on Mass Flow
the majority of flowmeters measure volumetric flow, three types of
flowmeters measure mass flow. These
three types are Coriolis, thermal, and multivariable flowmeters.
This article discusses the reasons for measuring mass flow, and
then looks at the advantages and disadvantages of Coriolis, thermal, and
Another type, mass flow controllers, both measure and control the
flow. Mass flow controllers are used both in the semiconductor
industry and in industrial markets.
learn more about multi-tech, go to:
flow is measured in a number of different ways.
Ultrasonic, magnetic, vortex, and turbine meters use various
methods for determining average speed or velocity of the flow at some
point in the flowstream. They
then multiply this velocity value by the cross-sectional area of the pipe
to yield volumetric flowrate.
displacement flowmeters measure volume directly by separating portions of
the flow into small containers of known volume, and counting how many
times this is done. This is a
highly accurate method of flow measurement, and positive displacement
flowmeters are widely used for custody transfer applications.
Why Mass Flow is
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.
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.
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
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.
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.
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
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.
thermal flowmeters also measure mass flow, they do so very differently
from Coriolis meters. Instead
of using fluid momentum, thermal flowmeters make use of the thermal or
heat conducting properties of fluids to determine mass flow.
While the majority of thermal flowmeters are used to measure gas
flow, they are also used to measure the flow of liquids.
origins of thermal flowmeters lie in hot wire anemometers.
These consist of a heated, thin wire element, and are very small
and fragile. Hot wire anemometers were used in velocity profile and
turbulence research. Because
they are susceptible to breakage and to dirt, they are not suited to
are several different thermal flowmeter technologies.
Some measure the speed with which heat that is added to the
flowstream disperses. Others
measure the temperature difference between a heated sensor and the ambient
flowstream. Thermal flowmeters typically require one or more temperature
sensors to measure the fluid temperature at specific points.
flowmeters have several main advantages.
One is a relatively low purchase price.
Secondly, thermal flowmeters can measure the flow of some
low-pressure gases that are not dense enough for Coriolis meters to
measure. Both of these
advantages give thermal flowmeters their own unique niche in flow
main disadvantage of thermal flowmeters is low accuracy.
While some thermal flowmeters may achieve accuracy levels of one
percent, other thermal flowmeters have accuracies in the three to five
percent range. It is the
accuracy level of thermal flowmeters that is the main barrier to
classifying them as new-technology flowmeters rather than traditional
technology meters. Users who
are considering thermal flowmeters need to balance their accuracy needs
with their cost requirements.
flowmeters measure mass flow by combining volumetric flow measurement with
density measurement. Density is usually measured either by consulting a table, or
by dynamically measuring pressure and temperature. This is called an inferred method, because a formula is used
to compute mass flow. The
main types of multivariable flowmeters are differential pressure (DP),
vortex, ultrasonic, and magnetic.
main advantage of multivariable DP flowmeters is that only one process
penetration is required to get three process readings: flow, temperature,
and pressure. This reduces
the chance of fugitive emissions, and also the number of leak points.
Another advantage of multivariable DP meters is that users who are
already measuring volumetric flow with a DP flowmeter can upgrade to a
multivariable DP meter with a minimum of changes.
disadvantage of multivariable flowmeters is that accuracy levels are not
as high as accuracy levels of Coriolis meters.
This is due to the number of variables involved, and to the fact
that it is an inferred method of computing mass flow.
On the other hand, the purchase price of multivariable flowmeters
is substantially below that of most Coriolis meters.
at least three main ways to measure mass flow, users are advised to
determine their accuracy requirements and their budgetary constraints
before making a decision about which type of flowmeter to select.
When considering cost, it is also advisable to consider the
lifetime costs of a flowmeter, rather than just the purchase price.
There are many high quality products available in all three