METABOLIC ANALYZER 

JOHN D. LEM 

Basic Design Configuration 

The metabolic analyzer (MA), figure A.I.g.-1, figure A.I.g.-2, was designed to support experiment M171. It operates on the so-called "open circuit" method to measure a subject’s metabolic activity in terms of oxygen consumed, carbon dioxide produced, minute volume, respiratory exchange ratio, and tidal volume or vital capacity. The system will operate in either of two modes.

This second mode was the prime mode for Skylab. Following is a brief description of the various subsystems and their operation. For a more comprehensive discussion, please refer to NASA Technical Memorandum No. 64797, December 26, 1973.

Major functional components of the metabolic analyzer (fig. A.I.g.-3) include the mass spectrometer, inspiration and expiration spirometers, a calibration assembly, breathing apparatus, and the electronics assembly. The mass spectrometer is of the single focusing magnetic sector type with ion collectors for oxygen, nitrogen, carbon dioxide, and water. An internal ion pump maintains the necessary vacuum within a sealed gas analyzer assembly. A more detailed description of the mass spectrometer is offered by paper 57, Spectrometry and Allied Topics, in the Proceedings of the American Society of Mass Spectrometry Twenty-first Annual Conference, held in San Francisco, California, on May 20-25, 1973.

Rolling-seal, dry spirometers are used to measure the inhaled and exhaled breath volumes. These have capacities of 4 and 7 liters, respectively, the expiration spirometer alone being used for vital capacity measurements thereby dictating the need for a larger capacity. A potentiometer connected by a cable to the spirometer piston provides a measurement of breath volume. A large electrically actuated, pneumatically operated ball valve controlled the spirometer emptying (exhalation) or filling (inhalation).

The calibration assembly consists of two cylinders of compressed gas, each with an absolute pressure regulator and solenoids for control of gas flow. One bottle is filled with a mixture of nitrogen, oxygen, and carbon dioxide; it is used for calibration of these same mass spectrometer outputs. The second cylinder contains a mixture of nitrogen and dideuteromethane (CH2D2). The CH2D2 has the same mass number as water vapor and is used to calibrate the water output from the mass spectrometer. Initial pressurization of the gas cylinders is approximately 10.7 MPa (1.07 X 10[7] N/m²).

The breathing apparatus consists of a mouthpiece, check valve assembly, hoses, and a nose clip. These are all of conventional design. The check valve assembly is a modified Lloyd valve. To minimize leakage, the valve seats are made of ground aluminum and the flappers of compression molded polycarbonate. The flat surfaces attainable with these materials permit achievement of very low leak rates.

The electronics assembly conditions all signals from the various transducers, provides control signals to the various valves and solenoids, and includes an analog computer for computation of metabolic data. 

Performance Specifications 

Measurement Range

      Oxygen consumption                                       0-4 liters/minute (STPD)

      Carbon dioxide production                              0-4 liters/minute (STPD)

      Minute volume                                                 0-150 liters/minute(BTPS)

      Vital capacity                                                   0-7 liters/minute (BTPS)

Accuracy

       O2 consumed, CO2 produced & min.vol.      ±3.5 % of value or .05 Liters/Min. if >

       Vital capacity                                                  ±2% of value or .05 liter whichever is greater

        Breathing resistance                                        747 Pa at peak flow rate of 5.83x10[-3]m³/s

        Power requirements                                        28 V dc at 4.46 A 

Acknowledgments 

The assistance of the following persons in the development and operation of this piece of equipment is gratefully acknowledged: O.K. Duren, William Lewter, and Cortes Perry at the Marshall Space Flight Center; Ralph Lehotsky at Perkin Elmer Corporation, Aerospace Division; and Doug Getchell at the Martin Marietta Corporation.

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