A flow tube apparatus provided for determining the lung Functional Residual Capacity, the apparatus comprising: a) a flow tube having a length defined between a distal end and a proximal end, having a substantially open lumen configured to allow air to flow from said proximal end to said distal end; said flow tube having a wall defined between an inner diameter and an outer diameter; b) a first recess disposed within said wall provided for receiving a flow-meter and a second recess disposed within said wall provided for receiving a pressure sensor; c) Said flow tube configured to associate with a flow deflecting apparatus comprising: i) an axle disposed across the diameter of said lumen at a distance from said proximal end, said axle having a first end disposed within said flow tube wall and a second end extending through and transcending said flow tube wall, wherein said second end is adapted for associating with a deflector driver disposed externally to said flow tube; ii) wherein said deflector driver is adapted to drive said axle within said flow tube lumen in at least one direction; iii) said axle associated with a deflector leaf member provided for deflecting airflow within said flow tube characterized in that said leaf is configured not to occlude airflow through said flow-tube and therein flow through said tube so as to maintain normal breathing physiology along said flow-tube; and wherein said deflector leaf member may be controllably manipulated about said axle to assume at least two non-occluding positions within said flow tube, a first leaf position provided to maintain said flow-tube lumen substantially open and a second leaf position provided to deflect airflow through said flow tube. a) a flow tube having a length defined between a distal end and a proximal end, having a substantially open lumen configured to allow air to flow from said proximal end to said distal end; said flow tube having a wall defined between an inner diameter and an outer diameter; b) a first recess disposed within said wall provided for receiving a flow-meter and a second recess disposed within said wall provided for receiving a pressure sensor; c) Said flow tube configured to associate with a flow deflecting apparatus comprising: i) an axle disposed across the diameter of said lumen at a distance from said proximal end, said axle having a first end disposed within said flow tube wall and a second end extending through and transcending said flow tube wall, wherein said second end is adapted for associating with a deflector driver disposed externally to said flow tube; ii) wherein said deflector driver is adapted to drive said axle within said flow tube lumen in at least one direction; iii) said axle associated with a deflector leaf member provided for deflecting airflow within said flow tube characterized in that said leaf is configured not to occlude airflow through said flow-tube and therein flow through said tube so as to maintain normal breathing physiology along said flow-tube; and wherein said deflector leaf member may be controllably manipulated about said axle to assume at least two non-occluding positions within said flow tube, a first leaf position provided to maintain said flow-tube lumen substantially open and a second leaf position provided to deflect airflow through said flow tube. i) an axle disposed across the diameter of said lumen at a distance from said proximal end, said axle having a first end disposed within said flow tube wall and a second end extending through and transcending said flow tube wall, wherein said second end is adapted for associating with a deflector driver disposed externally to said flow tube; ii) wherein said deflector driver is adapted to drive said axle within said flow tube lumen in at least one direction; iii) said axle associated with a deflector leaf member provided for deflecting airflow within said flow tube characterized in that said leaf is configured not to occlude airflow through said flow-tube and therein flow through said tube so as to maintain normal breathing physiology along said flow-tube; and wherein said deflector leaf member may be controllably manipulated about said axle to assume at least two non-occluding positions within said flow tube, a first leaf position provided to maintain said flow-tube lumen substantially open and a second leaf position provided to deflect airflow through said flow tube.