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SCFM ACFM Calculator
“Don’t Believe What Your Rotameter Tells You!”
In measuring air flow, problems can sometimes occur in distinguishing standard cubic feet per minute (SCFM) from actual cubic feet per minute (ACFM). The difference between what you see on the rotameter and what you may have expected to see may be SCFM vs. ACFM flow. Most rotameters read in SCFM when you may be expecting ACFM flow. The Rotameter is most likely accurate, but like other forms of flow measurement through a static device, the SCFM reading has to be adjusted for Pressure and Temperature. In remediation air and gas flow applications, the density changes with the pressure and temperature so is accounted for in the following calculations. SCFM is determined using “standard” conditions. The most common standard used in the United States is with “sea-level” properties (NIST and OSHA).
14.7 PSI Pressure
60° Fahrenheit Temperature
36% Relative Humidity
However, real life “actual” conditions are seldom “standard” conditions, so corrections must be made to assure that the blower or compressor will provide the proper amount of air for the process to function properly. As a rule of thumb, an SCFM result will be lower than an ACFM result during a vacuum application, and a SCFM result will be higher than an ACFM result during a pressure application. Blower performance calculations are based on actual (not standard) conditions existing at the inlet and outlet connections of the blower. To better understand the difference between SCFM and ACFM, consider these situations:
There is a balloon filled with 1 cubic foot of gas at ambient, or “standard” conditions. The balloon is taken into a hotter temperature, and the gas expands to 2 cubic feet because of reacting to the heat. The balloon now has 2 Actual Cubic Feet, 2 ACF, but still has only 1 Standard Cubic Foot, 1 SCF. This is true because if the balloon is taken back to standard or ambient conditions, the volume within the balloon will return to 1 ACF.
Suppose there is highly compressed gas in a tank. The gas takes up 1 SCF or .5 ACF within that tank. When the gas is released into a balloon, there will be more than .5 ACF because of the change of pressure, but still 1 SCF. In any situation, the actual volume can be changed by adjusting the pressure or temperature. The standard volume cannot be changed by adjusting the pressure or temperature. The only way to change standard volume is by adding or reducing mass. Back to the reading on your rotameter. The same concept for volume applies for rotameters, except that the volume of gas is moving. The following formula helps to figure out Actual Cubic Feet per Minute based on data from your Standard Cubic Feet per Minute Rotameter:
ACFM=SCFM* (460+Temperature (F))/530* 14.7/(14.7+psig)
If you read the gas flow at 100 SCFM at a temperature of 90 degrees F and a pressure of 40 psi, your ACFM flow is:
100* (460+90)/530* 14.7/(14.7+40)=27.9 ACFM
The difference between ACFM and SCFM for this scenario is significant, so be thoughtful when checking your rotameter. To get the ACFM flow, convert from SCFM to ACFM using this helpful calculator:
2010 - present
2010 - present