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When searching for an air compressor to meet my needs, the information I found at Thomas Industries made it a real pleasure to incorporate their product into this hobby project. I highly recommend spending some time on their web site to learn more about compressor operation in general. Below are a few points of information that I learned that could be an important factor in determining how large a compressor you need.
Free Air vs Operating Pressure CFM
A lot of people (and advertisers) refer to the "Free Air CFM" rating in reference to the delivery of a given air compressor. Most people also know that the delivery CFM drops off as the operating pressure rises. As can be seen for example in the specs to the left for the TA4101DC, it delivers 2.20 CFM with no back pressure (at sea level) and diminishes to 0.95CFM at 100psi (again at sea level).
Effects of Altitude
However, since many people operate at high altitude areas such as in Colorado, there is another significant factor to consider; that is, the effects of altitude on compressor output.
If you are operating your air system for example to cycle the compressor between 80 to 100psi, which would be within this manufacturer's specifications, these reference points provide for some interesting comparision of compressor output at sea level and at 5000 feet. As shown by the following link to a Thomas Technical Data Bulletin, this same compressor will be putting out only 65% of it's sea level capacity at 5000 feet and, at 100psi it will have dropped down to approximately 30%.
For a rugged 1/3 horsepower continuous duty compressor such as this unit, this merely means it will take longer to supply the same volume of air under these conditions. For the cheaper, non continuous duty compressors, this probably means a burn out, or extensive cool down cycles while trying to get your tires pumped up. You might want to consider the effects of altitude, depending on where you plan to operate.
Excerpt from Thomas Technical Data Bulletin