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Figure 1. In order to understand the characteristics of different
types of flowmeters, it is important to first review some basic facts
about how gas flows through a closed piping system.
Part I I : Flow met er Selection Str ategies
How to Choose the Right Technology for Gas Measurements
By John Frederick
In this second installment of a two-part article, we address the topics typically covered in a comprehensive gas flow measurement
course. The following describes common types of gas flowmeters and provides guidelines for their selection and usage. It also discusses
the influence of basic gas properties on meter performance and examines methods and equipment for accurate calibration. Part one of
this article covered similar topics in liquid flow measurement (Aug. 2010, pages 18-22).
Basic Gas Law s
In order to understand the characteristics of different types of
flowmeters, it is important to first review some basic facts about how
gas flows through a closed piping system. One of the key
characteristics that can help in this regard is the Reynolds Number,
which takes into account the relationship between the velocity of the
flow, the size of the pipe, and the density and viscosity of the gas.
Identifying the Reynolds Number relationship helps to understand
how a gas flows through the pipe and how various flowmeters are
influenced by the flow (Figure 1).
Gas flow measurement is somewhat more complicated than liquid
flow measurement because a gas is a compressible fluid and a liquid
is a non-compressible fluid. When we say that a liquid is non-
compressible, we mean that when the pressure is increased, the
density changes only a negligible amount. A gas is compressible, so
when the pressure changes, the density also changes. When the
absolute pressure on a gas is doubled (if the temperature is held
constant), the density will double. When a gas flows through a
system, the pressure continually changes resulting in density
changes. These density changes must be taken into account in gas
flow calculations.
It is also important to make accurate measurements of pressure and
temperature at the appropriate locations in the system in order to
determine the true flow characteristics.
The relationships among pressure, temperature, density, and flow are based on gas laws, which describe the relationship of pressure,
temperature and volume for a fixed quantity of gas in a closed container. Two gas laws that are useful in understanding gas flow are
Boyles Law (Robert Boyle) and Charless Law (Jacques Charles), named for the early experimenters who discovered their underlying
principles. The gas laws are based on using absolute pressures and absolute temperatures.
Boyles Law states that at a constant temperature, the volume occupied by a given quantity of gas varies inversely with the absolute
pressure (Figure 2).
Charless Law states that at a constant pressure, the volume occupied by a given amount of gas varies directly with the absolute
temperature (Figure 3).
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Figure 2 . Boyles Law for gas. Figure 3. Charless Law for gas.
These two laws can be combined into a single law, called the Perfect Gas Law, as follows:
where:
P = Absolute pressure
T = Absolute temperature
Q = Volume
Subscripts 1 and 2 represent two different conditions.
The previous equation can be rearranged to solve for each of the three quantities:
Popular Flowm eter Designs
Modern gas flowmeters can be classified into two general categories: quantity meters and rate meters. Quantity meters are those in
which the gas passes through the meter in successive, isolated quantities. Rate meters are those in which the gas passes though the
primary element in a continuous stream. The flowrate (amount of flow per unit of time) is derived from the interaction between the
flow stream and the primary element.
The most popular gas flowmeter designs in use by industry include:
Differential- Pressure Flowmet ers: When a gas flowing through a pipe encounters a restriction reducing the area of the pipe, the
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Figure 4. Turbine fl owm eters have a large installed base and remain a high
accuracy solut ion for gas applications.
gas must go faster past the restriction. This increase in speed results in a difference between the pressure upstream of the restriction
and the pressure downstream of the restriction. There is a definite relationship between this pressure difference and the flowrate of the
gas passing the restriction, which can be used to create a flow measuring device by purposely installing a restriction with known
characteristics into the pipe. Establishing the relationship between flowrate and differential pressure enables measurement of an
unknown flowrate in the pipe.
Differential pressure or differential head flowmeters fall into three general categories: orifice, nozzle and venture. The basic principle of
operation of each of these is the same, but the basic shapes are different.
Pitot Tube Flow meters: A pitot tube flowmeter is a small-diameter tube with one end bent 90 degrees to the stem of the tube. The
tube is inserted into a pipe so the bent end faces into the flow. A pressure gauge connected to the other end of the tube reads the total
or stagnation pressure of the flow impacting on the open end of the tube. This total pressure is greater than the static pressure, which
is measured at a pressure tap on the side of the pipe. The difference between the total pressure and the static pressure is called the
velocity pressure and is related to the flow velocity at the tube opening.
Turbine Flow meters: Turbine flowmeters are employed
extensively for high-accuracy flow measurements. They
can be built for use at very high pressures, as well as very
high and low temperatures. They are made in sizes from
to over 24 inches in diameter. The meters signal can
be used to measure the flowrate at any given time or thetotal amount of flow over a period of time (Figure 4).
A turbine flowmeter has a rotor suspended on low-friction
bearings in a meter body in such a way that the rotor can
spin freely as the gas passes through the device. A pickup
assembly mounted on the outside of the meter body
generates an electrical signal when the rotor turns and
produces one electrical pulse as each rotor blade passes
the pickup. The pickup may be a self-generating magnetic-
type or a carrier-excited nonmagnetic type.
Laminar Flow Elements: Laminar flow elements are
made with one or more small passages through which gas
flows. Pressure taps are provided to measure the
differential pressure developed by the flow going through
these small passages. The passages are designed to
produce a Reynolds Number small enough (less than 2000)
to always produce laminar flow. Under laminar flow
conditions, the relationship between the differential
pressure and volumetric flowrate is not the square-root
relationship found in other differential-pressure flowmeters, but a linear relationship.
Vortex-Shedding Flowm eters: Vortex-shedding flowmeters are made by installing a metal bar, called a bluff body, across a
diameter of the pipe. The bluff body creates an obstruction so that the flow must go faster in order to pass. This results in the creation
of swirls or vortices in the flow, which form first on one side of the bluff body and then on the other side. These vortices, called vonKrmn vortices (Theodore von Krmn), are generated at a frequency proportional to the flowrate of the gas past the bluff body. The
frequency can be measured in several ways, including pressure sensing, temperature sensing, and by detecting modulation of an
ultrasonic beam. Bluff bodies are made in a variety of shapes to produce the desired operating characteristics.
Ultrasonic Flow meters: Ultrasonic flowmeters utilize piezoelectric transducers mounted on the pipe in contact with the gas, or can be
mounted on the outside of the pipe (clamp-on transducers). The transducers generate high frequency sound waves that travel across
the pipe at the speed of sound in the gas.
Two basic ultrasonic meter principles are used: transit-time and Doppler. Transit-time meters have two or more pairs of transducers
that alternately send signals downstream and then upstream. The transit-time in both directions is measured and the time difference is
proportional to the average flow velocity along the sonic path. Transit-time meters are only suitable for relatively clean liquids, since
dirt, particles and gas bubbles can disperse the sound signals. Doppler meters send a sound beam into the flow stream where it is
bounced or scattered by a particle of dirt. The reflected signal will experience a frequency shift, called the Doppler shift, and the change
in frequency is proportional to the velocity of the particle. Doppler meters require some dirt to reflect the sound waves. They will not
work with very clean gases.
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Crit ical Flow Ventur is: In 1839, from studies of Daniel Bernoullis and Giovanni Battista Venturis works, Jean Claude Barr de Saint
Venant and Pierre Laurent Wantzel developed a general equation of the discharge of fluids from apertures by which the existence of a
sonic-flow limit could be inferred. In 1866, Julius Weisbach observed the phenomenon that the mass rate of a gas through a nozzle
reaches a maximum directly proportional to the inlet. In recent years, the sonic-flow Venturi has been used as a reference meter, as a
transfer standard, and as a control for regulating the flow of gas (See Fig. 5 & 6).
Figure 5. Smooth Approach Orifice (SAO). A form of a Venturi that offers
excellent stability while maintaining its original calibration, unless mechanically
abused. Frequently employed as standards for measuring critical gas flow,
SAOs provide high-efficiency recovery, resulting in less pressure loss. SAOs are
commonly used in the automotive industry for measuring exhaust flow.
Figure 6. Sonic Nozzles. Also known as a Critical Flow Venturi, these
instruments are, by design, a constant volumetric flowmeter. By using
a regulated pressure supply, the Sonic Nozzle becomes a precision
mass flowmeter. Sonic Nozzles are used primarily in the aerospace
industry and for flow transfer standard calibration systems.
Therm al Flow meters: Thermal flowmeters function on the principle of measuring heat transfer, where the Delta T is a function of
flow. Nature wants everything to exist in a state of equilibrium. If two objects of different temperatures are placed in contact with each
other, there will be a heat energy transfer until the two bodies obtain a temperature equilibrium. An analogy would be the human body.
If you stand in the wind and bare your arms by removing your jacket, you will feel chill on your arms. This chill is caused by the
temperature of the wind and by the mass of wind molecules taking heat energy away as the cooler air crosses your skin surface, which
is a heat transfer of energy from your warm body to the air molecules that impinge on your skin surface. Your brain controls your body
temperature at a constant 98 F. When air passes over your arms, your skin temperature is reduced by the thermal transfer of body heat
to the air molecules, and your brain, which instantly senses the skin heat loss and tells your body to burn more calories to maintain a
constant temperature. If your brain included a calorie meter, it could provide an indication of air flowrate, because skin heat loss is
proportional to the number of air molecules crossing your skin surface.
Common Application Crit eria
The choice of a flowmeter for a particular gas process involves an analysis of many factors. After evaluating the major application
criteria, the number of possibilities can be reduced and the final selection can be made based on the remaining factors.
Some or all of the following considerations should be taken into account when choosing what flowmeter to use:
1. What t ype of gas is to be met ered?
Clean or dirty
Corrosive or noncorrosive
2. What are the flow conditions?
Measuring flowrate or total flow or both
Normal flowrate
Minimum and maximum flowrate
Minimum and maximum temperature
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Minimum and maximum pressure
3. What are the install conditions?
Pipe size
Reynolds Number
Is there room for adequate length of piping?
Are flow conditioners needed?
Is pipe vibration a problem?
Is flow steady or pulsating?
4. What are the performance requirements?
Accuracy
Is accuracy required in all conditions?
5. What are the cost factors?
Initial cost of primary and secondary instrument
Cost of accessories
Installation cost
Reliability vs. maintenance cost
Energy cost for pumping
Availability of parts and repair service
Compatibility with existing equipment
Choosing a Calibration Solution
Many gas flowmeters require individual calibration to meet the required accuracy. Flowmeters are recalibrated periodically for quality
assurance and are also recalibrated after being repaired. The precision of a flowmeter is limited to the accuracy of the calibration
equipment used. In some industrial applications, accuracy is not critical, however, in many instances, meters must be calibrated to the
very best possible accuracy. Some of these applications are found in military, aerospace, electronics, and whenever flow measurement
is related to the sale of product.
Major independent flow laboratories offer flowmeter calibration services meeting both end-user and OEM production requirements.
These labs verify flowmeter accuracy by comparing the meter with a primary flow standard traceable to the U.S. Nati onal Inst itu te
of Standards & Technology (NIST). Their calibration services take into account key factors affecting flowmeter performance, such as
orientation of the meter, operating temperature and pressure, and the type of the flow medium and viscosity.
When it comes to flow calibration equipment, the volume vs. time principle of operation is used for most high-accuracy gas meter
calibrations. Some weight vs. time gas calibrators have been made, but they are highly specialized and are not very practical for most
calibration work.
The most common volume vs. time gas flowmeter calibrators are the Bell Prover and the Glass-Tube Piston Prover. (Figures 7 & 8).
Both calibrators are designed to collect a precise volume of gas for a measured length of time, after the gas has passed through the
flowmeter. The volume and time measurements establish the measured volumetric flowrate of the gas. Measurements of the absolute
pressure and temperature of the collected volume of gas inside the calibrator allow calculations of all required flow parameters (Figure
8).
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Figure 7. High Volume Bell Prover provides primary
standard calibrations from 0.1 SCFM to 1,000 SCFM with an
uncertainty of +/-0.2 percent as stated on NVLAP
Certificate of Accreditation to ISO/IEC 17025:2008 register
to Flow Dynamics NVLAP Lab Code: 200668-0.
Figure 8. Glass Tube Low Flow Piston Prover provides primary standard calibrations from
0.000035 SCFM to 1 SCFM with an uncertainty of +/-0.2 percent as stated on NVLAP
Certificate of Accreditation to ISO/IEC 17025:2008 register to Flow Dynamics NVLAP Lab
Code: 200668-0.
Another type of volume base primary flow standard for gas is the Pressure Volume Temperature Time, or PVTt, calibrator. NIST and
other laboratories have used PVTt systems as primary gas flow standards for more than 30 years. The PVTt system consists of a flow
source, valves for diverting the flow, collection tank, vacuum pump, pressure and temperature sensors, and a critical flow Venturi
(CFV), which isolates the meter under test from the pressure variations in the downstream piping and tank.
The vacuum pump is used to remove the gas from the collection tank. The starting pressure and temperature are recorded. The
diverting valves are switched, allowing gas to enter the collection tank. The gas is collected and the resulting temperature and pressure
are recorded. Then, using the Real Gas Law, the volume of gas may be calculated with reasonable accuracy.
John Frederickhas worked in precision metrology for the past 27 years, holding senior technical positions with the U.S. Navy,
Lockheed Martin, and Flow Dynamics Inc. For the past 14 years, John has focused on flow measurement in various positions at Flow
Dynamics, including Calibration Laboratory Manager, Vice President of Engineering, and Vice President of Business Development. John
has authored numerous papers on flow measurement, holds several patents on flow measurement technologies, and teaches flow
measurement and measurement uncertainty courses. John earned a bachelors degree in Physical Science from the State University of
New York and a masters degree in Business Administration from Central Michigan University. John can be reached at
j oh n. f rederick@flo w -dynamics.com or 480 948-3789, ext. 16.
www.flow-dynamics.com
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