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Which of the following statements is incorrect about the hot wire instruments?

  • The statement that "Their response is instantaneous" is incorrect about hot wire instruments.
  • Hot wire instruments, also known as thermal anemometers, are devices used to measure the velocity of fluids.
  • The velocity is determined by measuring the rate at which a heated wire or filament is cooled by the fluid flowing past it.
  • However, the response time of the instrument is not instantaneous and depends on the thermal time constant of the wire.
  • The thermal time constant is the time required for the wire to reach 63.2% of its steady-state temperature in response to a step change in fluid velocity.
  • Therefore, hot wire instruments have a finite response time and cannot measure rapid changes in fluid velocity.