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Date Posted: 20:09:02 02/28/02 Thu
Author: doc
Subject: more interesting
In reply to: doc 's message, "Interesting" on 19:54:22 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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