CTD CALIBRATION

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CTD CALIBRATION
Paul Ridout
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Why Calibrate?
OSIL,Ltd 2004
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Why Calibrate ?

• 2 2 ??

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Accuracy and Precision
Precision Accuracy True
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Basic Principle of Calibration

• Primary Standard - national or international
• Transfer Standard - recognised method
• Instrument for Calibration - functional
• (Ancillary equipment to maintain stable
  conditions)

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Accuracy

• WOCE requirements for CTD
• Temperature 0.002 degrees C
• Salinity 0.002 (PSS78)
• Pressure 3dBar(0.05FS)

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CTD Calibration
Temperature
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Temperature Calibration

• Primary Standard is triple point cell
• Transfer standard is PRT with resistance
  bridge
• Standard Resistor

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Measurements

• Temperature is measured according to the
  International Temperature Scale1990 ( ITS90)
• Oceanographic range Primary Standards
• Mercury -38.8344
• Water 0.010
• Gallium 29.7646
• No extrapolation

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Temperature Calibration

• Fluid Bath
• 2 X prt
• Bridge
• Std. Resistor
• Temperature controlled laboratory

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Data Recording
Data Recording

• PRT resistance ratio ( with Std. Resistor)
• Nominal temperature of bath
• Std. Resistor temperature
• Laboratory temperature
• Instrument raw data
• Serial nos., date, analyst, comments, etc.

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Calculations

• Temperature from resistance using
  calibrations from prt and std. Resistor with
  deviation functions for ITS90
• Polynomial fit of instruments data to standard
  data

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CTD Calibration
Pressure
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Pressure Calibrations

• Primary Standard comprises known masses and
  defined piston (NPL)
• Transfer Standard DWT mass (or quartz sensor)
• Dead weight tester (DWT)

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Measurements

• Pressure using dead weight tester (and quartz
  standard)
• Temperature
• Atmospheric pressure
• Relative heights pressure of standards and
  instruments

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Pressure Calibration

• DWT used to increment pressure
• Exercise sensor prior
• Quartz sensor values
• increasing/decreasing
• Changes with temperature
• Barometer
• Height adjustments

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Data recording

• Standard pressure
• Atmospheric pressure
• Height of instruments
• Temperature
• Instrument raw data
• Serial nos., analyst, notes, etc

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Calculations

• Adjustments for atmospheric pressure
• Adjustments for height
• Polynomial fit instrument data to standard data

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CTD Calibration
Conductivity
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Conductivity Calibration

• Comparison of the measured value with a
  recognised, traceable standard
• Salinity primary standard is KCl
• Transfer standard is IAPSO Standard Seawater

www.seawatersolutions.com
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Measurements

• Salinity - using salinometer to PSS78
• Temperature - using prt according to ITS 90
• Calculate IPTS68 temperature from ITS90
• (1.00024 ITS90 IPTS68, Saunders 1990)
• Calculate conductivity from the salinity
  (PSS78)
• temperature (IPTS68)

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Conductivity by temperature change

• Salinometer
• Temperature controlled seawater bath(high to
  low)
• Platinum thermometer
• Salinometer pump
• Lab Assistant software
• IAPSO Std. Seawater

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Conductivity by salinity change

• Seawater tanks of various salinities
• Salinometer
• Platinum thermometer
• Salinometer pump
• Lab Assistant software
• IAPSO Std. seawater

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Data Recording

• Salinometer salinity/conductivity ratio
• PRT temperature of bath
• Instrument raw data
• Laboratory temperature
• Atmospheric pressure
• Serial numbers
• Date, analyst, remarks

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Calculations

• IPTS68 temperature from ITS90 of PRT
• Conductivity (mS/cm) from salinity or
  conductivity ratio (Rt) and temperature
• Use polynomial curve fitting program to compare
  standard readings with instruments data

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Post Calibration Checks

• Temperature 2 points in range
• Pressure 0, 10, 50, 100
• Conductivity 2 points in range

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CTD Calibration
Paperwork
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Incoming Inventory
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Diagnostics
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Diagnostics (contd.)
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Service Report
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Datasheet
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Calibration Report
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Calibration Certificate
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Calibration Centre
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