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Calibration methods

Calibration methods. Calibration – experiment conducted to determine the correct value of the scale reading of an instrument Need to know Sensitivity Beam pattern Signal characteristics. How often?. Before and after each cruise standard for fisheries applications (legal protection)

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Calibration methods

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  1. Calibration methods • Calibration – experiment conducted to determine the correct value of the scale reading of an instrument • Need to know • Sensitivity • Beam pattern • Signal characteristics

  2. How often? • Before and after each cruise standard for fisheries applications (legal protection) • Necessary – Once/year or Twice/year in seasonal differences

  3. How • Tanks • Reflections (walls, surface, bottom) • Large tanks • Anechoic tanks • Baffles

  4. Calibrated hydrophone • Need transmitter/receiver with known response over the frequency range of interest Measure on axis sensitivity Measure receiving beam angle Known range and orientation Calibrated transmitter (known signal strength and frequency)

  5. Calibrated hydrophone • Need transmitter/receiver with known response over the frequency range of interest Measure transmission sensitivity, frequency response, beam angle Known range and orientation Calibrated receiver (known signal strength and frequency)

  6. Calibrated hydrophone comparison Compare levels received Source of unknown characteristics Range need not be known

  7. Calibrated hydrophone • Pros • Simple • Cons • Requires calibrated transmitter/receiver • Difficult to do at sea • Requires large tank

  8. Reciprocity technique • Based on electroacoustic reciprocity principle • To be reciprocal, transducer must be • Linear • Passive • Reversible • Satisfied by piezoelectric elements

  9. Reciprocity technique Projector Input known voltage ResponseH≈ (VTH VPH/VPT VT) Input same voltage (VT) Reciprocal transducer Hydrophone VPH VPT VTH

  10. Two transducer reciprocity • Two identical transducers (often determined by calibrated hydrophone comparison method) ResponseH≈ (VTH / VT) H V

  11. Self reciprocity • Only need transducer to be calibrated • Perfect reflector • Flat surface • Metal-backed corprene • Must use pulsed signals Perfect reflector ResponseH≈ (VTH / VT) H V

  12. Reciprocity technique • Pros • Does not require calibrated hydrophone • Self reciprocity good for measuring frequency response for broadband measurements • Very accurate measurements • Cons • Lengthy • Requires reciprocal transducer • Difficult to measure beam pattern

  13. Standard target method • Spheres • Orientation unimportant • Must minimize hardware for attachment • Pros • Accurate • Simple to apply in the field • Calibration same as field survey set up • Measure • Combined transmit-receive sensitivity (including gain and noise and frequency response) • Beam angle • Cons • Need to control target position relative to beam

  14. Spheres Tungsten carbide • Copper or tungsten carbide • Note difference in units • TS well understood and easily predicted based on radius and material Copper

  15. Field set up

  16. Simple field set up • Split-beam only so can measure position in beam • Calm currents

  17. Set up for towed body

  18. How far away? Ropt = 2d2f0/c • d largest width of transducer face • f0 is echosounder frequency • c speed of sound in seawater 38 kHz, 12º - 5.1 m Outside near field, but easy to control sphere position 70 kHz, 7º - 7.3 m 120 kHz, 7º - 5.2 m 200 kHz, 7º - 3.8 m

  19. Calibrating for echo energy integration • SA or SV correction, also called C Rt =c(th-tdel)/2 (target range) C =EtRt2/st Et = measured from the target sphere st = acoustic cross section of the target sphere

  20. Calibrating for single target measures • TS correction, also called C C =Et/st Et = measured from the target sphere st = acoustic cross section of the target sphere

  21. Equivalent beam angle y • Crucial for echo energy integration • Predictions from theoretical >20% off real measurements • Constant for a given transducer unless damaged • Difficult experiment for single beam transducers • Need to be ±2% • Measurements usually provided by manufacturer Measured beam patterns from 2 transducers with the same y

  22. Multibeam calibration

  23. Calibrating ADCPs • Tow tank • No current • Seed tank with backscattering particles • Tow ADCP at known speeds in different directions for relatively long distances • Mostly factory cal’d, not user • Calibration of gyro-compass • Backscatter measurement not intended • Techniques to calibrate backscatter counts not established

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