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Apparatus and methods relating to circulation of air within a plethysmographic measurement chamber are provided. An air circulation system, comprised of one or more pumps, is coupled to a plethysmographic measurement chamber using one or more inlet and exhaust tubes. The air circulation system renew

Apparatus and methods relating to circulation of air within a plethysmographic measurement chamber are provided. An air circulation system, comprised of one or more pumps, is coupled to a plethysmographic measurement chamber using one or more inlet and exhaust tubes. The air circulation system renews the air within the measurement chamber using ambient air, or air derived from a controlled temperature environment.

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What is claimed is: 1. A plethysmographic measurement system, comprising: a plethysmographic measurement chamber; plethysmographic measurement components coupled to the measurement chamber, the measurement components comprising a volume perturbation element and means for determining pressure within

What is claimed is: 1. A plethysmographic measurement system, comprising: a plethysmographic measurement chamber; plethysmographic measurement components coupled to the measurement chamber, the measurement components comprising a volume perturbation element and means for determining pressure within the chamber; and an air circulation system coupled between the measurement chamber and an atmospheric environment outside the measurement chamber, wherein the air circulation system renews air within the measurement chamber using air from the atmospheric environment outside the chamber during plethysmographic measurement. 2. The system of claim 1, wherein the measurement components are housed within the measurement chamber. 3. The system of claim 1, further comprising a chamber door coupled to the measurement chamber. 4. The system of claim 1, wherein the means for determining pressure comprises one or more pressure transducers. 5. The system of claim 1, wherein the volume perturbation element is an oscillating diaphragm. 6. The system of claim 1, wherein the volume perturbation element generates pressure oscillations within the measurement chamber at a first frequency. 7. The system of claim 6, wherein the first frequency is between 3 and 20 Hz. 8. The system of claim 6, wherein the air circulation system comprises an inlet tube coupled between the measurement chamber and a pump assembly, and further comprises an exhaust tube coupled between the measurement chamber and pump assembly, wherein the inlet tube and exhaust tube attenuate noise at a perturbation frequency generated by the volume perturbation element. 9. The system of claim 8, wherein the length of the inlet tube is one quarter of a wavelength corresponding to the perturbation frequency. 10. The system of claim 8, wherein the length of the exhaust tube is one quarter of a wavelength corresponding to the perturbation frequency. 11. The system of claim 1, wherein the air circulation system comprises: a pump assembly; an inlet tube coupled between the measurement chamber and the pump assembly, and an exhaust tube coupled between the measurement chamber and pump assembly. 12. The system of claim 11, wherein the pump assembly comprises an inlet pump and an exhaust pump. 13. The system of claim 12, wherein the inlet pump is placed at the end of the inlet tube distal from the measurement chamber. 14. The system of claim 12, wherein the exhaust pump is placed at the end of the exhaust tube distal from the measurement chamber. 15. The system of claim 11, wherein the pump assembly comprises one or more rotary pumps. 16. The system of claim 11, wherein the pump assembly comprises one or more turbine pumps. 17. The system of claim 11, wherein the pump assembly comprises one or more centrifugal pumps. 18. The system of claim 11, wherein the pump assembly comprises one or more fan pumps. 19. The system of claim 11, wherein the pump assembly comprises one or more diaphragm pumps. 20. The system of claim 11, wherein the pump assembly comprises one or more piston pumps. 21. The system of claim 11, wherein the pump assembly comprises one or more peristaltic pumps. 22. The system of claim 11, further comprising: a plurality of parallel inlet tubes. 23. The system of claim 11, further comprising a plurality of parallel exhaust tubes. 24. The system of claim 11, wherein the means for determining pressure comprises a pressure transducer; and the system further comprises a feedback circuit coupled between the pump assembly and the pressure transducer, wherein the pressure transducer measures a flow rate of the air circulation system, and wherein the feedback circuit maintains a constant flow rate in the air circulation system. 25. The system of claim 24, wherein the pressure transducer is coupled to the inlet tube. 26. The system of claim 24 wherein the pressure transducer is coupled to the exhaust tube. 27. The system of claim 24, wherein the feedback circuit varies the operation of the pump assembly. 28. The system of claim 27, wherein the pump assembly comprises one or more rotary pumps, and wherein the feedback circuit varies the angular velocity of the one or more rotary pumps. 29. The system of claim 1, wherein the air circulation system comprises: a pump assembly; an inlet manifold; an inlet manifold tube coupled between the pump assembly and the inlet manifold; at least one inlet chamber tube coupled between the inlet manifold and the measurement chamber; an exhaust manifold; an exhaust manifold tube coupled between the exhaust manifold and the pump assembly; and at least one exhaust chamber tube coupled between the exhaust manifold and the measurement chamber. 30. The system of claim 29 wherein there are a plurality of parallel inlet chamber tubes coupled between the inlet manifold and the measurement chamber. 31. The system of claim 29 wherein there are a plurality of parallel exhaust chamber tubes coupled between the exhaust manifold and the measurement chamber. 32. The system of claim 1, wherein the volume perturbation element generates volume perturbations at a first perturbation frequency. 33. The system of claim 1, further comprising: a controlled temperature environment, wherein the air circulation system pumps air from the controlled temperature environment into the chamber. 