One solution to this problem is to dispense with volumetric flow measurement altogether in favor
of mass flow measurement, constructing the flowmeter in such a way that the actual mass of the
gas molecules is measured as they pass through the instrument. This approach is explored in more
detail in section 22.7 beginning on page 1452. A more traditional approach to this problem is to
specify gas flow in volume units per time, at some agreed-upon (standardized) set of pressure and
temperature conditions. This is known as standardized volumetric flow measurement.
Referring to our pictorial example previously shown, imagine if we took a sample of the gas
flowing at a line pressure of 250 PSIG and let that sample expand to atmospheric pressure (0
PSIG) and ambient temperature (60 degrees Fahrenheit), measuring its new volume under those
new conditions. Obviously, one cubic foot of gas at 250 PSIG would expand to a far greater volume
than 1 ft3 at atmospheric pressure. This ratio of “standard volume” to “actual volume” (in the
pressurized pipe) could then be used to scale the flowmeter’s measurement, so that the flowmeter
registers in standard cubic feet per minute, or SCFM