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Date Posted: 20:14:46 02/28/02 Thu
Author: Lark
Subject: "If there are any sneaks from Roaring Forest I stop them/"
In reply to: doc 's message, "more interesting" on 20:09:02 02/28/02 Thu

>Strain-sensing materials, such as copper, change their
>internal structure at high temperatures. Temperature
>can alter not only the properties of a strain gage
>element, but also can alter the properties of the base
>material to which the strain gage is attached.
>Differences in expansion coefficients between the gage
>and base materials may cause dimensional changes in
>the sensor element.
> Expansion or contraction of the strain-gage element
>and/or the base material introduces errors that are
>difficult to correct. For example, a change in the
>resistivity or in the temperature coefficient of
>resistance of the strain gage element changes the zero
>reference used to calibrate the unit.
> The gage factor is the strain sensitivity of the
>sensor. The manufacturer should always supply data on
>the temperature sensitivity of the gage factor. Figure
>2-11 shows the variation in gage factors of the
>various strain gage materials as a function of
>operating temperature. Copper-nickel alloys such as
>Advance have gage factors that are relatively
>sensitive to operating temperature variations, making
>them the most popular choice for strain gage
>materials. Apparent strain is any change in gage
>resistance that is not caused by the strain on the
>force element. Apparent strain is the result of the
>interaction of the thermal coefficient of the strain
>gage and the difference in expansion between the gage
>and the test specimen. The variation in the apparent
>strain of various strain-gage materials as a function
>of operating temperature is shown in Figure 2-12. In
>addition to the temperature effects, apparent strain
>also can change because of aging and instability of
>the metal and the bonding agent.
> Compensation for apparent strain is necessary if the
>temperature varies while the strain is being measured.
>In most applications, the amount of error depends on
>the alloy used, the accuracy required, and the amount
>of the temperature variation. If the operating
>temperature of the gage and the apparent strain
>characteristics are known, compensation is possible.
>Stability Considerations
>It is desirable that the strain-gage measurement
>system be stable and not drift with time. In
>calibrated instruments, the passage of time always
>causes some drift and loss of calibration. The
>stability of bonded strain-gage transducers is
>inferior to that of diffused strain-gage elements.
>Hysteresis and creeping caused by imperfect bonding is
>one of the fundamental causes of instability,
>particularly in high operating temperature
>environments.
> Before mounting strain-gage elements, it should be
>established that the stressed force detector itself is
>uniform and homogeneous, because any surface
>deformities will result in instability errors. In
>order to remove any residual stresses in the force
>detectors, they should be carefully annealed,
>hardened, and stress-relieved using temperature aging.
>A transducer that uses force-detector springs,
>diaphragms, or bellows should also be provided with
>mechanical isolation. This will protect the sensor
>element from external stresses caused either by the
>strain of mounting or by the attaching of electric
>conduits to the transducer.
> If stable sensors are used, such as deposited
>thin-film element types, and if the force-detector
>structure is well designed, balancing and compensation
>resistors will be sufficient for periodic
>recalibration of the unit. The most stable sensors are
>made from platinum or other low-temperature
>coefficient materials. It is also important that the
>transducer be operated within its design limits.
>Otherwise, permanent calibration shifts can result.
>Exposing the transducer to temperatures outside its
>operating limits can also degrade performance.
>Similarly, the transducer should be protected from
>vibration, acceleration, and shock. Transducer Designs
>Strain gages are used to measure displacement, force,
>load, pressure, torque or weight. Modern strain-gage
>transducers usually employ a grid of four strain
>elements electrically connected to form a Wheatstone
>bridge measuring circuit.
> The strain-gage sensor is one of the most widely
>used means of load, weight, and force detection. In
>Figure 2-13A, a vertical beam is subjected to a force
>acting on the vertical axis. As the force is applied,
>the support column experiences elastic deformation and
>changes the electrical resistance of each strain gage.
