Paints & Coatings Resource Center

Ask the Expert Question-and-Answer Archive

by Ron Joseph

May, 2005

Determining Air Flow Rate for Paint Spray Gun

Q. Found your site through a search using the term "automotive painting air pressure calculations" in the All the Web search engines. My question is as follows:

I bought a brand new paint gun at a yard sale. It is a cheap gun but I bought it for five dollars so even if I cannot use it, I can get my money back when we have our own yard sale. If possible, I'd like to use it to paint an old BMW motorcycle I'm refurbishing.

The problem is, on the package, it says that the air consumption is "380/min at 4 BAR". Now I know I can convert BAR to PSI, but what is the 380/min? 380 What? per minute??? I'm used to requirements like 7 CFM at 40 psi, but I cannot find a converter to tackle this question. Perhaps if I knew what the 380 represented. So I'm "Asking the expert".

The package also states that the gun requires 50 to 70 psi to operate. I converted 4 BAR to psi and came up with 58 psi. My calculator, online converters and my brain just can't wrap around what the 380 represents. HELP!!!!!

Thanks in advance.

A. Clearly you are an engineer, going to online conversion tables and realizing that 380/min has little meaning. Using my own deductive powers I have concluded that it is highly probable that the unit should be 380 L/min, or 380 dm3/min. Here is how I reached this assumption. An HVLP spray gun usually has an air flow rate of approximately 20-25 ft3/min. By going online and using my own favorite conversion tables I played around with m3 and liters only to find that 380 m3/min would be way too high for an air atomizing gun, while 380 L/min would yield 13.4 ft3/min. (Conversion: 1 L/min = 0.0353 ft3/min) This is definitely the type of value I would expect for a conventional air atomizing spray gun, but it is too low for HVLP. Hence, my conclusion is that you are using a conventional air atomizing spray gun for which the volumetric air flow rate is 13.4 CFM at 58 psig.

I hope I'm right and get the prize!

Best wishes,

Ron Joseph

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