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 plateinthecenterofthereservoirgeneratesawatercascadewithintheheadspace chamber.Thebubblecoalescingchamberoftheunitissubmergedbelowthewater 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.