>By the use of a Wheatstone bridge, the value of the
>load can be measured. Load cells are popular weighing
>elements for tanks and silos and have proven accurate
>in many other weighing applications.
> Strain gages may be bonded to cantilever springs to
>measure the force of bending (Figure 2-13B). The
>strain gages mounted on the top of the beam experience
>tension, while the strain gages on the bottom
>experience compression. The transducers are wired in a
>Wheatstone circuit and are used to determine the
>amount of force applied to the beam.
> Strain-gage elements also are used widely in the
>design of industrial pressure transmitters. Figure
>2-13C shows a bellows type pressure sensor in which
>the reference pressure is sealed inside the bellows on
>the right, while the other bellows is exposed to the
>process pressure. When there is a difference between
>the two pressures, the strain detector elements bonded
>to the cantilever beam measure the resulting
>compressive or tensile forces.
> A diaphragm-type pressure transducer is created when
>four strain gages are attached to a diaphragm (Figure
>2-13D). When the process pressure is applied to the
>diaphragm, the two central gage elements are subjected
>to tension, while the two gages at the edges are
>subjected to compression. The corresponding changes in
>resistance are a measure of the process pressure. When
>all of the strain gages are subjected to the same
>temperature, such as in this design, errors due to
>operating temperature variations are reduced.
>
>Installation Diagnostics
>All strain gage installations should be checked using
>the following steps:
>
>
> 1. Measure the base resistance of the unstrained
>strain gage after it is mounted, but before wiring is
>connected.
> 2. Check for surface contamination by measuring the
>isolation resistance between the gage grid and the
>stressed force detector specimen using an ohmmeter, if
>the specimen is conductive. This should be done before
>connecting the lead wires to the instrumentation. If
>the isolation resistance is under 500 megaohms,
>contamination is likely.
> 3. Check for extraneous induced voltages in the
>circuit by reading the voltage when the power supply
>to the bridge is disconnected. Bridge output voltage
>readings for each strain-gage channel should be nearly
>zero.
> 4. Connect the excitation power supply to the bridge
>and ensure both the correct voltage level and its
>stability.
> 5. Check the strain gage bond by applying pressure
>to the gage. The reading should be unaffected.
>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. 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. 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. Transducer Types
>Figure 3-3 provides an overall orientation to the
>scientist or engineer who might be faced with the task
>of selecting a pressure detector from among the many
>designs available. This table shows the ranges of
>pressures and vacuums that various sensor types are
>capable of detecting and the types of internal
>references (vacuum or atmospheric pressure) used, if
>any.
> Because electronic pressure transducers are of
>greatest utility for industrial and laboratory data
>acquisition and control applications, the operating
>principles and pros and cons of each of these is
>further elaborated in this section. Strain Gage
>When a strain gage, as described in detail in Chapter
>2, is used to measure the deflection of an elastic
>diaphragm or Bourdon tube, it becomes a component in a
>pressure transducer. Strain gage-type pressure
>transducers are widely used.
> Strain-gage transducers are used for narrow-span
>pressure and for differential pressure measurements.
>Essentially, the strain gage is used to measure the
>displacement of an elastic diaphragm due to a
>difference in pressure across the diaphragm. These
>devices can detect gauge pressure if the low pressure
>port is left open to the atmosphere or differential
>pressure if connected to two process pressures. If the
>low pressure side is a sealed vacuum reference, the
>transmitter will act as an absolute pressure
>transmitter. Strain gage transducers are available for
>pressure ranges as low as 3 inches of water to as high
>as 200,000 psig (1400 MPa). Inaccuracy ranges from
>0.1% of span to 0.25% of full scale. Additional error
>sources can be a 0.25% of full scale drift over six
>months and a 0.25% full scale temperature effect per
>1000¡ F.
>
>Capacitance
>Capacitance pressure transducers were originally
>developed for use in low vacuum research. This
>capacitance change results from the movement of a
>diaphragm element (Figure 3-5). The diaphragm is
>usually metal or metal-coated quartz and is exposed to
>the process pressure on one side and to the reference
>pressure on the other. Depending on the type of
>pressure, the capacitive transducer can be either an
>absolute, gauge, or differential pressure transducer.
