Calibration methods Calibration – experiment conducted to determine the correct value of the scale...
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![Page 1: Calibration methods Calibration – experiment conducted to determine the correct value of the scale reading of an instrument Need to know –Sensitivity –Beam.](https://reader036.fdocuments.in/reader036/viewer/2022062304/56649d0e5503460f949e42d3/html5/thumbnails/1.jpg)
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
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How often?
• Before and after each cruise standard for fisheries applications (legal protection)
• Necessary – Once/year or Twice/year in seasonal differences
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How
• Tanks– Reflections (walls, surface, bottom)– Large tanks– Anechoic tanks– Baffles
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Calibrated hydrophone
• Need transmitter/receiver with known response over the frequency range of interest
Calibrated transmitter(known signal strength and frequency)
Known range and orientation
Measure on axis sensitivityMeasure receiving beam angle
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Calibrated hydrophone
• Need transmitter/receiver with known response over the frequency range of interest
Calibrated receiver(known signal strength and frequency)
Known range and orientation
Measure transmission sensitivity, frequency response, beam angle
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Calibrated hydrophone comparison
Range need not be known
Source of unknown
characteristics
Compare levels received
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Calibrated hydrophone
• Pros– Simple
• Cons– Requires calibrated transmitter/receiver– Difficult to do at sea– Requires large tank
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Reciprocity technique
• Based on electroacoustic reciprocity principle
• To be reciprocal, transducer must be– Linear– Passive– Reversible
– Satisfied by piezoelectric elements
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Reciprocity techniqueProjector
HydrophoneReciprocal transducer
Input known voltage
VPH VPT
Input same voltage (VT)
VTH
ResponseH ≈ (VTH VPH/VPT VT)
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Two transducer reciprocity
• Two identical transducers (often determined by calibrated hydrophone comparison method)
ResponseH ≈ (VTH / VT)
H V
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Self reciprocity• Only need transducer to be calibrated• Perfect reflector
– Flat surface– Metal-backed corprene
• Must use pulsed signals
ResponseH ≈ (VTH / VT)
H V
Perfect reflector
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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
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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
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Spheres
• Copper or tungsten carbide
• Note difference in units
• TS well understood and easily predicted based on radius and material
Tungsten carbide
Copper
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Field set up
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Simple field set up
• Split-beam only so can measure position in beam
• Calm currents
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Set up for towed body
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How far away?
• d largest width of transducer face• f0 is echosounder frequency• c speed of sound in seawater
Ropt = 2d2f0/c
38 kHz, 12º - 5.1 m
70 kHz, 7º - 7.3 m
120 kHz, 7º - 5.2 m
200 kHz, 7º - 3.8 m
Outside near field, but easy
to control sphere position
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Calibrating for echo energy integration
• SA or SV correction, also called C
Rt =c(th-tdel)/2 (target range)
C =EtRt2/t
Et = measured from the target spheret = acoustic cross section of the target sphere
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Calibrating for single target measures
• TS correction, also called C
C =Et/t
Et = measured from the target spheret = acoustic cross section of the target sphere
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Equivalent beam angle • 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
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Multibeam calibration
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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