August 25, 2012

Side cup vs. gravity feed cup


When reading information in postings on airbrush forums comparing the properties of airbrush guns with gravity feed paint cups with those of siphon feed cups, I feel a lot is being written that does not really have a sound rational foundation. A few of these I would like to elaborate on from my perspective in this blog entry.

Below this paragraph you see an image of a gravity feed airbrush (Iwata HP-BH) on top and a siphon feed airbrush (Iwata Custom Micron SB) underneath it. The solid red line indicates the paint level in the cups and the dotted blue line indicates the center of the needle / nozzle combination.

Gravity feed vs. siphon feed airbrushes



Please bear in mind that airbrushes are almost never in an absolutely horizontal position as seen in the image, but slightly tipped with the tip pointing downwards somewhat. In both constructions it elevates the paint level relative to the needle / nozzle plane. From this results that in both cases the needle / nozzle is submerged in paint when the paint cups are full. Gravity feed needle / nozzles will be flooded in paint as long as there is paint inside the cup. In siphon feed airbrushes the paint level may at some point drop below the level of the center of the needle / nozzle combination.

What this means is that artists can not remove the needle to remove tip dry residues from the needle when they use gravity feed airbrushes without allowing paint to flow out of the nozzle (which is fully open since the needle is removed to be cleaned). Artists that use a siphon feed airbrush however can remove the needle from the airbrush to clean the tip without spilling paint.

The paint cup supposedly blocking the artist's view is a myth since the airbrush is never held in a concentric position relative to their eyes. Unless of course an airbrush is fitted with a disproportionally huge paint cup. Most of the time the gravity feed paint cup will be beside the area between the artist's eyes and the detail that is being worked on.

An other myth is the speed of response. Gravity feed airbrushes are said to be more responsive to trigger action, which is not really an issue in the way I work, since I always have a piece of paper in the hand that is not holding the airbrush that I use to tune the spray which I want to be fine and consistent before spraying paint onto the airbrush paper, -board, -metal surface or canvas. Siphon feeds may become a little less responsive as the paint level drops, but this is not an issue when I decide to buy a certain type of airbrush. The slight delay in paint supply of siphon feed airbrushes may be an advantage because it allows artists to respond to the airbrush, because there is always an interaction between the artist and the airbrush which involves subtle periods of time before paint leaves the airbrush.

Furthermore the paint in gravity feed airbrushes is always pulled towards the nozzle cavity in front of which paint and air mix, which makes it available right away. The paint in siphon feed guns is sucked towards the nozzle by the reduced pressure that is created around the needle tip just outside the nozzle. It means that the supply of paint in such constructions is more gradual, but fast enough to not disturb the painting process.

Then there is the supposed matter of (in)balance. I have read and heard some artists say that siphon feeds are unbalanced since the paint cup is off center in the horizontal plane - beside the needle / nozzle center instead of right above it. But the weight of the air hose, usually minimizes this effect (if it is noticeable at all, I never could). When an air pressure reducer and / or a moisture trap is fitted between airbrush and hose the alleged imbalance will be minimized even further. In my experience this does not affect the the possibility of creating fine detail with the airbrush in any way.

Considering all properties of the different constructions and having had experience with both types that are in the image, I have a slight preference for the siphon feed, because it is easy to clean between paint refills or color changes and it allows to turn the paint cup allowing me to work in more extreme angles than the fixed gravity feed guns allow me to do. Finally siphon feed paint cups are often available in various sizes which could be an advantage to artists that create larger sized art work, even if I rarely make use of that possibility myself.

One nice thing to know for artists that own both the HP-BH and the Custom Micron SB, is that the needles are interchangeable. In fact I found that the HP-BH needle works better in the Micron SB than the original one... The other way around there's not a lot of difference in performance. A slight top angle difference of the needle appears to be an improvement in one case while not doing much in an other. This puzzles me somewhat, but I may find out why this is the case one day.