calibration methods l.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Calibration methods PowerPoint Presentation
Download Presentation
Calibration methods

Loading in 2 Seconds...

play fullscreen
1 / 23

Calibration methods - PowerPoint PPT Presentation


  • 347 Views
  • Uploaded on

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)

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Calibration methods' - lotus


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
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
How often?
  • Before and after each cruise standard for fisheries applications (legal protection)
  • Necessary – Once/year or Twice/year in seasonal differences
slide3
How
  • Tanks
    • Reflections (walls, surface, bottom)
    • Large tanks
    • Anechoic tanks
    • Baffles
calibrated hydrophone
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)

calibrated hydrophone5
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)

calibrated hydrophone comparison
Calibrated hydrophone comparison

Compare levels received

Source of unknown characteristics

Range need not be known

calibrated hydrophone7
Calibrated hydrophone
  • Pros
    • Simple
  • Cons
    • Requires calibrated transmitter/receiver
    • Difficult to do at sea
    • Requires large tank
reciprocity technique
Reciprocity technique
  • Based on electroacoustic reciprocity principle
  • To be reciprocal, transducer must be
    • Linear
    • Passive
    • Reversible
    • Satisfied by piezoelectric elements
reciprocity technique9
Reciprocity technique

Projector

Input known voltage

ResponseH≈ (VTH VPH/VPT VT)

Input same voltage (VT)

Reciprocal transducer

Hydrophone

VPH

VPT

VTH

two transducer reciprocity
Two transducer reciprocity
  • Two identical transducers (often determined by calibrated hydrophone comparison method)

ResponseH≈ (VTH / VT)

H

V

self reciprocity
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

reciprocity technique12
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
standard target method
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
spheres
Spheres

Tungsten carbide

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

Copper

simple field set up
Simple field set up
  • Split-beam only so can measure position in beam
  • Calm currents
how far away
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

calibrating for echo energy integration
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

calibrating for single target measures
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

equivalent beam angle y
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

calibrating adcps
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