Large diameter plethysmograph

1. A plethysmograph comprising:

a) a reference chamber having a first transducer port for connecting said reference chamber to a pressure transducer;

b) a test chamber having a second transducer port for connecting said test chamber to said pressure transducer; and

c) a manifold having a first airflow opening in communication with said reference chamber, a second airflow opening in communication with said test chamber, an exterior airflow opening, a passageway connecting said exterior airflow opening to said first and second airflow openings, and an air sampling port in communication with said passageway between said second airflow opening and said exterior airflow opening, whereby air closely approximating the air in the test chamber can be sampled as well as air entering the test chamber.

2. The plethysmograph of claim 1 , wherein said passageway is comprised of a first section in communication with said first and second airflow openings and a second section having an inner end in communication with said first section at a juncture between said first and second airflow openings and an outer end forming said exterior airflow opening.

3. The plethysmograph of claim 2 , wherein said sampling port is in communication with said passageway at the juncture of said first and second passageway sections.

4. The plethysmograph of claim 2 , wherein the juncture of said first and second sections is equidistant from said first and second airflow openings.

5. The plethysmograph of claim 1 , wherein a bottom wall of said reference chamber forms a top wall of said test chamber.

6. A plethysmograph comprising:

a) a reference chamber having a first transducer port for connecting said reference chamber to a pressure transducer, a top wall, a bottom wall, a continuous side wall with a lower edge, and a first continuous flange with a first interior face and a flange lower edge extending downwardly from said side wall lower edge;

b) a test chamber having a second transducer port for connecting said test chamber to said pressure transducer, a bottom wall and a continuous side wall with an upper edge, a second continuous flange including a second interior face extending upwardly from said upper edge, one of the first and second interior faces including an annular groove with an O-ring in said groove, the other of the first and second interior faces being tapered relative to the one of the first and second interior faces, said flanges being joinable to form an airtight seal with said O-ring being compressed between said flanges, the compression on said O-ring being diminished due to the other of the first and second interior faces being tapered relative to the one of the first and second interior faces as said chambers are separated to facilitate opening; and

c) a manifold having a first airflow opening in communication with said reference chamber, a second airflow opening in communication with said test chamber, an exterior airflow opening, a passageway connecting said exterior airflow opening to said first and second airflow openings, and a sampling port in communication with said passageway between said second airflow opening and said exterior airflow opening.

7. The plethysmograph of claim 6 , further including a footed base with an upper surface, said upper surface forming said bottom wall.

8. The plethysmograph of claim 6 , wherein said side walls are cylindrical and of the same diameter.

9. The plethysmograph of claim 6 , wherein said first flange is an interior flange and said second flange is an exterior flange.

10. The plethysmograph of claim 6 , wherein said test and reference chambers include finger engageable tabs extending outwardly from said side walls, whereby a user can push a reference chamber tab away from a test chamber tab to separate the reference chamber from the test chamber.

11. The plethysmograph of claim 6 , wherein one of said flanges is tapered from vertical by up to about 1°.

12. A plethysmograph comprising:

a) a reference chamber having a top wall, a bottom wall, a barrier wall spaced beneath said bottom wall, a continuous side wall having a lower edge, and a first continuous flange with a first interior face and a flange lower edge extending downwardly from said side wall lower edge, said first flange including an interior face and a flange lower edge below said bottom wall and above said barrier wall; and

b) a test chamber having a bottom wall and a continuous side wall with a lower edge joined to said bottom wall and a upper edge with an upwardly extending second continuous flange including a second interior face tapered relative to the other flange face, said flanges being joinable to form an airtight seal with said O-ring being compressed between said flanges, the compression on said O-ring being diminished due to the second interior face being tapered relative to the other flange face as said chambers are separated to facilitate opening; and

c) a manifold having a first airflow opening in communication with said reference chamber, a second airflow opening in communication with said test chamber, an exterior airflow opening, a passageway connecting said exterior airflow opening to said first and second airflow openings, and a sampling port in communication with said passageway between said second airflow opening and said exterior airflow opening.

13. The plethysmograph of claim 12 , wherein the interior face of one of said flanges including an annular groove with an O-ring in said groove.

14. The plethysmograph of claim 12 , wherein said first flange has a tapered interior face.

15. The plethysmograph of claim 12 , wherein said test chamber and reference chamber side walls have the same inner and outer diameters.

16. The plethysmograph of claim 12 , wherein said barrier wall is air permeable.

17. A plethysmograph comprising:

a) a reference chamber having a top wall, a bottom wall, a barrier wall spaced beneath said bottom wall, a continuous side wall having a lower edge, and a first continuous flange extending downwardly from said side wall lower edge, said first flange including an first interior face and a flange lower edge below said bottom wall and above said barrier wall; and

b) a test chamber having a bottom wall and a continuous side wall with an upper edge, and a second continuous flange including a second interior face extending upwardly from said upper edge, one of the first and second interior faces including an annular groove with an O-ring in said groove, the other of the first and second interior faces being tapered from vertical relative to the one of the first and second interior faces, said flanges being joinable to form an airtight seal with said O-ring being compressed between said flanges, the compression on said O-ring being diminished due to the other of the first and second interior face being tapered from vertical relative to the one of the first and second interior faces as said chambers are separated to facilitate opening; and

c) a manifold having a first airflow opening in communication with said reference chamber, a second airflow opening in communication with said test chamber, an exterior airflow opening, a passageway connecting said exterior airflow opening to said first and second airflow openings, and a sampling port in communication with said passageway between said second airflow opening and said exterior airflow opening.

18. The plethysmograph of claim 17 , wherein said sampling port is in communication with said passageway equidistant from said first and second airflow openings.

19. The plethysmograph of claim 17 , wherein said side walls are cylindrical and of the same diameter.

20. The plethysmograph of claim 17 , wherein said test and reference chambers include finger engageable tabs extending outwardly from said side walls, whereby a user can push a reference chamber tab away from a test chamber tab to separate the reference chamber from the test chamber.

21. The plethysmograph of claim 17 , wherein the interior face of said first flange includes an annular groove with an O-ring in said groove, and the interior face of said second flange is tapered from vertical by up to about 1° relative to the interior face of said first flange.