Development and Validation of a Space Suit Helmet Carbon Dioxide Washout Measurement System

Providing adequate washout of carbon dioxide (CO2) from within a space suit helmet is essential to maintaining a safe operating environment for astronauts using space suits. A valid and reliable method for quantification of inspired CO2 inside space suits is required to ensure the health and performance of suited crewmembers. With this objective, several different methods for measuring the concentration of CO2 in a space suit helmet were evaluated. A nasal cannula was compared with respirator style masks worn by suited test subjects, with air drawn into gas analyzers to measure the concentration of CO2 in the immediate vicinity of the mouth and nose. The respirator style masks, some being commercially available products and some novel designs, did not provide repeatable results based on initial pilot testing in three subjects. Based on the analysis of those studies, the decision was made to down-select to a commercially available nasal cannula as the primary sampling device to be used in follow-on testing in the MKIII (n = 8) and Z2 (n = 6) prototype space suits, with five subjects performing tests in both suits allowing for repeated measures comparisons. Subjects were tasked with achieving target metabolic rates of 293, 586, and 879 Watts (1000, 2000, and 3000 BTU/h) and at air supply flow rates of 3.4, 6.8, and 10.2 Am(sup 3)/hr. (2, 4, and 6 ACFM). Each test condition was performed twice; once with subjects instructed to breathe however they felt comfortable, and once with subjects instructed to breathe only through their nose. Inspired CO2 values were determined by the lowest points, or troughs, within each breath of the respiratory trace. This method provides multiple inspired CO2 samples at known metabolic rates for each test condition and provides a larger data set for analysis than possible through gross averaging of the minimum inspired CO2. Results indicate that reliable measures are achievable under both breathing conditions but that restricting subjects to breathing only through their nose provides for less variable CO2 measurements breath to breath, within +/- 66.7 Pa CO2 (+/- 0.5 mmHg) at a 95% confidence level than when breathing unrestricted, which resulted in +/- 173.3 Pa CO2 (+/- 1.3 mmHg) variability with 95% confidence. However, during normal operation of a space suit it is not realistic to enforce or monitor the breathing style of the crewmember and, although restricted breathing is reliable, it is not known whether measurement of inspired CO2 during nose-only breathing is valid for evaluation of a space suit washout. To verify that one or both of the breathing style nasal cannula sampling methods provide a valid inspired CO2 measurement, testing is planned in an environmental chamber during which the ambient CO2 concentration will be controlled. A nasal cannula will be worn by unsuited subjects and measurements will be taken under both restricted and unrestricted breathing conditions across a range of metabolic rates during which nasal cannula CO2 concentration measures will be compared against the ambient value.