Pressure?

limd21,

Thanks for those facts, that is interesting.

Mike

Mike,

I agree, since most all of us use the regulator pressure at the tank as our reference point and probably have setups that are all relatively comparable (< 10' of 1/8" dia hose). A few feet +/-, a few PSI +/-, it doesn't matter much. Where it does make sense to consider is if somebody was using an *abnormally* long run of airbrush hosehose - most of this stuff sold for airbrush use is only 1/8" I.D. (Pretty puny, really).

I found a air flow calculator at the Gates Rubber website and at 35 psi tank regulator pressure, 50' of 1/8" I.D. hose and flowing at .7 cfm (all "normal" values for an airbrush) will drop almost a full 9 psi! A 1/4" hose, in comparison, will only drop around 0.2 psi.

But for all practical purposes the pressure loss in the airhose poses no problem or even concern since we are mearly using the reading on the regulator gauge as a basis for where we want the tank pressure at. What literally comes out of the airbrush nozzle is irrelevant.

Mike

Yep - I'd also agree with Ron (driggc55) that the pressure drop due to the hose is neglible - probably no more than a couple PSI at pressures we use our ABs. It is interesting to note how strongly this pressure drop is correlated to hose diameter - if I remember my physics, it goes something like the *4th* power of the diameter ratio.

I am goin to have to agree with limd21 about the pressure drop in hose. Its do to the viscous nature of the fluid flow. Although in this context I would think it would be negligible.

Ron

I am goin to have to agree with limd21 about the pressure drop in hose. Its do to the viscous nature of the fluid flow. Although in this context I would think it would be negligible.

Ron

QUOTE: Originally posted by MikeV QUOTE: 3) pressure at the airbrush when spraying (less than #2, due to loss in the hose) I don't think so.

I'm going to humbly disagree here. In any fluid transfer through a hose/pipe/tube, there will be pressure drop. In designing air line sizing for commercial / industrial settings, the piping or tubing always has an engineered spec of pressure drop per unit of length. This is due to the inherent resistance the pipe/tube has on the fluid (air).

A more common example is the plumbing you and I have in our own homes. Water supply pipes are typically large where it enters the house, and step down at least once or twice (e.g. 1" supply, 3/4" distribution, 1/2" final). If it was 1/2" from supply to final, the pressure losses would be unacceptable. Another example is the air hoses I have in my garage shop that I run my air tools. If I use the 50' run of 3/8" hose, my impact wrench really struggles on tight bolts because the pressure at the tool is so reduced. If I use a bigger hose (1/2" dia) it fares much better. (No, there are no leaks!)

If you want to read more about this, I suggest you do your own Google search for "pressure drop air hose" - you'll find *lots* of stuff. Or, for a more specific reference, check out:
http://tinyurl.com/2od4o

Swanny, I have a 4 Gal. air tank on my compressor, but I still see a drop of about 2 psi from static to dynamic. Agree on the hose, wont get any change in psi.

I always refer to static pressure as that is what most people do when referring to airbrush pressures. If you have an air tank then the drop will only be a pound or two when you depress the airbrush trigger.

QUOTE: 3) pressure at the airbrush when spraying (less than #2, due to loss in the hose)

I don't think so.

Mike

QUOTE: Originally posted by limd21 Do you guys have the gauge/regulator at the tank or near the brush. I think most of us have the gauge at the tank - so that's what most mean when they speak of pressure. In reality, though, there's actually three pressures one could consider: 1) static pressure when not spraying 2) pressure at the regulator when spraying (less than #1) 3) pressure at the airbrush when spraying (less than #2, due to loss in the hose) It's all relative, of course,

Loss in the hose??? Only if it leaks. If your hose is of the proper diameter the only thing it will restrict is cfm, not psi.

Do you guys have the gauge/regulator at the tank or near the brush. I think most of us have the gauge at the tank - so that's what most mean when they speak of pressure. In reality, though, there's actually three pressures one could consider:
1) static pressure when not spraying
2) pressure at the regulator when spraying (less than #1)
3) pressure at the airbrush when spraying (less than #2, due to loss in the hose)

It's all relative, of course,

i too adjust my pressure with the trigger depressed. i spray broad coverage with a 60/40 paint to thinner ratio at 18 psi and drop under 10 psi with a 40/60 ratio for fine line work. i spray future thinned just a hair at 15 to 18 psi as well as my primers and clear coats. later.

I too am talking about actual spraying pressure. If you use a tank as a reservoir you will not experience that pressure drop, have an overall smoother airflow and a cleaner air stream.

Can't speak for others, but I always refer to the actual spraying pressure when using the AB. I get some drop from static pressure on my gauge as well, but I adjust to where I want it when running air through the brush.