The Repair Arm augment has the following description:
Repairs your hull every time you collect scrap, but reduces scrap collected by 15 percent.
Does anyone know by how much it repairs your hull? Should I assume from the description that it repairs the hull by just one HP each time scrap is collected, or might it vary (e.g. one HP for each 10 scrap collected)?
From personal experience (which seems backed up by this thread where the developer responded), the Repair Arm heals you 1 hull point, regardless of the amount of scrap collected.
From the developer via that thread as well:
We didn’t want it to be always a positive, obvious choice. The power of constant repairs after every fight is too useful without at least a few setbacks. […] I might sell it come sector 6 or 7 if I’m doing well since that 15% starts to hurt a lot more and the extra boost to finish off the boss is helpful.
The FTL Wikia seems to agree with this as well:
Repairs your hull by 1 point every time you collect scrap[.]