> Stainless steel is the most common diaphragm
>material used, but for corrosive service, high-nickel
>steel alloys, such as Inconel or Hastelloy, give
>better performance. Tantalum also is used for highly
>corrosive, high temperature applications. As a special
>case, silver diaphragms can be used to measure the
>pressure of chlorine, fluorine, and other halogens in
>their elemental state.
> In a capacitance-type pressure sensor, a
>high-frequency, high-voltage oscillator is used to
>charge the sensing electrode elements. In a two-plate
>capacitor sensor design, the movement of the diaphragm
>between the plates is detected as an indication of the
>changes in process pressure. As shown in Figure 3-5,
>the deflection of the diaphragm causes a change in
>capacitance that is detected by a bridge circuit. This
>circuit can be operated in either a balanced or
>unbalanced mode. In balanced mode, the output voltage
>is fed to a null detector and the capacitor arms are
>varied to maintain the bridge at null. Therefore, in
>the balanced mode, the null setting itself is a
>measure of process pressure. When operated in
>unbalanced mode, the process pressure measurement is
>related to the ratio between the output voltage and
>the excitation voltage.
> Single-plate capacitor designs are also common. In
>this design, the plate is located on the back side of
>the diaphragm and the variable capacitance is a
>function of deflection of the diaphragm. Therefore,
>the detected capacitance is an indication of the
>process pressure. The capacitance is converted into
>either a direct current or a voltage signal that can
>be read directly by panel meters or
>microprocessor-based input/output boards.
> Capacitance pressure transducers are widespread in
>part because of their wide rangeability, from high
>vacuums in the micron range to 10,000 psig (70 MPa).
>Differential pressures as low as 0.01 inches of water
>can readily be measured. And, compared with strain
>gage transducers, they do not drift much. Better
>designs are available that are accurate to within 0.1%
>of reading or 0.01% of full scale. A typical
>temperature effect is 0.25% of full scale per 1000¡ F.
> Capacitance-type sensors are often used as secondary
>standards, especially in low-differential and
>low-absolute pressure applications. They also are
>quite responsive, because the distance the diaphragm
>must physically travel is only a few microns. Newer
>capacitance pressure transducers are more resistant to
>corrosion and are less sensitive to stray capacitance
>and vibration effects that used to cause "reading
>jitters" in older designs. Potentiometric
>The potentiometric pressure sensor provides a simple
>method for obtaining an electronic output from a
>mechanical pressure gauge. The device consists of a
>precision potentiometer, whose wiper arm is
>mechanically linked to a Bourdon or bellows element.
>The movement of the wiper arm across the potentiometer
>converts the mechanically detected sensor deflection
>into a resistance measurement, using a Wheatstone
>bridge circuit (Figure 3-6).
> The mechanical nature of the linkages connecting the
>wiper arm to the Bourdon tube, bellows, or diaphragm
>element introduces unavoidable errors into this type
>of measurement. Temperature effects cause additional
>errors because of the differences in thermal expansion
>coefficients of the metallic components of the system.
>Errors also will develop due to mechanical wear of the
>components and of the contacts.
> Potentiometric transducers can be made extremely
>small and installed in very tight quarters, such as
>inside the housing of a 4.5-in. dial pressure gauge.
>They also provide a strong output that can be read
>without additional amplification. This permits them to
>be used in low power applications. They are also
>inexpensive. Potentiometric transducers can detect
>pressures between 5 and 10,000 psig (35 KPa to 70
>MPa). Their accuracy is between 0.5% and 1% of full
>scale, not including drift and the effects of
>temperature. Resonant Wire
>The resonant-wire pressure transducer was introduced
>in the late 1970s. In this design (Figure 3-7), a wire
>is gripped by a static member at one end, and by the
>sensing diaphragm at the other. An oscillator circuit
>causes the wire to oscillate at its resonant
>frequency. A change in process pressure changes the
>wire tension, which in turn changes the resonant
>frequency of the wire. A digital counter circuit
>detects the shift. Because this change in frequency
>can be detected quite precisely, this type of
>transducer can be used for low differential pressure
>applications as well as to detect absolute and gauge
>pressures.
