardous high-pressure piping.