the pressure drop. The differential pressure flow transmitter is a device that converts the differential pressure across its
ports into an analog signal. When the differential pressure flow transmitter has an
integral square root function, its output
signal can be linear with flow rate.
Some (multivariable) differential pressure flow transmitters can make multiple
measurements, such as differential pressure, pressure and/or temperature, from
which the flow rate can be calculated.
These transmitters are outside the scope
of this article.
Designs
In general, wetted parts (such as
diaphragms) that are in contact with the
fluid provide movement or force that is
related to the differential pressure across
the transmitter pressure ports. Differential pressure flow transmitters have been
designed using many technologies including, capacitance, differential transformer,
force balance, piezoelectric, potentiometer, strain gage and vibrating wire.
The quality of the differential pressure
flow transmitter signal can be described
by its performance specifications. Therefore, given performance specifications,
the (internal) sensing technology and
details of (internal) operation are not
overly important considerations when
considering the purchase of differential
pressure flow transmitters (although
some sensing technologies may have
superior reputations).
Due to the nature of the nonlinear
relationship between flow and differential pressure, relatively small differential pressure changes can result in
relatively large flow changes at low
flow rates. To reduce the noise associated with the flow signal at these flows,
some differential pressure transmitters
force their output signal to zero flow
when the signal falls below a certain
(preset) differential pressure. Some
transmitters with integral square root
extraction use a linear relationship
between flow and differential pressure
below a certain (preset) flow rate. Other
algorithms may be available to stabilize
the output signal at low flow rates.
Construction
The construction of differential pressure flow transmitters is such that its
wetted parts can be made from materials that can withstand corrosion. In
typical installations, impulse tubes are
installed such that no flow occurs at
the transmitter, so abrasion and wear
are usually not important concerns.
However, abrasion and wear can affect
the performance of a differential pressure primary flow element by affecting
its geometry. Differential pressure flow
transmitters can measure the flow of
many corrosive liquids, gases, and
vapors. Differential pressure primary
flow elements with appropriate geome-tries and materials of construction can
withstand abrasive fluids.
Differential pressure flow transmitters can be constructed of materials
that do not contaminate the fluid. However, they are not generally applied to
sanitary service because of limitations
on the ability to clean them.
Most differential pressure primary
flow elements have straight run requirements, so they are usually not applied
where limited straight run is available.
In addition, differential pressure primary flow element technology has
Reynolds number constraints, so it may
find limited application in low flow
applications, and where the liquid
exhibits high or varying viscosity.
Differential pressure flowmeters
measure velocity head, from which the
fluid velocity is inferred, after which the
volumetric flow rate is inferred. The differential pressure produced is a function
of the square of the velocity, so this
technology exhibits a relatively small
flow turndown as compared with other
flowmeter technologies. However, within
Reynolds number constraints, the range
of accurate flow measurement is relatively easy to change after installation.
Fluid Density
Differential pressure flowmeter measurements are inherently affected by fluid
density. Density changes in liquid applications are usually small because of
their non-compressible nature and
because (in many applications) process
temperature has a relatively small affect
on density. In gas and vapor applications, both temperature and pressure
can affect density and significantly
degrade the quality of the flow measurement. Notwithstanding the above, note
that changes in fluid composition can
affect the density of the fluid.
Flow computers can be used to compensate for density and other operating
parameters in applications where
degradation of the flow measurement
produces unacceptable flow measurement performance. A flow computer
can be implemented as a separate hardware device that calculates the compensated flow measurement from field
devices, such as differential pressure,
pressure and/or temperature instruments. These calculations can also be
performed in the process control system. In addition to measuring flow,
temperature and pressure, some multivariable differential pressure flow
transmitters can perform these calculations internally.
Multivariable flowmeters, such as
multivariable differential pressure flow
transmitters and other multivariable
flowmeter technologies, are outside the
scope of this article because even though
there is some overlap with information
contained here, additional parameters
are used to evaluate the relative performance of the different technologies.
Differential pressure flow transmitters generate an electrical signal that
represents the differential pressure at
its ports. Of importance is how well the
differential pressure flow transmitter
performs this function. Because performance is the prime concern in many
applications, the technology used to
effect the measurement is typically a
secondary or tertiary matter. VM
DAVID W. SPITZER, PE, is principal of Spitzer
Boyes, LLC, and has more than 30 years experience in instrumentation, process control, electrical and utility engineering. He is the author of
several textbooks used in the industry and has
taught numerous training seminars. Reach him at
www.spitzerandboyes.com. This article was
excerpted from “The Consumer Guide to Differential Pressure Flow Transmitters.”