> The most significant advantage of the resonant wire
>pressure transducer is that it generates an inherently
>digital signal, and therefore can be sent directly to
>a stable crystal clock in a microprocessor.
>Limitations include sensitivity to temperature
>variation, a nonlinear output signal, and some
>sensitivity to shock and vibration. These limitations
>typically are minimized by using a microprocessor to
>compensate for nonlinearities as well as ambient and
>process temperature variations.
> Resonant wire transducers can detect absolute
>pressures from 10 mm Hg, differential pressures up to
>750 in. water, and gauge pressures up to 6,000 psig
>(42 MPa). Typical accuracy is 0.1% of calibrated span,
>with six-month drift of 0.1% and a temperature effect
>of 0.2% per 1000¡ F. Piezoelectric
>When pressure, force or acceleration is applied to a
>quartz crystal, a charge is developed across the
>crystal that is proportional to the force applied
>(Figure 3-8). The fundamental difference between these
>crystal sensors and static-force devices such as
>strain gages is that the electric signal generated by
>the crystal decays rapidly. This characteristic makes
>these sensors unsuitable for the measurement of static
>forces or pressures but useful for dynamic
>measurements. (This phenomenon also is discussed in
>later chapters devoted to the measurement of dynamic
>force, impact, and acceleration.)
> Piezoelectric devices can further be classified
>according to whether the crystal's electrostatic
>charge, its resistivity, or its resonant frequency
>electrostatic charge is measured. Depending on which
>phenomenon is used, the crystal sensor can be called
>electrostatic, piezoresistive, or resonant.
> When pressure is applied to a crystal, it is
>elastically deformed. This deformation results in a
>flow of electric charge (which lasts for a period of a
>few seconds). The resulting electric signal can be
>measured as an indication of the pressure which was
>applied to the crystal. These sensors cannot detect
>static pressures, but are used to measure rapidly
>changing pressures resulting from blasts, explosions,
>pressure pulsations (in rocket motors, engines,
>compressors) or other sources of shock or vibration.
>Some of these rugged sensors can detect pressure
>events having "rise times" on the order of a millionth
>of a second, and are described in more detail later in
>this chapter. The output of such dynamic pressure
>sensors is often expressed in "relative" pressure
>units (such as psir instead of psig), thereby
>referencing the measurement to the initial condition
>of the crystal. The maximum range of such sensors is
>5,000 or 10,000 psir. The desirable features of
>piezoelectric sensors include their rugged
>construction, small size, high speed, and
>self-generated signal. On the other hand, they are
>sensitive to temperature variations and require
>special cabling and amplification.
> They also require special care during installation:
>One such consideration is that their mounting torque
>should duplicate the torque at which they were
>calibrated (usually 30 in.-lbs). Another factor that
>can harm their performance by slowing response speed
>is the depth of the empty cavity below the cavity. The
>larger the cavity, the slower the response. Therefore,
>it is recommended that the depth of the cavity be
>minimized and not be deeper than the diameter of the
>probe (usually about 0.25-in.).
> Electrostatic pressure transducers are small and
>rugged. Force to the crystal can be applied
>longitudinally or in the transverse direction, and in
>either case will cause a high voltage output
>proportional to the force applied. The crystal's
>self-generated voltage signal is useful where
>providing power to the sensor is impractical or
>impossible. These sensors also provide high speed
>responses (30 kHz with peaks to 100 kHz), which makes
>them ideal for measuring transient phenomena. Figure
>3-9 illustrates an acceleration-compensated pressure
>sensor. In this design, the compensation is provided
>by the addition of a seismic mass and a separate
>"compensation crystal" of reverse polarity. These
>components are scaled to exactly cancel the inertial
>effect of the masses (the end piece and diaphragm)
>which act upon the pressure-sensing crystal stack when
>accelerated.
