I understand the psi at the wall makes a difference and I'll check that! But is there an explanation for how the psi at wall changes things when you are regulating it down at the gun?..just curious, like I said I know it does make a difference but just like to learn all I can from the guys that's more experienced than me!
A little late to the party, so...apologies and sorry for what is a long answer but this is something I have to deal with all the time in my job.
Nothing flows without a change in pressure. Pressure and flow are closely related. The greater the change in pressure, the greater the flow (air is a little bit of an exception because there is a pressure drop beyond which you don't get more flow (google critical pressure)). The 30 psi you adjust at the gun is the pressure (and therefore flow) you are allowing at the outlet of the regulator when the trigger is squeezed. This also happens to be the inlet to the spray gun. We want to control the amount of air
into the gun so this is a great place for it.
If you have 120psi at the wall, you would also like to have 120 psi in the hose just before the regulator such that when you squeeze the trigger, the pressure drop across the regulator is high (120psi -30psi=90psi) and therefore you have good flow across it and into the gun due to the nice change in pressure. Unfortunately hoses have friction, fittings have restrictions and you do not get the same pressure at the end of the hose when air is flowing as you had at the beginning. Hose diameter influences pressure drop too. Smaller diameter hoses loose more pressure than large ones. If you were using a
ridiculously long,
ridiculously small hose, it could lose 90psi from start to finish when you pull the trigger. In this case, you would only get 30 psi in the hose just before the regulator when everything was flowing. Pulling the trigger in this case would produce no change in pressure across the regulator and you would get no flow at all. You would get a big initial puff until things equalized though. Large hoses also contain more air than smaller ones. This means that when you do make that nice pressure drop by using the spray gun you literally have more air available to deliver in a large hose as compared to a small one.
So in summary, you want to use a large hose with a bunch of air in it to carry as much pressure as you can all the way to where you need it.