Article – Electronic Differential Pressure
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No More Horses: The Benefits of
Electronic Differential Pressure
Henry Ford is famously credited with saying, “If I
had asked people what they wanted, they would
have said faster horses.” Whether or not Ford
actually made this claim is up for debate, but the
lesson of this adage remains true: Manufacturers
must be more imaginative and innovative than
the markets they serve. History has proven this
logic true. Our televisions, our mobile phones,
and—Yes, Mr. Ford—even our automobiles, have
advanced in ways consumers never imagined.
What’s more is these technological improvements
have become the standard; it’s impossible to
imagine life without them.
However, this brand of innovation-first thinking has yet to take
hold throughout the process control industry. Take differential
pressure transmitters for example. DP transmitters have been
around forever, and that’s the biggest reason users like them.
They’re comfortable, flawed, but comfortable. Anyone who
has used a traditional differential pressure instrument has
had his fair share of bad experiences. Moreover, the market
expects difficulties and develops ways to work around them.
When it comes to DP transmitters, users are conditioned to
finding ways to improve a flawed solution. They’re still asking
for faster horses.
Article – Electronic Differential Pressure
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Background
The pursuit of pressure measurement has been around since
Galileo’s day, and the first pressure transducer didn’t hit the
market until the 1930s. At that time, transducers converted
the movements of diaphragms and bourdon tubes into
electrical impulses relating to a given pressure. Since then
advancements have developed quickly, and today we have
many different technologies for measuring pressure.
Customers use pressure technologies to monitor level,
interface, and density. The most commonly used device is the
tried and true differential pressure or DP measurement. Using
the density of the measured product as a reference, one can
monitor the pressure created by the fluid to indicate its level
inside the vessel. Since most vessels have additional pressure
blankets on top, users must monitor both pressures to make
sure they only look at the pressure created by the fluid.
The traditional differential pressure measurement system
consists of a dual-sided diaphragm that senses pressure from
the bottom of the vessel on one side and from the top of
the vessel on the other. This arrangement allows the system
to “remove” the upper pressure from the measurement,
and evaluate only the pressure generated by the fluid. The
connections to the vessel are traditionally made with either
impulse lines or capillary lines.
Traditional differential pressure
device with capillary lines for
connection to the vessel
Pressure Points
Important Moments in Pressure
Measurement History
1594 -
Galileo Galilei patents a machine to pump water
from a river. The machine uses a tube mechanism
similar to a syringe and plunger. He finds he can
only pull about ten meters of water at a time. This
phenomeon baffles him.
1643 –
Italian physicist Evangelista Torricelli notices the
level of mercury drops in a sealed tube when he sets it
in a basin with the open end down. He calls the space
of the liquid “vacuum” and explains it as the result of
an unknown force exerting itself on the earth.
1648 –
Blaise Pascal joins the search for the cause of Galileo
and Torricelli’s findings. He theorizes that the
weight of the air above the earth is responsible for
the vacuum space. To prove this theory, he takes
Torricelli’s experiment to a mountain peak and finds
that the height of the mercury decreases, and uses
that height difference to calculate the weight of the
air. He calls this force “pressure” and hypothesizes
that it acts uniformly in all directions.
1654 –
Otto von Guericke invents the vacuum pump. In
a publicity stunt that would impress P.T. Barnum,
Guericke vacuum-seals two hemispheres together and
harnesses eight horses to each side. The horses pull
and struggle, but cannot separate the hemispheres.
1662 –
Impulse lines are usually hard lines that let the fluid or
gas in the process directly contact the diaphragm in the
measurement device. Capillary lines are usually flexible,
armor-coated, oil-filled lines that have a flange with a metal
diaphragm that mounts to the vessel. These “remote seals”
are used to remove the transmitter from a potentially high
process temperature or to separate the measuring diaphragm
from a caustic process material allowing the vessel’s pressure
to transmit to the measuring cell without harm.
Robert Boyle, after years of experimentation,
finds that the pressure and volume of an ideal gas
are inversely proportional. We know this today as
Boyle’s Law, the first of three gas laws that eventually
give us the Ideal Gas Law.
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Problems with Traditional Differential
Pressure Systems
Using traditional differential pressure as a method of level measurement
has its challenges. Installing capillaries or impulse lines can be tricky and
time consuming. Bleeding the lines to remove air may require additional
valves and could potentially release harmful or expensive product.