> Because quartz is a common and naturally occurring
>mineral, these transducers are generally inexpensive.
>Tourmaline, a naturally occurring semi-precious form
>of quartz, has sub-microsecond responsiveness and is
>useful in the measurement of very rapid transients. By
>selecting the crystal properly, the designer can
>ensure both good linearity and reduced temperature
>sensitivity.
> Although piezoelectric transducers are not capable
>of measuring static pressures, they are widely used to
>evaluate dynamic pressure phenomena associated with
>explosions, pulsations, or dynamic pressure conditions
>in motors, rocket engines, compressors, and other
>pressurized devices that experience rapid changes.
>They can detect pressures between 0.1 and 10,000 psig
>(0.7 KPa to 70 MPa). Typical accuracy is 1% full scale
>with an additional 1% full scale per 1000¡ temperature
>effect.
> Piezoresistive pressure sensors operate based on the
>resistivity dependence of silicon under stress.
>Similar to a strain gage, a piezoresistive sensor
>consists of a diaphragm onto which four pairs of
>silicon resistors are bonded. Unlike the construction
>of a strain gage sensor, here the diaphragm itself is
>made of silicon and the resistors are diffused into
>the silicon during the manufacturing process. The
>diaphragm is completed by bonding the diaphragm to an
>unprocessed wafer of silicon.
> If the sensor is to be used to measure absolute
>pressure, the bonding process is performed under
>vacuum. If the sensor is to be referenced, the cavity
>behind the diaphragm is ported either to the
>atmosphere or to the reference pressure source. When
>used in a process sensor, the silicon diaphragm is
>shielded from direct contact with the process
>materials by a fluid-filled protective diaphragm made
>of stainless steel or some other alloy that meets the
>corrosion requirements of the service.
> Piezoresistive pressure sensors are sensitive to
>changes in temperature and must be temperature
>compensated. Piezoresistive pressure sensors can be
>used from about 3 psi to a maximum of about 14,000 psi
>(21 KPa to 100 MPa).
> Resonant piezoelectric pressure sensors measure the
>variation in resonant frequency of quartz crystals
>under an applied force. The sensor can consist of a
>suspended beam that oscillates while isolated from all
>other forces. The beam is maintained in oscillation at
>its resonant frequency. Changes in the applied force
>result in resonant frequency changes. The relationship
>between the applied pressure P and the oscillation
>frequency is: where TO is the period of oscillation
>when the applied pressure is zero, T is the period of
>oscillation when the applied pressure is P, and A and
>B are calibration constants for the transducer.
> These transducers can be used for absolute pressure
>measurements with spans from 0-15 psia to 0-900 psia
>(0-100 kPa to 0-6 MPa) or for differential pressure
>measurements with spans from 0-6 psid to 0-40 psid
>(0-40 kPa to 0-275 kPa). Inductive/Reluctive
>A number of early pressure transducer designs were
>based on magnetic phenomena. These included the use of
>inductance, reluctance, and eddy currents. Inductance
>is that property of an electric circuit that expresses
>the amount of electromotive force (emf) induced by a
>given rate of change of current flow in the circuit.
>Reluctance is resistance to magnetic flow, the
>opposition offered by a magnetic substance to magnetic
>flux. In these sensors, a change in pressure produces
>a movement, which in turn changes the inductance or
>reluctance of an electric circuit. Figure 3-10A
>illustrates the use of a linear variable differential
>transformer (LVDT) as the working element of a
>pressure transmitter. The LVDT operates on the
>inductance ratio principle. In this design, three
>coils are wired onto an insulating tube containing an
>iron core, which is positioned within the tube by the
>pressure sensor.
