I'm installing a water pressure gauge on my mobile home water system and the gauge will be on a stub end of the pipe (as opposed to mounted directly on or near a tee). I'm going to be mounting the gauge to the end of a 48" long pipe (the water line is below the mobile home and I want to be able to read the pressure from inside the home).

How much do I need to be concerned about air in the line affecting the accuracy of the gauge? In other words, when I install the new stub pipe, should I try to fill the pipe with water as full as possible before I mount the gauge on it?

 

No need to be concerned about air in the piping, the gauge will work just fine. I do suggest that you install a valve (ball valve) just before the gauge so that you can quickly isolate the gauge in the unlikely event that it self destructs.