How many leaks have been fixed on the ISS, roughly?

Question

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! "

Answer 1

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.

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

• 2018: A leak through a drilled hole in the orbital module of a Soyuz docked to the station. Several posts on this site mention this incident such as Where is the hole now? (ISS/Soyuz spacecraft hole) The hole was patched and the problem went away completely when the Soyuz undocked.

• 2020: A slow leak is isolated to the Service Module after various troubleshooting steps including isolating the crew in certain areas of the ISS. This incident has also been asked about on this site: Tea bags to find the leak on the International Space Station? and was a developing story this year: October 6 2020 October 16 2020 October 21 2020 The leak may have been slowed down but this is probably not the final resolution.

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