> Alternating current is applied to the primary coil
>in the center, and if the core also is centered, equal
>voltages will be induced in the secondary coils (#1
>and #2). Because the coils are wired in series, this
>condition will result in a zero output. As the process
>pressure changes and the core moves, the differential
>in the voltages induced in the secondary coils is
>proportional to the pressure causing the movement.
> LVDT-type pressure transducers are available with
>0.5% full scale accuracy and with ranges from 0-30
>psig (0-210 kPa) to 0-10,000 psig (0-70 MPa). They can
>detect absolute, gauge, or differential pressures.
>Their main limitations are susceptibility to
>mechanical wear and sensitivity to vibration and
>magnetic interference.
> Reluctance is the equivalent of resistance in a
>magnetic circuit. If a change in pressure changes the
>gaps in the magnetic flux paths of the two cores, the
>ratio of inductances L1/L2 will be related to the
>change in process pressure (Figure 3-10B).
>Reluctance-based pressure transducers have a very high
>output signal (on the order of 40 mV/volt of
>excitation), but must be excited by ac voltage. They
>are susceptible to stray magnetic fields and to
>temperature effects of about 2% per 1000¡ F. Because
>of their very high output signals, they are often used
>in applications where high resolution over a
>relatively small range is desired. They can cover
>pressure ranges from 1 in. water to 10,000 psig (250
>Pa to 70 MPa). Typical accuracy is 0.5% full scale.
>Optical
>Optical pressure transducers detect the effects of
>minute motions due to changes in process pressure and
>generate a corresponding electronic output signal
>(Figure 3-11). A light emitting diode (LED) is used as
>the light source, and a vane blocks some of the light
>as it is moved by the diaphragm. As the process
>pressure moves the vane between the source diode and
>the measuring diode, the amount of infrared light
>received changes.
> The optical transducer must compensate for aging of
>the LED light source by means of a reference diode,
>which is never blocked by the vane. This reference
>diode also compensates the signal for build-up of dirt
>or other coating materials on the optical surfaces.
>The optical pressure transducer is immune to
>temperature effects, because the source, measurement
>and reference diodes are affected equally by changes
>in temperature. Moreover, because the amount of
>movement required to make the measurement is very
>small (under 0.5 mm), hysteresis and repeatability
>errors are nearly zero.
> Optical pressure transducers do not require much
>maintenance. They have excellent stability and are
>designed for long-duration measurements. They are
>available with ranges from 5 psig to 60,000 psig (35
>kPa to 413 MPa) and with 0.1% full scale accuracy.
>
>Practical Considerations
>In industrial applications, good repeatability often
>is more important then absolute accuracy. If process
>pressures vary over a wide range, transducers with
>good linearity and low hysteresis are the preferred
>choice.
> Ambient and process temperature variations also
>cause errors in pressure measurements, particularly in
>detecting low pressures and small differential
>pressures. In such applications, temperature
>compensators must be used.
>
> Power supply variations also lower the performance
>of pressure transducers. The sensitivity (S) of a
>transducer determines the amount of change that occurs
>in the output voltage (VO) when the supply voltage
>(VS) changes, with the measured pressure (Pm) and the
>rated pressure of the transducer (Pr) remaining
>constant: In a pressure measurement system, the total
>error can be calculated using the root-sum-square
>method: the total error is equal to the square root of
>the sums of all the individual errors squared.
>
>Selection Criteria
>Pressure transducers usually generate output signals
>in the millivolt range (spans of 100 mV to 250 mV).
>When used in transmitters, these are often amplified
>to the voltage level (1 to 5 V) and converted to
>current loops, usually 4-20 mA dc.
> The transducer housing should be selected to meet
>both the electrical area classification and the
>corrosion requirements of the particular installation.
>Corrosion protection must take into account both
>splashing of corrosive liquids or exposure to
>corrosive gases on the outside of the housing, as well
>as exposure of the sensing element to corrosive
>process materials. The corrosion requirements of the
>installation are met by selecting corrosion-resistant
>materials, coatings, and by the use of chemical seals,
>which are discussed later in this chapter.
