the entirety of the test period this may be accomplished with a relatively small maximum bag volume because a the bag volume has been substantially decreased during phase 1 of the test and 2 the exhalation enhanced gas concentration has been substantially increased prior to initiating the closed loop system if however the photocell were to indicate that the maximum bag volume was reached or were to be exceeded prior to the conclusion of the test it is within the concept of the present invention to revert to phase 1 and thereby open valves 26 and 28 to allow for the escape of gas from peep valve 36 gas volume in the closed loop system and the bag could thus be reduced for example from 5 liters to 4 liters and then phase 2 returned and the closed loop process continued without interrupting the test also if the bag volume were to decrease below a desired minimal value e g 1 0 liters during phase 2 valve 26 may be temporarily opened so that pump 58 would add room air to the system fig2 illustrates a suitable embodiment of the apparatus according to the present invention a portable cabinet 82 is provided.