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VOLUME, MASS AND STANDARD
There are several basic categories of flowrate units. This newsletter will focus on the broad categories of VOLUME, MASS, and STANDARD. The units a flow meter will use to display flowrate depends mostly on the fluid being measured. In general, water is usually measured in volume units, steam in mass units, and natural gas in standard units.
Volume flowrate refers to flowrate in terms of volume per unit of time, for instance, gallons per minute or liters per second. Other examples of volume flowrates are acre feet per day or cubic meters per minute. These units are straightforward and relate to most people's basic understanding of flowrate...velocity. A user can divide by the cross-sectional area of the pipe and get the fluid velocity in units such as feet per second. Learn More.
Mass flowrate refers to flowrate in terms of mass per unit time, such as pounds per second or kilograms per second. These units are helpful when the fluid moving through the pipe is compressible or changing composition. For instance, a user wants to account for steam moving through his plant. Volume units will not be sufficient because a cubic foot can have more or less steam depending on whether it is under more or less pressure. A certain flow of steam at 100 cubic feet per second can have dramatically different amounts of steam depending on the pressure and temperature. In this case, mass flowrate units are more suitable because they "adjust" for pressure and temperature changes. With mass flowrate units, the users can account for the amount of steam circulating in pounds rather than cubic feet of steam. This is desirable because the heating value of steam is related to the mass of the steam, not the volume alone. Learn More.
Standard units can be related to both volume and mass units. Standard units are volume units at a certain pressure, temperature, and compressibility. Examples are standard cubic feet per hour (SCFH) or normal cubic meters per minute. These units are used only with compressible gasses, such as natural gas. Every measurement is "converted" to a standard or base set of conditions. The base conditions ensure that every volume measured has the same amount of product in it. In other words, a cubic foot of natural gas at standard conditions will always have the same density. Because the density at standard conditions is constant, it can be multiplied into the standard flowrate to get mass flowrate. For example, 100 SCFH multiplied by 1 pounds-mass per cubic foot equals 100 pounds-mass per hour. One very important note about standard units of flow: standard conditions vary between industries, companies, countries, etc. Make sure the standard units used are consistent throughout the calculations. Learn More.