Characterization of variability sources associated with measuring inspired CO2 in spacesuits

NASA seeks a validated, standardized methodology for measuring the inspired carbon dioxide gas (CO2) in spacesuits to verify that ventilation designs maintain safe levels of CO2 during suited operations. To date, several studies have been performed to assess the CO2 washout capability of different spacesuits using a variety of in-suit sampling techniques and devices, while different approaches are used to test breathing masks for applications such as firefighting and diving. This study reviews existing methodologies for measuring CO2 washout and then describes a series of systematic evaluations conducted to characterize sources of variability associated with spacesuit CO2 washout measurement equipment and methods so that calculations of inspired CO2 may be appropriately adjusted or interpreted to account for the known measurement errors. To systematically isolate and identify the contributions of variability associated with each component of measurement equipment and methods, a technique was developed using 4% CO2 calibration gas and 1% CO2 calibration gas to simulate perfect washout with respiratory traces of exactly known expired CO2 levels. Using this technique, sample line length, line inner diameter, effect of fittings, and placement of flow controllers such as rotameters were tested sequentially to quantify their effects on the resulting simulated respiratory trace. The results of this testing indicate that unsuited, ambient sampling for CO2 should be performed with small diameter, short length tubing, at high flow rates with minimal flow interrupters (e.g., fittings, valves) in order to minimize errors associated with loss of data integrity. Of the conditions tested, data integrity was best maintained when sampling at 1000 mL/min, using a practical sample tube of 3.0 m (10 ft) length and 1.6 mm (0.063 in) inner diameter, with no flow interrupters between the CO2 source and the CO2 sensor. In the near-term, the results of this study will inform a follow-up study, the objectives of which are to define a validated, standardized methodology for measuring inspired CO2 in pressurized spacesuits, and to characterize intra-subject and inter-subject variability during human-in-the-loop (HITL) testing of CO2 washout in the extravehicular mobility unit (EMU) spacesuit.