Fig. 1. Cut-away view of the head space unit used to measure the partial pressure of dissolved carbon dioxide in saltwater. Water is directed into the unit’s head box at a rate that maintains a slight overflow (approximately 10–11Lmin−1). An orifice plate in the center of the reservoir generates a water cascade within the head space chamber.The bubble coalescing chamber of the unit is submerged below the water level of the sump which is maintained by the position of five overflow ports. Gas samples are drawn from the hose barb assembly at the top of the instrument. The vent tube shown in the same view prevents changes in gauge pressure within the head space during equilibration or sampling.