This quantity of fuel can be measured directly by a respirometer, a device that measures the rate at which oxygen is combined with food to produce carbon dioxide and work.    From Wordnik.com.  [Net energy analysis] Reference

To get back to my thesis research, the pieces of equipment that I have been using in the research are fairly standard in physiological research: a Beckman spectrophotometer, a Coleman photometer, a van Slyke amino nitrogen apparatus, a Warburg respirometer, pH meters, Kjeldahls.    From Wordnik.com.  [On Handling the Data] Reference

Photo of blue crab being monitored for metabolism using a respirometer, courtesy of Louis and Karen Burnett.    From Wordnik.com.  [Newswise: Latest News] Reference