CHAPTER 19 SYSTEMS: ENVIRONMENTAL CONTROL AND LIFE SUPPORT SYSTEM—SUPPORTING THE HUMAN ELEMENT OF THE ISS 336 total pressure and trigger an alarm if the pressure gets too high or low, the PCA can automatically introduce O 2 and N 2 from the external tanks on the airlock. As discussed in Chapter 3, pressure above a critical limit can rupture the shell of the ISS, thereby introducing a catastrophic leak. Too low of a pressure can cause the astronauts to lose consciousness and die. If the pressure gets too high (777 mm Hg or 15.03 psi), the PCA uses its Vent and Release Assembly (VRA) to release gas outside of the spacecraft. In case of a problem with the VRA, the Positive Pressure Release Assembly (PPRA) can also vent the atmosphere. The trigger point of the PPRAs (778 mm Hg or 15.05 psi) is set higher than the PCA and would only vent in a significant emergency. The PPRAs are essentially large vents on the ISS, and they can release about 68 kg/hr (~150 lbs/hr) of gas. These might also be used in an emergency response (see below). However, the flight control team monitors the atmosphere closely since any gas that is vented is a waste of a critical commodity. Similar devices are present in the Columbus Module and JEM. Careful measurement of the O 2 and N 2 quantities in the atmosphere is required for the PCA or the ETHOS flight controller to know whether either levels need to be adjusted. Composition of the atmosphere is measured by the Major Constituent Analyzer (MCA). The MCA consists of a mass spectrometer that can measure O 2 , N 2 , CO 2 , H 2 , water, and methane in the atmosphere. The USOS is lined with tubes that make up the Sample Delivery System (SDS). The MCA draws in a small sample of atmosphere from each module and measure the constituents. An alarm will be annunciated if any component is outside of the expected limits. If this occurs, the MCA will repeatedly sample the atmosphere in that module so the crew and ground can monitor the situation. Otherwise, the system will move on to the next module and keep cycling. Several handheld devices can also be used to measure atmospheric contaminants. These devices consist of the Carbon Dioxide Monitor (CDM), the Compound Specific Analyzer-Combustion Products (CSA-CP), and the Chip Measurement System (CMS). See Figure 2. All three devices work essentially the same way by pulling in cabin air and measuring the constituents. The CDM is mainly used in situations when a localized area needs to be monitored e.g., if a crew is working in an area where ventilation is poor. The CSA-CP is the main tool to determine the constituents of smoke or whether there is a fire inside of a rack. It measures the levels of carbon monoxide, hydrochloric acid, and hydrochloric cyanide—typical and dangerous by-products of a fire for the type of materials used on the ISS. The CSA-CP has a long tube attachment that can be inserted into holes in the racks to measure the presence of smoke that may not be visible. Finally, the CMS on the USOS measures ammonia, whereas the Russian CMS detects formaldehyde, benzene, styrene, ozone, phosgene, carbon monoxide, ammonia, and nitrous fumes. After O 2 and N 2 , CO 2 is the next- biggest atmospheric concern. Even low levels of CO 2 can impair the mental acuity of an astronaut, especially if the exposure occurs over a long period of time. The amount of a gas is measured in terms of partial pressure, which is the amount of the pressure that a specific gas contributes to the total pressure. The average partial pressure of CO 2 at the surface of the Earth is less than 1 mm Hg, or about one-tenth of a percent of the total pressure. On the ISS, the level is maintained to be less than 4 mm Hg and is typically around 3.5 mm Hg. Some astronauts reported headaches when the levels went above this amount. Exposure to values above 20 mm Hg can lead to headaches, increased respiratory rate, reduced performance decrement, and possible depression of the central nervous system. Recent research may also indicate that a person’s sensitivity may change in weightlessness. The primary way of removing CO 2 on the USOS is via the Carbon Dioxide Removal Assembly (CDRA). See also Figure 3. A similar device is located on the RS. In the event of failure, the crew can load Lithium Hydroxide (LiOH) canisters into a fan assembly to filter the CO 2 as a backup. The LiOH canisters absorb CO 2 in a chemical reaction. However, this is a contingency plan only, as it uses a non-regenerative consumable. Even maintaining low levels is not adequate to keep crew members healthy. Gravity causes warm gases to rise and cooler ones to sink. This helps mix atmospheric gases, dispersing those such as CO 2 . In the absence of gravity, fans are needed to perform this function on the space station or as an astronaut breathes in one place, since a local pocket of toxic gas can build up. The ventilation on the ISS is designed to keep levels of CO 2