Clamp-on type ultrasonic flow meter and a temperature and pressure compensation method therein

A temperature and pressure compensation method in a clamp-on type ultrasonic flow meter comprising

• a pair or more of ultrasonic transducers each including an ultrasonic oscillator and an oblique wedge, and mounted on the outer peripheral surface of a pipe in which a fluid is flowing, and
• a measurement control unit for calculating for measuring propagation times of ultrasonic waves propagating in a flow direction and in the reverse direction of the fluid flow, and outputting a flow amount of the fluid based on the propagation times,
• said method comprising the steps:
• (A) preparing, as given known data:
• (a) a distance between opposite inner wall surfaces or outer wall surfaces on a plane of the pipe through which ultrasonic waves transmit, and a thickness of the pipe wall;
• (b) a length of a propagation path of ultrasonic waves through the oblique wedge of the ultrasonic transducer projected onto a plane perpendicular to the center axis of the pipe;
• (c) a length of a propagation path of ultrasonic waves between the ultrasonic transducers projected onto the center axis of the pipe;
• (d) sonic speed in the oblique wedges at a reference temperature;
• and
• (e) sonic speed in the pipe wall of the pipe at the reference temperature;
• (B) calculating an approximate convergence value of a sonic speed/refraction angle ratio, which is a ratio of sonic speed to a sinusoidal value at the refraction angle of the ultrasonic wave from the pipe to the fluid in the fluid existing in the pipe at temperature and pressure under measuring conditions, based on the measured propagation times of the ultrasonic waves in the forward direction and backward direction with respect to the fluid flow, using said given known data, by a gradual approximate calculation;
• and
• (C) applying an incident angle of the ultrasonic waves to the fluid existing in the pipe derived by said gradual approximate calculation and the propagation time of the ultrasonic waves passing through the oblique wedge and the pipe wall to a calculation equation for deriving a flow velocity of the fluid existing in the pipe from the measured propagation times of the ultrasonic waves in the fluid flowing direction and in the reverse direction to calculate a flow amount of the fluid existing in the pipe at temperature and pressure under the measuring conditions.