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Leaks come in all sizes. Very small ones might be ignored or remain undetected, slightly faster ones might be identified and dispositioned; if they are slow and going to remain stable, they might not be repaired. Leaks that are faster-still, or that might be unstable however are likely to be fixed immediately.

The video below, found in Spaceflight Insider's How was the exact location of the recent ISS air leak found? shows clips of use of an ultrasonic leak detector aboard the ISS and at least one instance where it seems a leak was repaired.

After astronauts determined from which of the modules the leak is coming from, in this case the upper section of the Soyuz MS-09 spacecraft, they used a device called an ultrasonic leak detector (ULD) to find the precise location of the Soyuz spacecraft that was leaking atmosphere.

Question: Roughly how many leaks have been actively repaired (fixed) on the ISS?

If it's possible to answer, I'm also curious if the existence of any were initially discovered by ULD use, or if the leaks that were repaired were all first sought after a drop in pressure was recognized.

below: screenshot from a recent Roscosmos tweet of cosmonaut Sergey Prokopyev(presumably) talking about the lead detection and repair and showing an ULD? (I can't speak Russian, but presumably this is correct.) Translating the text using Google:

"Friends, I decided to shoot a video to answer your numerous comments and dispel rumors. Everything is calm on the ISS! "

cosmonaut Sergey Prokopyev and ultrasonic leak detector

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  • $\begingroup$ I've added the russia tag in hopes of getting this double-checked by someone who speaks Russian, thanks! $\endgroup$
    – uhoh
    Sep 17, 2018 at 6:56
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    $\begingroup$ The ULD isn't brought out until a leak is confirmed; it's not routinely used to check the cabin. Leaks are found by pressure or dp/dt sensor trends.The only other "big" ISS leak I'm aware of was in the USOS at a window, IIRC because the crew had been using a piece of tubing as a handhold and loosened it. $\endgroup$ Sep 17, 2018 at 11:49
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    $\begingroup$ Ah. I rarely watch videos in questions, and didn't this time. $\endgroup$ Sep 17, 2018 at 11:59
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    $\begingroup$ He demonstrates the ULD and the box of sealant with which the hole was sealed. $\endgroup$
    – A. Rumlin
    Nov 22, 2020 at 18:45
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    $\begingroup$ @uhoh The ULD - UL101 Ultrasound Sensor manufactured by CTRL Systems has several attachments for leak detection, including the conical shaped attachment shown in the picture and the small metal tip shown in the video. $\endgroup$ Feb 8 at 18:52

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I have found four instances of cabin air leaks on the ISS being publicly discussed.

  • 2001: A leak in the air lines used to scavenge air out of the airlock prior to opening the hatch for EVAs. This resulted in less air being scavenged than desired.

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.

The Ultrasonic Leak Detector (ULD) was used to locate the leak and it was fixed by tightening fittings.

  • 2004: a leak through a hose used to maintain vacuum between two panes of the large window in the US Lab. This picture shows the ULD in use for that issue.

enter image description here

Information on both of these issues is from The International Space Station - Operating an Outpost in the New Frontier

The crew then patched the leak using polyimide masking tape (aka. Kapton tape), an industrial tape that is extremely resistant to extremes in temperature

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  • $\begingroup$ Very nice history of leaks. The "Where it the hole now?" question shows what I believe is a ULD, but I'm not sure if it was used to find the leak, or was just being tested to see if the leak registered on it after it was found. $\endgroup$
    – uhoh
    Nov 22, 2020 at 18:40
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    $\begingroup$ @uhoh I think you are right. The instrument there has a conical fitting on the "front" of it instead of the tubular one on the ULD in the picture above, but the body looks similar. $\endgroup$ Nov 22, 2020 at 18:43
  • $\begingroup$ okay thanks, there's also this comment just now posted under the question. $\endgroup$
    – uhoh
    Nov 22, 2020 at 18:48

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