text: The key elements of any FOCE experimental units are perspex, partially open, chambers, a CO mixing system, sensors to continuously monitor ambient and chamber pH, and a control loop to regulate the addition of gases or liquids to each experimental chamber.
  The carbonate chemistry of seawater can be manipulated using different approaches to mimic future conditions. It is possible to directly inject gases (pure CO or CO-enriched air) but this is more difficult than delivering water to achieve precise pH control. Current FOCE systems lower pH using metered addition of CO-enriched seawater into the experimental chambers. pH is controlled as a constant pH offset relative to ambient values, maintaining natural variability, or as a constant value.
  Other approaches have been used to manipulate the seawater carbonate chemistry in the field. In pelagic mesocosm experiments, the carbonate chemistry is generally altered at the beginning of the experiment and subsequently drifts as a function of biological processes and air-sea gas transfer. CO bubbling in open water has also been used. This approach does not enable precise control of the carbonate chemistry because it does not include a device to ensure full equilibration of added CO in seawater and its precise control. There are no experimental chambers to regulate water flow, and thus allows for natural near-bottom flow conditions, but it generates highly variable pH under variable current speed or direction. This approach is therefore more similar to natural CO vents than to FOCE systems. This approach can be useful when organisms can not be enclosed in chambers and when they inhabit environments such as estuaries where pCO2 levels are naturally hyper-variable. The approach has inherent limitations but may allow greater replication, at lower cost. 
  Current users of FOCE systems have organized to release guidelines and best practices information for future users. Furthermore, the Monterey Bay Aquarium Research Institute will release an open source package to transfer FOCE technology to interested researchers ([http://www.xfoce.org xFOCE]). This package will comprise all engineering information required to develop cost effective FOCE systems.
  Future development of FOCE systems will include the study of the combined effects of ocean acidification and other environmental factors such as temperature or the concentration of dissolved oxygen.