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The World Market for Pressure Transmitters, 3rd Edition
Now Shipping!! Released in August 2011!
Differential Pressure Articles
Pressure
Articles
Links to DP Transmitter Suppliers
The World Market for Pressure Transmitters, 2nd Edition
Released in October 2007
The World Market for Pressure Transmitters, 1st Edition
2004 - Provides historical reference
The World Market for Differential Pressure (DP) Flowmeters and Primary Elements
January 2007
What is Pressure?
Pressure is a state or condition we face everyday. We speak of being “under pressure”, and talking about putting pressure on another person or situation. Fundamentally, the idea of pressure is the idea of the continuous exertion of force. From a measurement perspective, pressure is force per unit area. The idea of atmospheric pressure also plays an important role in pressure measurement by pressure transmitters. In process control, pressure measurement occurs in a variety of contexts, especially including the measurement of the pressure exerted by liquid, steam, and gas.
Pressure
is one of the most widely measured variables in the process industries.
In the oil & gas industry, wellhead pressures are measured in
subsea and topside locations. Pressure
measurement is used in pipelines to help create a
liquid interface detector for liquids in pipelines.
In water towers, pressure is measured to monitor and control water
levels. Pressure sensors are
used to monitor the water pipe pressure in automatic sprinkling systems.
And one of the most common measurements is barometric pressure,
which is used to predict the weather.
Just as
there are many applications for pressure measurement, there are also many
different instruments used for pressure measurement.
These include pressure gauges, pressure sensors, pressure
transducers, and pressure transmitters.
There are also specialized devices for measuring pressure extremes,
such as vacuum pressure.
Pressure
Transmitters
While
pressure sensors can detect variations in pressure by converting changes in
resistance or capacitance to electrical values, these electrical signals
are relatively weak. These
signals are not strong enough to be transmitted over distances, or sent to
a controller that may be located at a long distance from the pressure
sensor. To accomplish this, a
pressure transmitter is required. A
pressure transmitter contains a pressure sensor, plus a signal conditioner
to amplify the output from the pressure sensor.
Pressure transmitters also typically contain a display that shows
the pressure value that is conveyed by the pressure sensor and convert into
a standardized output. This
output is typically either 4-20mA or a digital signal.
There are
four main types of pressure transmitters: absolute, gauge, differential
pressure, and multivariable.
Absolute
Pressure Transmitters
Absolute
pressure transmitters, which sense absolute pressure, are referenced to a
full vacuum. The pressure of the measured media is compared against the
reference pressure of an absolute vacuum in a sealed reference chamber. Absolute
pressure transmitters are used to measure pressures that are not influenced
by atmospheric pressures. Vacuum transmitters are often used where high
accuracy is required, and are used for applications such as low pressure
measurement of vacuum distillation columns.
Gauge
Pressure Transmitters
Gauge
pressure transmitters, which sense gauge pressure, are referenced to
atmospheric conditions. Gauge
transmitters are vented to atmospheric pressure rather than to an absolute
vacuum. Gauge pressure
transmitters are widely used in the process industries to measure the
pressures of liquid, gas, and steam.
Both
absolute and gauge pressure transmitters are growing in part by replacing
pressure gauges, switches, and transducers.
Pressure transmitters are more stable and reliable than pressure
transducers. As end-users
increasingly automate their operations, and also build new plants, they are
more likely to choose pressure transmitters over pressure transducers.
Absolute and gauge pressure transmitters also have the advantage
that, unlike differential pressure transmitters used for flow, they do not
have another technology that is replacing them.
Differential
Pressure Transmitters
Differential
pressure (DP) transmitters measure the difference between two pressures.
They are used for both flow and level applications.
When used for flow applications, they measure the difference between
pressures on the upstream and downstream side of a constriction in the
pipe, called a primary element. This
difference in pressure is used to compute flowrate.
DP
transmitters have been used for more than 100 years to measure flowrate.
They have a pronounced advantage over other types of flowmeters in
installed base. Despite this,
they are being displaced in some applications by new-technology flowmeters
such as Coriolis and ultrasonic. This
is especially the case when higher accuracy and greater reliability is
required.
Multivariable
Pressure Transmitters
Multivariable
pressure transmitters measure two or more process variables.
In many cases, they measure volumetric flow, along with pressure and
temperature, and use these values to compute mass flow.
Multivariable transmitters are becoming more widely used for steam
and gas flow measurement. While
they are more expensive than single variable DP transmitters, they
typically cost less than buying a DP transmitter, along with single
pressure and temperature transmitters.
Multivariable transmitters were first introduced in 1992 by Bristol
Babcock, now part of Emerson Process Management.
Pressure
Sensors
Pressure
sensors operate by sensing pressure and converting it into an electrical
quantity. Two of the main types
used are piezoresistive and capacitive.
Piezoresistive is the most commonly used.
Piezoresistive
materials change resistance to the flow of current when they are strained
or compressed. A piezoresistive
pressure sensor consists of a micro-machined silicon diaphragm that has a
piezoresistive strain gauge diffused into it.
The diaphragm is fused to a silicon or glass backplate.
The sensor contains resistors that are typically arranged in the
form of a Wheatstone Bridge Circuit. As
pressure increases on the silicon, it is more resistant to current passing
through it. As a result, the
output of the Wheatstone Bridge is proportional to pressure.
Capacitive
pressure sensors, use a thin diaphragm as one plate of a capacitor.
This diaphragm is usually metal or metal-coated quartz.
The diaphragm is exposed to a reference pressure on one side and to
the process pressure on the other. Pressure
changes cause changes in the capacitance.
Pressure changes can be determined by monitoring the changes in
capacitance values.
Pressure
Transducers
Pressure
transducers resemble pressure transmitters, but they are generally lower in
cost and smaller than pressure transmitters.
They often have loose wires at one end and do not perform at the
same level as pressure transmitters. Pressure
transducers are widely used in the discrete industries such as automotive
and plastics. They are less
typically used in the process industries.
Pressure
Gauges
Pressure
gauges are relatively inexpensive mechanical devices that, for the most
part, are read manually. One of
the most well-known types is called the Bourdon gauge, which was patented
in
France
by Eugene Bourdon in 1849. In
1852, Edward Ashcroft bought the American rights to the design of the
Bourdon gauge and founded the Ashcroft Manufacturing Company to make
pressure gauges for steam engines. This
was the beginning of the company that is today known as Ashcroft, Inc.
Ashcroft is located in
Stratford
,
Connecticut
, and it still sells pressure gauges, along with a variety of other
products including pressure transducers and transmitters, pressure and
temperature switches, and test instruments.
Bourdon
gauges contain a thin-walled metal tube that is typically threaded into the
compartment whose pressure is being measured.
As pressure increases in the tube, the tube begins to straighten. On
the other end of the tube is a lever system that contains a pointer.
As the tube straightens, the pointer moves around a dial, indicating
pressure in pounds per square inch (psi).
Common tube shapes include curved or C-shaped, spiral, and helical.
This is a mechanical device that is manually read.
Another type of mechanical gauge that operates in a similar fashion
and also contains a pointer is called a diaphragm gauge.
Traditional
gauges such as the Bourdon and diaphragm gauges are sensitive to vibration
and condensation. Another type
is called a “filled” pressure gauge, and it is filled with a viscous
oil. This design has fewer
moving parts than the traditional pressure gauges, and is more reliable.
This design dampens pointer vibration and is not susceptible to
condensation.
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