Technical criteria for contract to build Underway pCO2 system

The system to measure surface seawater carbon dioxide concentrations must fulfill stringent performance criteria as determined by community consensus at the workshop on underway pCO2 systems, held in Miami Oct 2 and 3, 2002.).  The systems will be certified by AOML that they meet the criteria listed below.  The specifications of parts are based performance, robustness and extensive experience with our current systems in operation.

Overall system accuracy:

The systems should measure ambient air CO2 levels with an accuracy of better than 0.2 ppm (parts per million) out of 370 ppm.  The equilibrated surface water values should be accurate to 2 uatm.  This will necessitate pressure measurements to within 0.2 mB and water temperature measurements to 0.01 C.

Overall system description:

The system must be patterned after those in operation and described in Feely et al, 1998; Wanninkhof et al, 1993.  In short, it must operate fully automatically and consist of an equilibrator that partitions CO2 in seawater into a headspace gas to be analyzer; an infrared analyzer to quantify the CO2 content in equilibrator headspace and ambient air; a valve box to select, monitor and control flows of equilibrator, ambient air, and gas standards; and a computer controlled interface to log data and control valves.  Pressures in infrared analyzer and equilibrator must be measured, along with gas and water flow, and temperature of water in equilibrator.  Water flow should be 1.5 L/min or less, and gas flows 30 mL/min.  

Parts: The following components should be used in production unless is can be unequivocally shown that alternate parts improve accuracy and robustness.

Analyzer: LICOR 6262.  The analyzer will be provided by AOML The infrared analyzer has a water vapor channel, which is essential to measure the water vapor concentration in the air stream in order to determine the proper mole fraction of CO2

Multiport valve: A 6-port Valco valve modelECMT2SD6MWE must be used to select gas flows from one of 4 compressed gas standards, equilibrator headspace or ambient air.

Pressure transducer: Setra model 270 or equivalent

Equilibrator: two units must be provided; a Liquicell unit and a small volume showerhead equilibrator. Water must be filtered with a 50 micron filter prior to entering the equilibrators

Tubing material; 1/8” 316 stainless steel tubing should be used

Enclosures: The equilibrator and valve and IR units should be built in two watertight enclosures.

Instrument interface: A Lab View instrument control and data acquisition software package must be produced as part of this package which automatically controls the instrument operations and logs data from IR, flowmeters, thermometer, and barometer.  The interface must include the option to mirror the display panel from a remote site.

 

Feely, R.A., R. Wanninkhof, H.B. Milburn, C.E. Cosca, M. Stapp, and P.P. Murphy, A new automated underway system for making high precision pCO2 measurements onboard research ships, Analytica Chim. Acta, 377, 185-191, 1998.

Wanninkhof, R., and K. Thoning, Measurement of fugacity of CO₂ in surface water using continuous and discrete sampling methods, Mar. Chem., 44 (2-4), 189-204, 1993.