Advanced Aircraft Pressure Measuring Devices and Indicators

Pressure Measurement Fundamentals and Device Classifications

• Definition of Gauge Pressure (psig): Gauge pressure is effectively a differential pressure measurement that calculates the difference between the prevailing atmospheric pressure (standardized at $14.7\,psia$) and the internal pressure of a referenced system.
• Definition of Differential Pressure (psid): This is the difference between two unrelated pressures. Measuring this typically requires a bellows or a diaphragm mechanism.
• Application Limitations: Capsules, bellows, and diaphragms are inherently unsuitable for measuring very high pressures.
• High-Pressure Measurement: For extremely high-pressure environments, such as hydraulic systems, the Bourdon tube type gauge is the industry standard.
• Device Sensitivity Hierarchy (Highest to Lowest):
  • Aneroid Capsule
  • Diaphragm
  • Bellows
  • Bourdon Tube

Barometers and Atmospheric Monitoring

• Household Barometer Construction: These are simple aneroid barometers that utilize an aneroid capsule to detect changes in atmospheric pressure.
• Calibration: Most barometer instruments are calibrated in Millibars ($mbar$).
• Measurement Range: These devices have a very limited range, typically spanning from $980$ to $1040\,mbar$.
• Transportation Warning: A barometer would be damaged if transported by air. This is because the internal pressure changes inside an aircraft would cause the aneroid capsule's range of movement to exceed the mechanical limits of the pointer, leading to physical failure.
• Meteorological Implications:
  • Low Atmospheric Pressure: Results in stormy conditions, such as tropical lows and cyclones.
  • High Atmospheric Pressure: Associated with stable, clear weather (referred to as "great fishing weather").

Aneroid Capsule Design and Applications

• Core Functions in Aviation: Aneroid capsules are used to measure atmospheric pressure altitude and other parameters relative to a vacuum or absolute pressure.
• Primary Instruments Based on Aneroid Capsules:
  • Barometers
  • Altimeters
  • Manifold pressure gauges
• Accuracy Enhancement: A stack of aneroid capsules is often used to increase the sensitivity of an instrument, thereby improving its overall accuracy.
• Operating Characteristics:
  • Capsules may be evacuated (true aneroid) or filled with a specific pressure of inert gas.
  • They are designed to measure very small pressure changes with reference to a vacuum.
  • Operating Range: Generally from $0\,psia$ (absolute vacuum) up to approximately $17\,psia$.

Bellows and Diaphragm Mechanisms

• Bellows Operation: Similar to an aneroid capsule but generally more rugged and not evacuated. An increase in pressure causes the bellows to expand.
• Mechanical Properties: A bellows is considered an extension of the corrugated diaphragm principle and resembles a helical compression spring in operation. It can be used for high, low, or differential pressure measurement.
• Spring-Rate Support: In some applications, an internal or external spring is used to increase the "spring-rate" and assist the bellows in returning to its natural length once pressure is removed.
• Construction: Manufactured from a length of seamless metal tube with end fittings for pressure connection or hermetic sealing.
• Bellows Types:
  • Differential Bellows: Two bellows fed from different sources; the center point movement is proportional to the pressure difference.
  • Sylphon Bellows: One bellows is fed from a pressure source while the other is evacuated (acting as an aneroid bellows). The resulting measurement is Absolute Pressure ($psia$).
• Aviation Example: The manifold pressure gauge is a typical example of a Sylphon type bellows system where an evacuated bellows provides a vacuum reference.
• Diaphragms: These are corrugated metal discs used primarily for low-pressure measurement. Pressure is applied to each side, and the mechanical output drives a pointer or a warning light contact.
• Corrugations: The purpose of the corrugations is to allow for larger deflections than a flat disc for a given thickness. The number and depth of corrugations control sensitivity and response; more corrugations result in a more linear deflection and higher sensitivity.

The Bourdon Tube

• Historical Context: Developed and patented in $1850$ by a Parisian watchmaker (Bourdon).
• Geometry: The element is a metal tube extruded into an elliptical cross-section and shaped into a "C".
• Sensitivity Factors: Sensitivity is determined by the ratio of the major axis (width) to the minor axis (flatness) of the elliptical tube. A wide, flat tube is more sensitive than one with a circular cross-section.
• Principles of Operation: When pressure is applied internally, the tube attempts to change from an elliptical cross-section back to a circular one. This causes the tube to "straighten out" as it seeks its original shape.
• Volume and Radius Theory:
  • An elliptical tube has a smaller volume than a circular one of equivalent length/perimeter. Adding pressure forces the tube to accommodate more volume by becoming circular.
  • In the C-shape, the inner wall is forced toward the center (decreasing radius) and the outer wall is forced away (increasing radius).
  • The reactions to these stretching and compressive forces result in a displacement that is maximized at the "free end" of the tube.
• Proportionality: The displacement of the free end is proportional to the applied pressure within the material's limit of proportionality. Because displacement is small, a quadrant and magnifying gear system is used to drive the pointer.
• Materials and Range:
  • Materials: Phosphor-Bronze, Beryllium-Bronze, Beryllium-Copper, or Stainless Steel.
  • Stainless steel is used for the highest-pressure applications.
• Safety Hazards: Beryllium is a known carcinogen. It can enter the body through wounds or inhalation of dust. Mechanics are advised never to handle broken capsules, bellows, or Bourdon tubes directly.

