While the majority of flowmeters measure volumetric flow, there is a type of flowmeter measure mass flow. One reason to measure mass flow is to achieve greater accuracy. Because the quantity of a fluid varies with temperature and pressure, fluid flow can vary with changing temperatures and pressures. This is most notable for gases. Pressure and temperature variations have minimal effects on liquids, so these effects are often disregarded when measure liquid flows. However, temperature and pressure have a much more pronounced effect on gases, so much mass flow measurement is measurement of gases.
In the process industries, it is sometimes desirable to measure mass flow for greater accuracy and to accommodate measurement standards. Chemical reactions often refer to mass rather than volume, so mass flow is often measured in the chemical industry. Some products are sold by weight rather than volume, and in these cases it is necessary to measure mass flow. Gas flow is widely measured in the process industries.
There is a close relation between volumetric flow and mass flow measurement. If the volumetric flow of a fluid is known, multiplying this value by the density of the fluid yields mass flow. Some flowmeters, such as multivariable flowmeters, compute volumetric flow and then determine mass flow by using a calculated density value.
What percent of the total flow measurements are volumetric as opposed to mass flow? In a recent worldwide survey of conducted by Flow Research and Ducker Worldwide, 75% of flow measurements were volumetric and 25 percent were of mass flow. It is clear, then, that mass flow accounts for a significant percentage of total flow measurements.
Coriolis flowmeters use fluid momentum to measure mass flow directly. The fluid enters the meter and passes through one or more vibrating tubes, and accelerates as it reaches the point of maximum vibration. As the fluid leaves this point, it decelerates. This causes a twisting motion in the tubes. The Coriolis meter measures this twisting motion, and mass flow is directly proportional to the amount of twist.
While it is natural to think that users choose Coriolis meters because of their ability to measure mass flow, user surveys show differently. In the previously mentioned user survey, respondents were asked why they are using Coriolis meters. The leading answer given was accuracy, which was mentioned by 63 percent of respondents worldwide. Reliability was the second leading reason, and was mentioned by 14 percent of respondents. Only a small percentage measure “ability to measure mass flow.”
Coriolis flowmeters are among the most accurate meters. Their main limitations are line size and cost. Over 90 percent of Coriolis flowmeters are used on line sizes of two inches and less. Coriolis meters become very large and unwieldy, especially in sizes from four to six inches. Cost also increases with size. Even smaller size meters are generally more expensive than other comparable new-technology flowmeters. Users who are considering Coriolis flowmeters need to balance their need for accuracy and reliability against purchase price. Some users select Coriolis meters despite their higher initial cost, because low maintenance requirements reduces their cost over the life of the meter.