The Underway Pumping System
The intercomparison exercise was almost entirely based on continuous underway sampling of surface seawater. All participating groups operated their underway fCO2 systems simultaneously on the same seawater pumping system. Like most up-to-date research vessels, the R/V Meteor provides a special seawater pumping system for scientific purposes. However, from experience, it is known that the use of this kind of pumping system for measurements of dissolved gases may be hampered by a number of problems. Pump action may cause cavitation when underpressure is applied to the water flow, thus making undisturbed gas measurements nearly impossible. Because of the location of the seawater intake close to the bow on R/V Meteor, air bubbles are introduced into the water lines in a rough sea. This again possibly biases the concentration of dissolved gases or even makes seawater sampling technically impossible in such cases. Furthermore the unavoidable warming of seawater during its travel from the bow intake to the user may be quite significant. In the case of the fCO2 intercomparison exercise, it was desirable to keep the temperature change as small as possible. As a result of the sluggish exchange of CO2 between the gas phase and the water phase, sampling for CO2 measurements (e.g., fCO2, pH, CT) is less susceptible to biases caused by inadequate pumping techniques than is sampling of reactive gases like oxygen. Nevertheless a careful sampling technique was an important aspect of the exercise. For this reason, a simple and reliable underway pumping system (see also Koertzinger et al. 1996b) was designed for use in the "moon pool" of R/V Meteor. The system consisted of a small CTD probe (ECO type, ME Meerestechnik-Elektronik GmbH, Trappenkamp, Germany) for measuring in situ seawater temperature and salinity at the intake as well as a submersible pump, both of which were installed in the shell plating at the bottom of the "moon pool." Figure 2 shows a schematic drawing of this underway pumping system. The system also includes a separate Global Positioning System (GPS) receiver (GPS 120, Garmin/Europe Ltd., Romsey, Hampshire, U.K.). Navigational data from the GPS system as well as CTD data were continuously logged on a computer.
The moon pool of R/V Meteor is specially designed for sampling purposes so that no cooling or wastewaters are emitted ahead of it and even at full speed or in a very rough sea no air bubbles reach it. Seawater was pumped through the moon pool from below the ship by means of a large submersible pump (multivane impeller pump, type CS 3060, ITT Flygt Pumpen GmbH, Langenhagen, Germany) at a pumping rate of about 350 L/min (pump head approx. 12 m). The CTD probe was installed next to the submersible pump. All underway fCO2 systems were assembled in the geology lab of R/V Meteor. Two seawater supply lines (port and starboard) were teed-off from the main bypass and laid through the lab. All underway systems were hooked-up to these supply lines which delivered the necessary flow rates of seawater to each system (approx. 1-15 L/min).
The wastewater from the systems was collected in three 100-L carboys and from there was disposed of continuously through the floor drains of the geology lab. In case of (occasionally observed) clogging of the lab's floor drains as a result of rough sea conditions, small submersible pumps (multivane impeller pump, type GS 9565, ITT Flygt Pumpen GmbH, Langenhagen, Germany) were at hand to pump the wastewater actively out of the lab. These pumps did not have to be used during this cruise, however.