34. The system of claim 33, wherein the air circulation system comprises: a pump assembly coupled to the controlled temperature environment; an inlet tube coupled between the measurement chamber and the pump assembly, and an exhaust tube coupled between the measurement chamber and pump assembly. 35. The system of claim 34, wherein the controlled temperature environment comprises; a heater element; and temperature monitoring circuitry coupled to the heater element. 36. The system of clam 35, wherein the temperature monitoring circuitry maintains a constant temperature within the controlled temperature environment. 37. The system of claim 36, wherein the temperature monitoring circuit controls the operation of the heater element. 38. The system of claim 1, further comprising: a pump assembly coupled to a controlled temperature inlet; an inlet tube coupled between the measurement chamber and the pump assembly; an exhaust tube coupled between the measurement chamber and the pump assembly; and temperature monitoring circuitry coupled to a heater element, wherein the temperature monitoring circuitry is configured to vary the operation of the heater element to maintain a constant temperature within the controlled temperature inlet. 39. A method of conducting plethysmographic measurement of a test subject, comprising: placing the subject to be measured in a plethysmographic measurement chamber, sealing said chamber; conducting plethysmographic measurement of the subject, and renewing air within the chamber using air from an atmospheric environment outside the chamber, wherein conducting plethysmographic measurement comprises: generating pressure oscillations within the chamber, measuring the amplitude of the pressure oscillations, calculating a volume of air within the chamber based on the measured pressure oscillations, and determining a body composition of the test subject based on the calculated volume of air within the chamber. 40. The method of claim 39, wherein generating pressure oscillations comprises: oscillating a diaphragm at a fixed frequency. 41. The method of claim 39, wherein renewing air within the chamber comprises: pumping air through an inlet tube into the chamber; and pumping air through an exhaust tube out of the measurement chamber. 42. The method of claim 41, wherein pumping air through an inlet tube comprises: establishing a controlled temperature environment; and pumping air from the controlled temperature environment through the inlet tube. 43. The method of claim 42, wherein establishing a controlled temperature environment comprises: measuring the temperature of an enclosed environment; comparing the measured temperature to a desired control temperature; varying the operation of a heater element in response to the comparison of the measured temperature to the desired temperature. 44. The method of claim 41 further comprising; heating the air with the inlet tube to a controlled temperature. 45. The method of claim 44, wherein heating the air with the inlet tube to a controlled temperature comprises: measuring the temperature of air within the inlet tube; comparing the measured temperature to a desired control temperature; and varying the operation of a heater element in response to the comparison of the measured temperature to the desired temperature. 46. The method of claim 39, further comprising: maintaining a constant flow rate of air being circulated through the chamber. 47. The method of claim 46, wherein maintaining a constant flow rate comprises: measuring a flow rate; comparing the measured flow rate to a desired flow rate, and varying the operation of an air pump in response to comparing the measured flow rate to the control flow rate. 48. A plethysmographic measurement system, comprising: a plethysmographic measurement chamber; plethysmographic measurement components coupled to the measurement chamber; and an air circulation system coupled between the measurement chamber and an atmospheric environment outside the measurement chamber, wherein the air circulation system renews air within the measurement chamber using air from the atmospheric environment outside the chamber during plethysmographic measurement, wherein the air circulation system comprises: a pump assembly, an inlet tube coupled between the measurement chamber and the pump assembly, and an exhaust tube coupled between the measurement chamber and pump assembly. 49. A plethysmographic measurement system, comprising: a plethysmographic measurement chamber; plethysmographic measurement components coupled to the measurement chamber; and an air circulation system coupled between the measurement chamber and an atmospheric environment outside the measurement chamber, wherein the air circulation system renews air within the measurement chamber using air from the atmospheric environment outside the chamber during plethysmographic measurement, wherein the air circulation system comprises: a pump assembly, an inlet manifold, an inlet manifold tube coupled between the pump assembly and the inlet manifold, at least one inlet chamber tube coupled between the inlet manifold and the measurement chamber, an exhaust manifold, an exhaust manifold tube coupled between the exhaust manifold and the pump assembly, and at least one exhaust chamber tube coupled between the exhaust manifold and the measurement chamber. 50. A plethysmographic measurement system, comprising: a plethysmographic measurement chamber; plethysmographic measurement components coupled to the measurement chamber; an air circulation system coupled between the measurement chamber and an atmospheric environment outside the measurement chamber, wherein the air circulation system renews air within the measurement chamber using air from the atmospheric environment outside the chamber during plethysmographic measurement; and a controlled temperature environment, wherein the air circulation system pumps air from the controlled temperature environment into the chamber. 51. A method of conducting plethysmographic measurement of a test subject, comprising: placing the subject to be measured in a plethysmographic measurement chamber, sealing said chamber; conducting plethysmographic measurement of the subject; renewing air within the chamber using air from an atmospheric environment outside the chamber; and maintaining a constant flow rate of air being circulated through the chamber.