The "pressure at the tip" numbers have been more of a marketing figure than a meaningful performance measurement.
You'd have to know the history of liquid spray guns and be familiar with the intro of HVLP to the painting industry many years ago to fully understand that. (which is horribly boring)
I'll try to lay out the nickel tour for you.
During what was possibly the peak of liquid coatings usage in the US for OEM's (manufacturing), the focus on efficiencies became elevated for what I'll call fuel mileage for paint. Basically the measure of how much of your quart of paint goes on your car vs how much of it ends up as overspray collected in your booth's filers.
In the paint world we call it "transfer efficiency" as in, how efficiently do you transfer the paint from your gun to the intended target.
A spray gun isn't a complicated tool, we use the gun to shape our spray into a pattern that works well for transferring it to the part (lets say car), to the car without using a brush or a roller or some other sort of surface contact tool.
We know that we need to atomize our paint and basically throw it at the car in a controlled manor. Droplet size plays heavily into the overall efficiency of the transfer process. The bigger the droplet size the better the transfer rate as a rule. Now this can all be done with a conventional spray gun by simply turning the atomizing air pressure down.
But, engineers took it upon themselves to failsafe efficient paint transfer by designing a gun that would not allow you to "over atomize" your paint. The design of the HVLP gun was unleashed on the paint world and marketed as this paint saving magic gun.
Folks, all they did is they made the damn air holes bigger. Take your garden hose and hold your thumb over the end to create a spray or squirt stream that you can shoot really far. That's restriction and velocity, that's pressure at work. Now take your thumb off the end of the hose with the water running, that's high volume/low pressure.
HVLP is training wheels for painters, training wheels for efficiency.
The numbers you see advertised for air cap pressures are to appeal to paint operations managers. They let that paint operations manager say "There, now lets see them waste paint by over atomizing it!".
Unfortunately that is ALL there is to it. It isn't a magic gun that body shops need or should have, it's just a gun that is compromised in it's ability to atomize and shape your spray pattern.
I tell everybody, buy a conventional gun and feed it low air pressure if you want HVPL characteristics. Then, if you choose to be more precise with your paint applications you will have those abilities on reserve in your gun. You can spray efficiently with a conventional gun. And a properly adjusted conventional gun will use much less air than an HVLP gun that is adjusted for it's maximum performance/efficiency.
So there's the low down on that air cap pressure number, it's nothing for you to be concerned about and it's nothing for you to try to measure or comply with. It is a marketing figure.
If you are running conventional spray gun with paint viscosities in a common range for light bodied primers, bases, clears.. I'd suggest starting at around 25psi to see if you can get away with it. If it's a gravity feed then chances are you can get away with it, often times even with a siphon gun (conventional) you can get away with 25 psi at the inlet.
HVLP, you will need to feed it more air because those air holes are so damn big.
You will see guns out there labeled “compliant”, compliant guns came out after HVLP (god I hate HVLP guns lol), compliant design took one step back towards conventional design. If you have a compliant gun then you have a gun that’s only half as compromised in it’s abilities as an HVLP gun.
I don’t paint for a living any more, I currently manage a powder coating line. But liquid painting used to be my bread and butter, I’ve sprayed no less that 25 thousand gallons of paint through a gun in my hand. It would have been shameful for me not to have picked up a decent understanding of liquid spray guns during that time. :grin: