Tools, Sharpening and Power Machinery - Replacing an air compressor's pressure switch

I was given an air compressor from a co-worker that has a broken pressure switch. The broken switch, which isn't the original, is a Siemens/Furnas, but I am planning on replacing it with a Square D pressure switch with the same pressure ratings. The switch I will be purchasing is:

Square D 9013FHG12J52M1X Air Compressor Pressure Switch (http://www.drillspot.com/products/121970/Square_D_9013FHG12J52M1X_Air_Compressor_Pressure_Switch)

I have two questions:

1. How exactly is the unloader valve attached to the copper pipe? It appears as if it is some type of crimp connection. I briefly studied the valve that is currently connected to my compressor, but I couldn't figure out how to remove it or how it was originally installed.

2. The compressor doesn't currently have a safety valve, so I am planning on adding one of those as well. The pressure switch will cut off at 125 PSI, so do I get a safety valve that releases at 125 PSI or should I go with something like 150 PSI?

1. This is usually a compression type fitting. A brass ring is crimped around a tube by a compression nut. Commonly available.

2. Is your tank ASME certified? There should be a metal tag on it stating the MAWP. Match the pop-off valve rating to the tank pressure rating. If not an ASME tank, go as low as you can to prevent an accident.

Yep, it is a compression nut fitting.

For the safety valve, I don't think I would want to go lower than my pressure switch (125PSI). Would I want to match the pressure switch, or go just above it?

Eh, go about 10-15 psi above your PS cutoff. When you replace the PS, if you salvage the unloader tube, make sure it seals tight to the new PS unloader. These compression fittings are somewhat tricky when reused. If it leaks, your compressor will short cycle like crazy. Good luck.

Eh, go about 10-15 psi above your PS cutoff. When you replace the PS, if you salvage the unloader tube, make sure it seals tight to the new PS unloader. These compression fittings are somewhat tricky when reused. If it leaks, your compressor will short cycle like crazy. Good luck.

What do you mean by reusing a compression fitting? Are you saying physically removing the nut and ferrule, or is simply unscrewing the nut (while leaving the nut and ferule on the pipe) and then screwing it back on also problematic?

The second thing :) Because the ferrule seals to the male part of the fitting on the pressure switch, it basically retains its shape and does not always want to seal to a new fitting without some persuasion. I've destroyed many an old compression fittings by overtightening them, trying to get them to seal. Not a problem, though, you can always cut another tube and use a fresh fitting, but that is a bit more work.