Electrical Temperature Indicators and Moving-Coil Systems

• Fundamental Principle: Electric current flowing through a conductor produces a magnetic field. Movement is generated by the interaction between a free-moving conductor's field and a permanent, uniform magnetic field.
• Force Equation: The force exerted on a current-carrying conductor in a magnetic field is in direct proportion to flux density, current, and the length of the conductor.
• Field Interaction: Magnetic lines flowing in the same direction attract; those in opposite directions repel. This distorts the main field, pushing the conductor toward the weaker field.
• Moving-Coil Indicator Construction:
  • Torque: For a single rotating coil, total torque is $2Fr$.
  • Components: Permanent magnet (high remanence/coercive force), soft-iron core (low reluctance), and a moving coil.
  • Magnetic Field Shaping: Pole pieces and the soft-iron core are shaped cylindrically to create a radial magnetic field that is intense and uniform in the air gap.
  • Hairsprings: Two flat-coil hairsprings supply current and provide a controlling torque. This system ensures deflections are proportionate to current and returns the pointer to zero when power is removed. They are made of phosphor-bronze or beryllium-bronze and are coiled in opposite directions to minimize temperature error.
  • Damping: An aluminum former (the frame the coil is wound on) moves through the magnetic air gap, setting up eddy currents that produce an opposing force (retarding force) proportional to velocity. This prevents oscillation and overshoot (dead beat indication).
  • Shielding: The movement is enclosed in a soft-iron case to provide screening from external magnetic fields.

Ratiometer Systems

• Definition: A temperature-indicating system with two coils moving together in a non-uniform permanent-magnet field.
• Circuit Design:
  • Arm 1: Coil A in series with a fixed resistance $R_1$.
  • Arm 2: Coil B in series with fixed resistance $R_2$ and the sensing element $R_x$.
• Operation: Current flows through the coils in opposite directions. Forces produced by the coils oppose each other. Because the field is non-uniform, the coil with the greater current moves toward the weaker field area until balance (equilibrium of torques) is achieved.
• Temperature Response: As sensing element temperature increases, its resistance increases (Positive Temperature Coefficient - PTC). This decreases current in Coil B. Coil A then rotates the assembly until balance is restored at a new current ratio.
• Key Advantage: Ratiometers are independent of power supply voltage fluctuations because a change in supply affects both coils equally, maintaining the same current ratio.
• The Zero Spring: While ratiometers don't need controlling springs for accuracy, a light spring is used solely to move the pointer to an "off-scale" position if power is disconnected.

DC Synchronous and AC Inductor Systems

• DC Synchronous (Micro-Desynn):
  • Used when mechanical movement is very small.
  • Utilizes a toroidal resistor arrangement. $45$ degrees of mechanical movement of the wiper arms translates to $360$ degrees of electrical movement to reduce friction errors.
• AC Inductor System:
  • Power Supply: $26\,V$ at $400\,Hz$.
  • Operation: A bellows moves two laminated armature cores (A and B) relative to stator coils. Increasing pressure moves core A further into its stator (increasing inductance and impedance $X_L$) and core B further out (decreasing $X_L$).
  • Indicator: Uses copper shading rings at air gaps to induce eddy currents in cam-shaped discs. The movement is determined by the ratio of currents in the coils.
  • Compensation:
    • Frequency: Capacitors are connected in parallel with each coil to counteract reactance changes caused by frequency shifts.
    • Temperature: A high-temperature-coefficient resistor is connected across the coils to prevent under-reading at elevated temperatures.

Pressure Switches

• Purpose: To provide visual warnings (lights) for high or low pressure hazards or to indicate when a specific operating pressure has been reached in systems where continuous measurement isn't needed.
• Internal Protection: Sensors often feature corrugated diaphragm halves that "nest" together when contracted. This turns the capsule into a virtual solid disc, preventing damage from overload pressure.
• Configurations:
  • Can be plain entry holes, O-ring spigots, or threaded connectors.
  • Can be designed as differential pressure switches by subjecting both sides of a diaphragm to pressure.
  • Sometimes integrated directly into a pressure transmitter unit.