# Hannes Schmidt – Realisation of a Substitution Method

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Baku, 20.-25.05.2013
Physikalisch-Technische
Bundesanstalt
Realisation of a Substitution Method to Perform High
Precision Density Measurements of Seawater
Hannes Schmidt
Henning Wolf
Egon Hassel
Hannes Schmidt
Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig
Universität Rostock, Lehrstuhl für Technische Thermodynamik, Albert-Einstein-Straße 2, 18059 Rostock
Content
• Introduction
• Importance of thermo physical properties of seawater
• Simplifications in calculation of thermo physical properties of seawater
• Motivation
• Practical salinity and conductivity
• Possible inconsistency
• Approach
• Traceability of salinity to density
• Substitution method
• Technical realisation
• Demands on equipment
• Density at atmospheric pressure
• Density at high pressure
• Results
• Measurement results
• Summary
Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig
2
Importance of thermo physical properties of seawater
/2000-years-of-global-temperatures-small.gif
Measurement data is used for understanding and prediction
of climate on long term scales.
Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig
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Importance of thermo physical properties of seawater
Deviations in density – meaning salinity and temperature – are
driving forces of subsurface currents (below 400 m).
Surface currents are mainly driven by wind.
Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig
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Simplification in calculation of
thermo physical properties of seawater
Source
Measurand
Resulting relation
e.g. heat capacity
Theoretical
Pressure,
Temperature,
Conductivity
Pressure,
Temperature,
N-1 amounts of substances
cp  f ( p, T , SP ( )) c p  f ( p, T , w1 ,.., wN 1 )
wi
mass fraction
Conductivity replaces information on amounts of substances in calculations.
Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig
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Possible inconsistency
Sources
Simplification by using
conductivity
Practical salinity
determination
salt composition is changing
salt composition is changing (!)
not all dissociated salts are
contributing to conductivity
(silicates,..)
reference substance not
certified (purity,..)
Especially a change in salt composition can cause inconsistency.
Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig
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Substitution Method
Procedure
Sea water density (Sample)
Ultrapure water density (Reference)
Seawater density
Ultrapure water density
…10 times…
1 bottle (200 ml) = 1 temperature = 8 h
Ultrapure water
Seawater
 Sample  Reference, Literature  Sample,Measured  Reference,Measured
Substitution method decreases measurement uncertainty about factor 10.
Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig
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Density at atmospheric pressure
Measurand
Specification range
Temperature
0..40 °C
1,01325
0,1 MPa
RS232
AIR
IN
CAN
ON
AIR
IN
SEC
FAN
ETH
S-BUS
0,1 MPa
M
FRONT
M
998,205
A. Paar DMA 5000M
SSW
S-BUS
H2O
REAR
VGA USB
VAISALA
SSW Standard Sea Water
H2O Ultrapure Water
Measurand
Measurement
uncertainty (k = 2)
Reproducibility
(Std. deviation)
Temperature
5 mK
1 mK
Pressure
0.007 kPa
0.002 kPa
Density
0.002 kg/m3
0.0005 kg/m3
Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig
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Density at high pressure (2)
DHI RPM4
Pressure
measuring unit
DMA HP
Densitometer
Teledyne
ISCO 100DM
Syringe pump
and controller
DMA 5000M
master device
Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig
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Measurement results
Density (Water) in kg/m3
Ultrapure water
Seawater
Substitution
result
0.4 ppm
Reference
Literature
 998.204 mkg3
Density (Seawater) in kg/m3
1.2 ppm
Time in s
Seawater measurement values were corrected by appr. 0.006 kg/m3.
Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig
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Summary
• Calibration and measurement routines for seawater have a risk of data
inconsistency when data is collected over decades.
• The solution is tracing back salinity to density directly by high precision
density measurements and electrolytic conductivity measurements.
• A fully automated density measurement device was built and measurement
uncertainties of 2 ppm at 1 bar (20 ppm at > 1 bar) were reached.
• Atmospheric density measurements are finished and high pressure density
measurements will start soon.
Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig
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Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig
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