portion 16 the check valves each comprise an annulus 30 whose outside diameter is slightly larger than the unstretched diameter of the elastomer tubular portion 16 in which it resides hence to place the check valve within the cross piece portion of the patient valve it is necessary to slightly stretch the elastomeric material which insures a secure gas tight seal between the outer diameter of the annulus 30 and the wall of the tube 16 the annulus 30 has a central opening 32 extending through its thickness dimension and overlaying this opening is a gas impervious flap 34 the flap 34 has an outer diameter which is greater than the diameter of the opening 32 is anchored to the annulus 30 at a single point as at 36 in fig2 the flap 34 is preferably formed from an elastomeric film such as a polystyrene material having a thickness in the range from about 0 003 inch to 0 008 inch and the attachment between it and the annulus 30 at point 36 is preferably achieved using a sonic spot weld as can be seen from fig1 the one way check valves 26 and 28 are positioned within the cross piece member 16 so that the flaps 34 and 35 open relative to the opening 32 in the annulus 30 by swinging to the left when viewed in fig1 consider the operation of the improved patient valve during the course of a lung diffusion test initially the patient is asked to breathe normally through the mouthpiece connected to the end 20 of the stem 14 at this time the clamping means not shown associated with.