The pressure decay poses no threat to the crew's safety or to the continued operation of the Station and its systems, but Russian and U.S. engineers are conducting a thorough investigation of the decrease, which appears to have begun about Dec. 22. The decline occurs at a rate so small, only a few hundredths of a pound per square inch (psi) of pressure per day, that it is difficult to detect.
This week, Foale and Kaleri checked a variety of valves and seals throughout the Station using an ultrasonic leak detection system and found no leaks. Today, Kaleri checked a Russian system, called Vozdukh, that removes carbon dioxide from the cabin as well as several other Russian systems for leaks and found none.
To continue the effort to diagnose the source of the pressure decay, flight controllers in Russia and the U.S. plan to ask the crew to shut off portions of the Station periodically in coming days. In the next few days, hatches will be closed for periods ranging between 12-24 hours to seal off various modules to check if any element within them could be the source of a leak. Those modules may include the Progress cargo vehicle, the Pirs Docking Compartment and Soyuz spacecraft, and the Quest airlock.
If those steps do not detect the source of the leak, then the crew may be asked to move into the Russian living quarters module for several days and shut hatches separating the Russian living quarters and other modules from the rest of the station for several days. Those actions would likely not take place any earlier than Wednesday. Engineers are continuing to work on potential plans for those steps to diagnose the leak and to review the number of hatches that would be closed at that time.
The decay in pressure over the past few weeks aboard the station has amounted to a decrease from the normal pressure of 14.7 psi, a pressure equivalent to sea level on Earth, to a pressure today of about 14 psi, a pressure equivalent to the normal air pressure in Oklahoma City. The changes in pressure do not present a concern for the health of the crew. Also, plentiful supplies of air, oxygen and nitrogen, are aboard the station -- enough that the current rate of decay could be sustained for six months without further supplies aboard if required. However, engineers are confident they will identify and correct the source of the decay as they continue the diagnostic work onboard.
Flight controllers may feed more nitrogen into the Station atmosphere late Sunday or Monday to increase the overall air pressure and maintain the cabin atmosphere in the optimal range for the operation of equipment aboard the complex. Russian flight controllers also are continuing to evaluate the possible replacement of parts of the Station's oxygen-generating Elektron system. The Russian system generates oxygen by recycling wastewater aboard the complex. It has failed, but spare parts are aboard that engineers are confident can bring it back to full operation and they are developing plans to perform that work possibly next week. While the Elektron failure is being evaluated, the crew has used Solid Fuel Oxygen Generators, canisters that are heated to produce oxygen, to replenish oxygen on the Station.
Despite the leak detection activities, engineers are not certain the fluctuation and slight decline in pressure aboard the Station is the result of a leak from the complex. Evaluations continue to determine if it instead could result from or be significantly contributed to by troubleshooting and intermittent Elektron operation, SFOG oxygen generation activities, recent changes in temperature and sun angles, the accuracy of various pressure measuring systems, or other factors.