Pressure Altimeter

Wg Cdr Rajagopal

Pressure Altimeter

Index

Introduction to Pressure Altimeter

  • Pressure altimeter is the basic instrument for measuring altitude
  • It is basically an aneroid barometer which measures pressure
  • Altimeter senses change in static air pressure and displays as altitude

Principle of Pressure Altimeter

  • Static or atmospheric pressure reduces with height at a non uniform rate
  • This is due to the reduction in weight of air column at higher altitudes
  • Static pressure is also affected by temperature and pressure variations
  • These need to be understood and corrected during height measurement

Construction of a Simple Altimeter

  • Altimeter consists of a sealed instrument case and an expanding capsule
  • Static source is fed to a a sealed instrument case
  • Sealed case has a aneroid capsule with fixed pressure
  • As height increases, static pressure decreases and the capsule expands
  • Altimeter displays the altitude where the observed pressure exists in ISA

Manual Setting Knob in Simple Altimeter

  • Manual setting knob is used to set the datum altitude by turning the pointer
  • On ground, the pointer can be set to zero for the altimeter to read height above ground or airfield elevation to read height above mean sea level

Construction of a Sensitive Altimeter

  • Sensitive altimeter is an improved form of pressure altimeter
  • Has multiple aneroid capsules for sensitivity and jeweled bearings
  • Has a temperature compensating device & vibrators to overcome friction
  • Variable sub-scale is provided to set the required datum pressure level
  • Multiple pointers are provided for accuracy like 100’s or 1000’s of feet

Sub-Scale of Sensitive Altimeter

  • A changes in altitude or surface pressure has no effect on subscale reading
  • Normally sub scale settings vary from 800 to 1050 hPa

Construction of a Servo Assisted Altimeter

  • Capsule movement in a simple altimeter is quite less at higher altitude due to reducing change in pressure with height
  • Servo assisted altimeter uses an electro-magnetic system called E-I bar

Advantages of a Servo Assisted Altimeter

  • E-I bar generates an electrical error signal even for small changes in capsule
  • Eliminates lag error
  • Improves accuracy
  • Provides faster response rate
  • Enables a digital readout

Calibration of Altimeter

  • Altimeter is calibrated as per International Standard Atmosphere (ISA)
  • Altimeter reads correctly only where ISA conditions exist
  • Pressure altitude is that altitude in ISA where the prevailing pressure exists
  • For example,
  • If the altimeter senses 1013.25 hPa it would indicate zero feet altitude

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