CHAPTER 16 DAY IN THE LIFE: IN-FLIGHT MAINTENANCE 272 The new Joint Airlock, named Quest, was brought to the ISS on board STS-104/ISS-7A (2001). This proved to be an ambitious assembly mission. The first two of three EVAs to install the new ISS airlock were successfully conducted from the airlock of the Space Shuttle orbiter. The mission called for the final EVA to be conducted from the new airlock to prove that all airlock systems functioned correctly. A feature of the airlock design is that a depress pump can pump cabin air from the airlock back into Node 1. Instead of having to vent the cabin air overboard when depressurizing the airlock, and thus losing the valuable air resource, the air can be saved by putting it back in the ISS stack. This would only work as long as the hatch seal between Node 1 and the airlock did not leak. It also meant that the seals in the air lines between the depress pump and Node 1 could not leak otherwise, cabin air would leak back from Node 1 into the airlock. It was quickly discovered that a leak existed in the air lines when the depress pump was first turned on, thus allowing air from Node 1 to leak back into the airlock. This leak prevented crew members from being able to keep the airlock at the lower pressure they needed. Fortunately, a new diagnostic tool—an Ultrasonic Leak Detector (ULD)—was flown to the ISS on this mission. The ULD is a tool used widely in industry to find leaks in pressure vessels by converting the ultrasonic noise created by the leaking, turbulent gas into an audible sound. The more directly the microphone of the tool is pointed at the leak point, the louder the sound gets. The crew successfully used the ULD during the mission to locate a leaking fitting on the depress pump. The fitting was tightened and, ultimately, the final EVA of the mission was completed from the brand new airlock. The primary reason for sending the ULD to the space station was to help the crew find leaks of cabin air to space. This method of using the ULD was first put to the test during Expedition 8 (2004). The ground teams detected a small leak (approximately 1.2 mm Hg [0.02 psi] pressure drop per day) however, the point of the leak could not be easily determined. The leak rate was slow enough that crew members did not need to perform their emergency depressurization procedures nonetheless, they did need to determine the source of the leak in order to stop it and prevent additional air from escaping. Over a number of weeks, the crew and the ground used the ULD and other techniques to isolate the leak to somewhere in the US Laboratory. The crew then used the ULD to survey the laboratory. The crew ultimately determined that the loudest source of ultrasonic noise was coming from a vacuum hose attached to the large window in that module (see Figure 21 in Chapter 3). The crew demated the hose from the vacuum source, which stopped the air from leaking. Figure 3. Expedition 8 Commander Mike Foale uses the Ultrasonic Leak Detector to try to pinpoint a small cabin leak to space near the window in the US Laboratory. A small probe is attached to the microphone of the ULD (near the astronaut’s hand), which detects the ultrasonic noise of leaking air. That ultrasonic noise is converted to audible noise that the astronaut listens to in the headphones. The louder the noise, the closer the ULD probe is pointed toward the leak point.