Torque Meter and Calibrator Mechanics

Gas Laws

• All pressure and temperature values are absolute.
• Absolute pressure = atmospheric pressure + gauge pressure.
• Equivalent units of Standard Atmospheric Pressure:
  • 14.69 PSI = 1013.25 mBar = 1.013 BAR = 101.3 KN/M² = 29.92 in. Hg = 760 mm. Hg
• Absolute temperature = (°Celsius + 273)K

Boyle's Law

• The volume of a fixed mass of gas is inversely proportional to the pressure, providing the temperature remains constant.
• PV = CONSTANT
• P₁V₁ = P₂V₂

Charles' Law

• At constant pressure, the volume of a gas decreases 1/273°C per °C fall in temperature from 0°C.
• In theory, all gases have zero volume at -273°C but since they become liquid before this temperature is reached, it cannot occur in practice.

Combined Gas Law

• If pressure and temperature are plotted against each other at constant volume, they produce a pressure law that states that P/T is a constant.
• From the Gas Laws:
  • Boyle's Law: PV = Constant
  • Charles Law: V/T = Constant
  • General Gas Equation: PV/T = Constant

The Atmosphere

• Gases percentages in the Earth's atmosphere (by volume) are given.
• Water vapor content per ounce, density and pressure for the atmosphere are given.

Earth's Surface

• The I.C.A.O. Standard Atmosphere is shown; temperature decreasing rate in millibars is given.

Effects of Temperature on the Atmosphere

• As temperature decreases, density increases but this is offset by decreasing pressure.
• Lower pressure causes a further decrease in density and a decrease in temperature.
• Air rises towards lower pressure levels.
• Heat is conducted to the air at the surface causing expansion and decrease in density.

Pressure Sensing Elements

• Pressure sensing elements convert pressure energy into a mechanical movement.
• Diaphragms: A circular plate, held around its edge. Pressure deflects it to measure difference.
• Capsules: Two diaphragms sealed at the joining edges.
  • Pressure Capsule
  • Aneroid Capsule
• Bellows: Corrugations are on the sides of the bellows as well as the surface. They are used to sense higher pressures.
• Sensitivity of Diaphragms, Capsules, and Bellows depends on material type, thickness, and surface area, and number/depth of corrugations.

Effects of Changes in Ambient Temperature

• An increase in ambient temperature decreases elasticity, which may increase the instrument's distension for a given applied pressure.
• Temperature compensation for mechanical gauges is normally achieved with a bi-metal material.

Pressure Gauge Calibration

• Pressure gauge accuracy is checked using known pressures and comparing indicator readings.
• Most calibrators use the Brahm's principle.
  • A piston assembly with a known weight and cross-sectional area.
  • Gradually increase pressure until the piston moves.
  • Calibration weights can be reduced according to the piston area.

Engine Pressure Instruments

• Direct reading pressure gauges are connected to the pressure source, typically using pipelines.
• Some gauges have outward relief valves for safety.
• Gauge and pressure source connection is often by capillary tubing.
• Pressure enters the bourdon tube causing changes in pressure and tube movement.

Electrical Pressure Switch for Sensing Differential Pressure

• Measures pressure difference of fuel/air mixture in the induction chamber of an internal combustion engine.
• Pressure difference drives the indicator.

Manifold Pressure Gauge

• Diagram of the manifold pressure gauge (mechanism assembly, pointer, handstaff, case, diaphragm bellow, and evacuated bellows) is shown.

Super Charger

• Diagram of a supercharger, showing the manifold pressure gauge connections to the system.

Turbo Engine Pressure Sections and Pressure Variations

• Pressure variations within a turbo engine are shown graphically. The E.P.R. gauge is an electrically operated instrument which shows pressure ratio (Pt7/Pt1).

Synchro Torque Pressure System

• Details of how a synchro torque pressure system works

Thrust Meters

• Thrust indicators are sensitive differential pressure gauges that display the percentage thrust output of a turbojet engine.
• The thrust output is displayed over a range of 50% to 100% and may consist of a pressure capsule and an aneroid capsule, or one pressure capsule with a mechanical compensation facility.

Torque Meters

• Torque meters are supplements to engine power indications and are fitted on supercharged piston and turbo-prop aircraft. There are three basic types which show basic operation diagrams.

Testing/Checks of Instruments

• Visual checks: Security of attachment, discolouration of pointers, cleanliness, leakage.
• Static engine conditions: Gauge should correspond to prevailing tolerances.
• Engine running conditions: Readings must match the thrust settings.
• Calibration: Compare readings to boost gauges and manometers.