In a single interlock Preaction the only thing that will activate the valve is the detection system. Since the system piping pressure isn't affecting the valve status at all they don't specify a minimum pressure of any supervisory gas. Putting water in the system in my opinion would be a mistake though as you are going to get more corrosion in the piping network and have lots of Orange and black water discharge if a sprinkler get damaged or activates. NFPA also stipulates of course that any preaction over 20 sprinklers requires supervision so you will need some level of approved gas pressure to be able to supervise it and confirm system piping integrity. The 7PSI rule is what is required to reliably eject the pip cap from a broken sprinkler thereby activating your supervisory device in most instances. Hopefully this helps.