Air Hose Loss
You may have noticed that I use the inexpensive plastic Badger hose. I started out with this because it was cheap. But, I stay with it because I like it. It's very light and flexible and doesn't get in the way. However, it's also very small. The outside diameter is just a little over 1/8 inch, so the hole in the middle can't be very large. I got to wondering how much pressure drop occurs between the compressor and the airbrush. So I dug into my stash of junk and came up with this setup.
Believe it or not, it doesn't leak. I checked it with soapy water, and no bubbles.
I connected each of my brushes to this rig, and adjusted my regulator so that the static pressure at the airbrush was 20 psi. Then I pressed the trigger and recorded the pressure under use. In each case, the reading back at the regulator dropped just one psi. Here are the results at the airbrush:
With the Patriot, I tried different static pressures and found that the percentage drop was a constant 20 percent until I got down to 10 psi, and there it rose to 25 percent.
So, what conclusions might we draw from this?
1. Yep, there is some pressure loss from the hose.
2. Percent wise, it varies from airbrush to airbrush. Brushes with more airflow, like the 250 and Paasche will cause a larger drop. In the case of the Paasche, it gets pretty significant.
3. This hose is 6 feet long. A longer hose would lose even more.
4. Remember, one psi, or 5 percent of this loss occurs back at the regulator.
I expect that a braided hose would have a larger bore and less drop. But, without trying one, I don't know if this is true. If anyone has done a similar experiment with this type of hose, I would be interested in the results.
Note: I was able to repeat this experiment with a six foot braided Badger hose, and the results were completely different. Air hose loss was reduced to less than one psi, even with the Paasche airbrush. So, I guess the real lesson to be learned here is that there is a difference in hoses. I still like the light plastic hose, but there is a price to pay for that flexibility.
Significance?
Probably not much. The drop can be compensated for at the regulator, and pressure should be set for results anyway, not some arbitrary number. But it's worth being aware of. Suppose you had a garden hose with a trigger type nozzle attached to it. Now, suppose you turned the water on at the faucet just a little. Pressure in the hose will build to full mains pressure. What will happen when you squeeze the trigger? You'll get a blast of water that immediately drops to a low flow. Something similar happens with your airbrush. You may have read that it's a good idea when doing close work to keep the trigger depressed, and control paint on and off strictly by moving the needle. This could be one reason why.
From what I can gather, a MAC valve (Micro Air Control) on an airbrush seems to be just an adjustable restriction which adds to this loss in a controllable manner.
Propel
Some of the lower price airbrushes like the Badger 250 and 350 come with an adapter for canned gas. Badger's brand is called Propel. The canned gas lets you start airbrushing on a shoe string. But it quickly gets to be expensive and it is not the ideal source to drive an airbrush. For one thing, pressure can vary widely during use. Here is a can of Propel at 79 degrees F.
Notice that the pressure is 41 psi. Now, here is the can cooled to 32 degrees F.
Notice that the pressure has dropped to 13 psi. The thing is, like any can containing compressed gas, it gets cold when it is used. So you may start out with something like 40 psi, but you will soon be getting much less. This is not the best situation for airbrushing. That is why you may see recommendations that the can should be placed in a container of warm water during use. But it shouldn't be too warm, or the can might explode. Propel says on the label not to exceed 120 degrees F. So, yes, it is possible to airbrush with canned gas. But it really isn't a practical source of air. That's why I recommend that anyone wanting to airbrush should acquire a regulated source of compressed air before they even start looking at airbrushes. Feedback
Ron Nath pointed out that there is something I should mention about the Badger vinyl hose. Badger sells two adapters to connect this hose to a standard 1/4" compressor output. The only difference between them is that P/N 50-021 has a bleed hole in the side and P/N 50-023 does not. Badger says the bleed is needed when connected to a diaphragm type compressor with no tank. Apparently, some diaphragm compressors do not like being connected to a stopped off line. But, if an air tank is used, it acts as a cushion to reduce the shock to the compressor. Badger says it is not a problem for their compressors. I suspect that it is not a problem for most airbrush specific compressors either. The braided Badger air hose connects directly to a 1/4" line and it has no bleed.