Here is an experiment to see what kind of siphoning power the 155 has.
The compressor was set to 15psi and the liquid is water.
The water level in the jar is about one inch above the bench.
By pressing the trigger and pulling back as if I were drawing a fine line, the water would rise to 3 1/2 to 4 inches or about 2 1/2 to 3 inches above the level in the jar.  But it didn't pop right up there.  It rose very slowly.  It appears that when the opening at the needle is very small, so is the venturi effect and so is the amount of liquid that can be lifted.
Maximum height was obtained with the trigger approximately vertical.  The water went all the way to the bottom of the brush and did so very quickly.  That's a rise of about 17 inches above the level in the jar.
When the trigger was pulled all the way back, the water dropped to about 9 inches above the table.  Apparently the larger opening weakens the venturi effect.
Since I have no way of accurately positioning the trigger, I can't produce a graph.  But, my impression is that it would look like an upside down U.
I also observed that pulling the trigger back without pressing down allowed the water to flow back in the jar.

So what does it show?  Well, it's the vacuum created by the venturi effect that draws the paint out of the airbrush so that it can be atomized.  As you can see, this effect is weakest when trying to paint a fine line.  The thicker or more viscose the liquid, the worse the problem.  Ink might work fine, but something like enamel might not.  So, if you want to make fine lines with paint, you need to thin it more than usual.

Gravity fed airbrushes like the model 100 have a slight advantage.  In a gravity fed brush, there is a small positive pressure in the paint chamber which helps in ejecting the paint.  A siphon fed brush on the other hand, always has a negative pressure in the paint chamber. It has to in order to draw paint up from below.  This works against the venturi effect.  So if you're going to do really fine work, a gravity fed brush is usually recommended.

You can increase the venturi effect by increasing the air pressure and therefore the velocity of the escaping air.  This will improve paint flow, but it can also increase overspray.  Generally, lower pressure is advised for fine lines.

Another thing that this experiment shows is that it is not a good idea to pull back the trigger without pressing down for air. With a siphon brush, the paint will run back into the cup or jar.  So the next time you want to spray, you'll get a pause while it's being pulled back up.  With a gravity fed brush, you might get a paint build-up that will splat the next time you press the trigger, or even paint dripping from the needle.

As much as I like my 155, I have to admit that I cannot produce pencil thin lines with enamel paint.  But, they are fine enough for my needs.  Besides, my hands aren't steady enough to do pencil-fine work anyway.  I can do best by thinning the paint more than usual and setting the air pressure to 25psi., which is as much as my little compressor can produce.

Here is a sample of my best effort with Floquil flat black thinned about 1:2 paint to thinner.




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