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Mechanical Pressure Elements |
Base on
pressure acting on a surface area inside the element to provide a force
that causes a mechanical deflection
Common elements used are:
Mechanical
methods of measuring pressure have been known for centuries. U-tube
manometers were among the first pressure indicators. Originally, these
tubes were made of glass, and scales were added to them as needed. But
manometers are large, cumbersome, and not well suited for integration
into automatic control loops. Therefore, manometers are usually found in
the laboratory or used as local indicators. Depending on the reference
pressure used, they could indicate absolute, gauge, and differential
pressure.
Differential pressure transducers often are used in
flow measurement where
they can measure the pressure differential across a venturi, orifice, or
other type of primary element. The detected pressure differential is
related to flowing velocity and therefore to volumetric flow. Many
features of modern pressure transmitters have come from the differential
pressure transducer. In fact, one might consider the differential
pressure transmitter the model for all pressure transducers.
"Gauge" pressure is defined relative to atmospheric conditions. In those
parts of the world that continue to use English units, gauge pressure is
indicated by adding a "g" to the units descriptor. Therefore, the
pressure unit "pounds per square inch gauge" is abbreviated psig. When
using SI units, it is proper to add "gauge" to the units used, such as
"Pa gauge." When pressure is to be measured in absolute units, the
reference is full vacuum and the abbreviation for "pounds per square
inch absolute" is psia.
Figure 3-1: Bourdon Tube Designs
Often, the terms pressure gauge, sensor, transducer, and transmitter are
used interchangeably. The term pressure gauge usually refers to a
self-contained indicator that converts the detected process pressure
into the mechanical motion of a pointer. A pressure transducer might
combine the sensor element of a gauge with a mechanical-to-electrical or
mechanical-to-pneumatic converter and a power supply. A pressure
transmitter is a standardized pressure measurement package consisting of
three basic components: a pressure transducer, its power supply, and a
signal conditioner/retransmitter that converts the transducer signal
into a standardized output.
Pressure transmitters can send the process pressure of interest using an
analog pneumatic (3-15 psig), analog electronic (4-20 mA dc), or digital
electronic signal. When transducers are directly interfaced with digital
data acquisition systems and are located at some distance from the data
acquisition hardware, high output voltage signals are preferred. These
signals must be protected against both electromagnetic and radio
frequency interference (EMI/RFI) when traveling longer distances.
Pressure transducer performance-related terms also require definition.
Transducer accuracy refers to the degree of conformity of the measured
value to an accepted standard. It is usually expressed as a percentage
of either the full scale or of the actual reading of the instrument. In
case of percent-full-scale devices, error increases as the absolute
value of the measurement drops. Repeatability refers to the closeness of
agreement among a number of consecutive measurements of the same
variable. Linearity is a measure of how well the transducer output
increases linearly with increasing pressure. Hysteresis error describes
the phenomenon whereby the same process pressure results in different
output signals depending upon whether the pressure is approached from a
lower or higher pressure.
From Mechanical to Electronic
The first pressure gauges used flexible elements as sensors. As pressure
changed, the flexible element moved, and this motion was used to rotate
a pointer in front of a dial. In these mechanical pressure sensors, a
Bourdon tube, a diaphragm, or a bellows element detected the process
pressure and caused a corresponding movement.
A Bourdon tube is C-shaped and has an oval cross-section with one end of
the tube connected to the process pressure (Figure 3-1A). The other end
is sealed and connected to the pointer or transmitter mechanism. To
increase their sensitivity, Bourdon tube elements can be extended into
spirals or helical coils (Figures 3-1B and 3-1C). This increases their
effective angular length and therefore increases the movement at their
tip, which in turn increases the resolution of the transducer.
Figure 3-2: Pressure Sensor Diaphragm Designs
The family of flexible pressure sensor elements also includes the
bellows and the diaphragms (Figure 3-2). Diaphragms are popular because
they require less space and because the motion (or force) they produce
is sufficient for operating electronic transducers. They also are
available in a wide range of materials for corrosive service
applications.
After the 1920s, automatic control systems evolved, and by the 1950s
pressure transmitters and centralized control rooms were commonplace.
Therefore, the free end of a Bourdon tube (bellows or diaphragm) no
longer had to be connected to a local pointer, but served to convert a
process pressure into a transmitted (electrical or pneumatic) signal. At
first, the mechanical linkage was connected to a pneumatic pressure
transmitter, which usually generated a 3-15 psig output signal for
transmission over distances of several hundred feet, or even farther
with booster repeaters. Later, as solid state electronics matured and
transmission distances increased, pressure transmitters became
electronic. The early designs generated dc voltage outputs (10-50 mV;
1-5 V; 0-100 mV), but later were standardized as 4-20 mA dc current
output signals.
Because of the inherent limitations of mechanical motion-balance
devices, first the force-balance and later the solid state pressure
transducer were introduced. The first unbonded-wire strain gages were
introduced in the late 1930s. In this device, the wire filament is
attached to a structure under strain, and the resistance in the strained
wire is measured. This design was inherently unstable and could not
maintain calibration. There also were problems with degradation of the
bond between the wire filament and the diaphragm, and with hysteresis
caused by thermoelastic strain in the wire.
The search for improved pressure and strain sensors first resulted in
the introduction of bonded thin-film and finally diffused semiconductor
strain gages. These were first developed for the automotive industry,
but shortly thereafter moved into the general field of pressure
measurement and transmission in all industrial and scientific
applications. Semiconductor pressure sensors are sensitive, inexpensive,
accurate and repeatable. (For more details on strain gage operation, see
Chapter 2.)
Many pneumatic pressure transmitters are still in operation,
particularly in the petrochemical industry. But as control systems
continue to become more centralized and computerized, these devices have
been replaced by analog electronic and, more recently, digital
electronic transmitters.
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