Citing an instrument manual I am analysing data from a commercial instrument. I need to set a threshold that has to do with the precision of the instrument, so I looked up the value from the manufacturer's online specifications. How do I cite it?

The university does not have any official policy, and other papers in the field either don't mention it explicitly, or give the value without citation, but I believe this is actually a quite relevant number, and thus should be specified and quoted.
Basically, you can cite this user's manual just as you would any book-like publication. The only difference is that you don't cite the name of an author:


  User's Manual for Foo Instrument, Model Bar. Random City: XYZ Corporation (Year).


or whatever is the appropriate reference style for your work.