Capillary lines, though small in diameter, need to be carefully mounted
and possibly insulated so they aren’t damaged and don’t move. Most
customers accept these challenges as expected inconveniences.
In addition, lines mounted outside the vessel are subject to environmental
changes independent of the process. Since pressure and temperature are
related, users must account for any external changes that can influence
the fluid or gas in the impulse lines or the oil within the capillaries.
This vulnerability to temperature puts users in the unenviable position of
asking themselves whether external factors are impacting measurement.
When using remote seals, users must first consider
whether the diaphragm material is compatible
with the process fluid or gas. They also need to
evaluate the static and dynamic pressure expected
in the process, as well as the planned temperature
swings. Selecting a fill fluid for the capillary
that is compatible with expected pressure and
temperature ranges is critical to reliable transfer of
the pressure from the vessel to the measurement
cell under all conditions.
Just as Henry Ford searched for a faster means
of travel, there has always been someone looking
for better and more efficient ways of measuring
pressure. Today, new advances in differential
pressure measurements are helping minimize the
ongoing concerns associated with environmental
influences and installation. The newest of these
technologies is Electronic Differential Pressure.
EDP: Reimagining Differential Pressure
Utilizing standard cell measurement technologies
and electronic interconnections in place of
capillaries and impulse lines is making pressure
measurement easier. A pair of standard pressure
devices mounted directly to the vessel and
interconnected with a small electrical cable is
replacing the traditional dual-sided transducer.
This new arrangement is more installation and
maintenance friendly than its predecessor.
Electronic differential pressure devices from VEGA provide a
reliable, accurate output, even in an unpredictable environment
Along with these concerns, customers must allocate time for calibration
and maintenance of their differential pressure systems. Frequently
correcting or calibrating for changing external conditions and internal
process changes will keep the maintenance staff busy. However, it will
leave the operator questioning the overall accuracy of the measurement.
Setting up a traditional differential pressure system should be as simple
as installing a pressure transducer and allowing the vessel to output the
pressure. Unfortunately, it’s more complicated than that.
Minimizing
or
completely
removing
environmental influences on the measurement
provides a more reliable, more accurate output.
Eliminating the need for oil to transfer pressure
to the cell improves overall response time, resulting
in quicker indications of changing process
conditions. Electronic differential pressure uses
two independent pressure transmitters, making
available more real-time information about the
process. In addition to the differential pressure,
users can monitor the head pressure within
the vessel as well as basic process temperature in
some circumstances.
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An additional benefit to electronic differential pressure
measurement is the opportunity to select different materials
and measurement technologies that best fit both ends of
the application. In essence, the user can custom-build his
pressure instrument. Instead of a traditional remote seal with
a thin metal diaphragm and oil filling, the user can select a
dry ceramic cell without a fill fluid that utilizes capacitive
measurement technology. This robust material allows for
longer life in abrasive environments along with minimal to no
recalibration. This is a huge benefit when considering repair
and/or replacement costs.
As with all new technologies, there are tradeoffs. Henry
Ford’s Model T may have been faster than a horse, but it
couldn’t cross the rolling hills of the High Plains. The same
goes for differential pressure systems. Traditional differential
pressure devices with dual sided diaphragms can maintain
tight accuracy on small changes in level even with large static
pressures. Electronic differential pressure systems have the
same ability but the measurement resolution may be impacted
when that difference becomes extreme. This is commonly
known as “turndown” and is calculated as the ratio of the
sensor’s full span to the customer’s needed measurement span.
Electronic differential pressure technology works well in
most level applications, and users enjoy continual reliability
with reduced costs for installation, maintenance, and repair.
With two independent transmitters, customers should expect
and demand easy setup of standard applications within
the device including DP, level, flow, interface, and density.
Communication protocols and a broad selection of output
units should also be available upon set up.
Combine any two sensors from
the VEGABAR 80 Series to create
a reliable electronic differential
pressure system
Like cars, pressure measurement technology has come a
long way thanks to decades of innovation and imagination.
Electronic differential pressure saves the market time
and money in repairs and maintenance, gives accurate
measurements regardless of external conditions, and comes
equipped with several standard applications. Traditional DP
devices can’t claim that. Perhaps it’s only a matter a time
before the market’s imagination catches up to technology and
customers stop asking for faster horses.
Author: Jeff Brand
Product Management
VEGA Americas, Inc.
JB/tb