> If the installation is in an area where explosive
>vapors may be present, the transducer or transmitter
>and its power supply must be suitable for these
>environments. This is usually achieved either by
>placing them inside purged or explosion-proof
>housings, or by using intrinsically safe designs.
> Probably the single most important decision in
>selecting a pressure transducer is the range. One must
>keep in mind two conflicting considerations: the
>instrument's accuracy and its protection from
>overpressure. From an accuracy point of view, the
>range of a transmitter should be low (normal operating
>pressure at around the middle of the range), so that
>error, usually a percentage of full scale, is
>minimized. On the other hand, one must always consider
>the consequences of overpressure damage due to
>operating errors, faulty design (waterhammer), or
>failure to isolate the instrument during
>pressure-testing and start-up. Therefore, it is
>important to specify not only the required range, but
>also the amount of overpressure protection needed.
>Most pressure instruments are provided with
>overpressure protection of 50% to 200% of range
>(Figure 3-12). These protectors satisfy the majority
>of applications. Where higher overpressures are
>expected and their nature is temporary (pressure
>spikes of short duration--seconds or less), snubbers
>can be installed. These filter out spikes, but cause
>the measurement to be less responsive. If excessive
>overpressure is expected to be of longer duration, one
>can protect the sensor by installing a pressure relief
>valve. However, this will result in a loss of
>measurement when the relief valve is open.
> If the transmitter is to operate under high ambient
>temperatures, the housing can be cooled electrically
>(Peltier effect) or by water, or it can be relocated
>in an air-conditioned area. When freezing temperatures
>are expected, resistance heating or steam tracing
>should be used in combination with thermal insulation.
> When high process temperatures are present, one can
>consider the use of various methods of isolating the
>pressure instrument from the process. These include
>loop seals, siphons, chemical seals with capillary
>tubing for remote mounting, and purging. Maintenance
>Without exception, pressure sensors require scheduled,
>periodic maintenance and/or recalibration. It is
>necessary to periodically remove the transducer from
>the process and to make sure that this procedure does
>not require shutting down the process and does not
>cause injury or damage. Because the process fluid may
>be toxic, corrosive, or otherwise noxious to personnel
>or the environment, it is necessary to protect against
>the release of such fluids during maintenance. A
>three-way manifold (Figure 3-13) can provide such
>protection. In the illustration, valve P is used to
>isolate the process and valve D serves to discharge
>the trapped process fluid from the instrument into
>some safe containment. The purpose of valve T is to
>allow the application of a known calibration or test
>pressure to the instrument. As all the components of
>the manifold are pre-assembled into a compact package,
>space and field assembly time are saved and chances
>for leaks are reduced. Calibration
>Pressure transducers can be recalibrated on-line or in
>a calibration laboratory. Laboratory recalibration
>typically is preferred, but often is not possible or
>necessary. In the laboratory, there usually are two
>types of calibration devices: deadweight testers that
>provide primary, base-line standards, and "laboratory"
>or "field" standard calibration devices that are
>periodically recalibrated against the primary. Of
>course, these secondary standards are less accurate
>than the primary, but they provide a more convenient
>means of testing other instruments.
> A deadweight tester consists of a pumping piston
>with a screw that presses it into the reservoir, a
>primary piston that carries the dead weight, and the
>gauge or transducer to be tested (Figure 3-14). It
>works by loading the primary piston (of cross
>sectional area A), with the amount of weight (W) that
>corresponds to the desired calibration pressure (P =
>W/A). The pumping piston then pressurizes the whole
>system by pressing more fluid into the reservoir
>cylinder, until the dead weight lifts off its support.
>Today's deadweight testers are more accurate and more
>complex than the instrument in Figure 3-14, but the
>essential operating principles are the same.
>Sophisticated features include temperature
>compensation and the means to rotate the piston in its
>cylinder to negate the effects of friction.
>
> Info from: omega.com/literature/transactions

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