Realisation of a Substitution Method to Perform High Precision Density Measurements of Seawater Hannes Schmidt Henning Wolf Egon Hassel

Content Introduction Motivation Approach Technical realisation Results 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

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 prediction of climate on long term scales.

Importance of thermo physical properties of seawater Thermohaline circulation Conveyor belt http://www.nasa.gov/images/content/436189main_atlantic20100325a-full.jpg Deviations in density – meaning salinity and temperature – are driving forces of subsurface currents (below 400 m). Surface currents are mainly driven by wind.

Simplification in calculation of thermo physical properties of seawater Source Theoretical Measurand Pressure, Temperature, Conductivity N-1 amounts of substances Resulting relation e.g. heat capacity Conductivity replaces information on amounts of substances in calculations.

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.

Substitution Method Procedure Sea water density (Sample) Ultrapure water density (Reference) Seawater density Ultrapure water density …10 times… Ultrapure water 1 bottle (200 ml) = 1 temperature = 8 h Seawater Substitution method decreases measurement uncertainty about factor 10.

Density at atmospheric pressure Measurand Specification range Temperature 0..40 °C VAISALA 1,01325 0,1 MPa REAR M M S-BUS USB 0,1 MPa FRONT S-BUS VGA 998,205 FAN RS232 ETH AIR IN AIR IN CAN A. Paar DMA 5000M SEC ON H2O SSW 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

Density at high pressure (2) Teledyne ISCO 100DM Syringe pump and controller DHI RPM4 Pressure measuring unit DMA HP Densitometer DMA 5000M master device

Density (Seawater) in kg/m3 Density (Water) in kg/m3 Measurement results 1.2 ppm Ultrapure water Seawater Substitution result Density (Seawater) in kg/m3 Density (Water) in kg/m3 0.4 ppm Time in s Seawater measurement values were corrected by appr. 0.006 kg/m3.

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.