The term for the amount of water vapor in air is called humidity. Just to be clear, “humid air” is not the same as “moist air” (which is a mixture of water vapor and other substances or particles in the air). Humidity is defined in terms of the water part of this mixture, it is called the "absolute humidity". It is commonly referred to as relative humidity, expressed as a % (like on household humidistats). It is called relative because it measures the absolute humidity relative to the maximum. Imagine a cube of 1 metre by 1 metre by 1 metre filled with air. The temperature is 16° Celsius, and the weather is fair. That cube contains one cubic metre of air, which is also saturated with water vapor.

The size of the compressor tank, usually measured in liters or m³, should be determined by the overall type of use. For short quick concentrated bursts (e.g. air nailer), a small tank size is sufficient. If the unit is to be used during longer periods of time (e.g. board sander, impact wrench or sandblasting work), a larger tank size will be required. Quite often, a combination of the two types is needed. In that case, the larger air receiver is the best option. It will also have a positive influence on the compressor start/stop or modulation control.