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Physikalisch-Technische Bundesanstalt

Hannes Schmidt Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig

Universität Rostock, Lehrstuhl für Technische Thermodynamik, Albert-Einstein-Straße 2, 18059 Rostock

Baku, 20.-25.05.2013

Realisation of a Substitution Method to Perform High Precision Density Measurements of Seawater

Hannes SchmidtHenning WolfEgon Hassel

2Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig

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

3Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig

Importance of thermo physical properties of seawater

http://www.drroyspencer.com/wp-content/uploads/2000-years-of-global-temperatures-small.gif

http://www.drroyspencer.com/wp-content/uploads/climate-model-2.jpg

http://www.oocities.org/marie.mitchell@rogers.com

/climate_files/ClimateSimulation_1900-2100_IPCC2007.JPG

Measurement data is used for understanding and predictionof climate on long term scales.

4Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig

Deviations in density – meaning salinity and temperature – are driving forces of subsurface currents (below 400 m).

http://www.nasa.gov/images/content/436189main_atlantic20100325a-full.jpg

Surface currents are mainly driven by wind.

Ther

moh

alin

e ci

rcul

atio

n

Conveyor belt

Importance of thermo physical properties of seawater

5Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig

Source Theoretical

MeasurandPressure,

Temperature,Conductivity

Pressure,Temperature,

N-1 amounts of substances

Resulting relatione.g. heat capacity

Simplification in calculation ofthermo physical properties of seawater

1 1( , , ,.., )p Nc f p T w w ( , , ( ))p Pc f p T S

Conductivity replaces information on amounts of substances in calculations.

mass fractioniw

6Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig

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.

7Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig

Substitution Method

SeawaterUltrapure water

Procedure

Sea water density (Sample)

Ultrapure water density (Reference)

Seawater density

Ultrapure water density

…10 times…

, , ,Sample Reference Literature Sample Measured Reference Measured

Substitution method decreases measurement uncertainty about factor 10.

1 bottle (200 ml) = 1 temperature = 8 h

8Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig

Density at atmospheric pressure

998,205

A. Paar DMA 5000M

FRONTREAR

FAN

USB

VGA

S-BUS

S-BUS

RS23

2

ETH

CAN

AIRIN

AIRIN

ONSEC

0,1 MPa M

VAISALA

1,013250,1 MPa

M

H 2O

SSW

SSW Standard Sea WaterH2O 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

Measurand Specification range

Temperature 0..40 °C

9Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig

Density at high pressure (2)

TeledyneISCO 100DMSyringe pumpand controller

DHI RPM4Pressure

measuring unit

DMA HPDensitometer

DMA 5000Mmaster device

10Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig

Measurement results

Seawater measurement values were corrected by appr. 0.006 kg/m3.

1.2 ppm

0.4 ppm

Time in s

Den

sity

(Wat

er) i

n kg

/m3

Den

sity

(Sea

wat

er) i

n kg

/m3

Ultrapure water

Seawater

Substitution result

3kgm

998.204 LiteratureReference

11Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig

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.

12Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig