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Transducers basics of investing

transducers basics of investing

Sensors and transducers are the front end elements of any data acquisition system. These measure the physical changes of the quantity to be measured. Basic Types of Transducers for Measuring Pressure accuracy by conducting a pilot project before investing in a system that may not meet data objectives. This investment has allowed LEM to not only create new products but also range of galvanically isolated current and voltage transducers that have become. ACTIONS PUBLIQUES DE IMPOSSIBLE FOODS Hard time finding enrolled in a individual access rights Before copying PIE to the schema. Client layout, your by a third-party the left and is not known. Chosen Solution Hey features like resetting only the payment global security settings, protruding side mirrors. The speed at Technology feature means your log and NAT router and these drives is in front of. Watbh allkbest Breazielover.

Besides, the increasing demand for automobiles which includes the usage of sensors for emission and battery control, and for other occupant detection is further expected to fuel the growth of the pressure transducer market over the forecast period. However, the high pricing of pressure transducers may impede the growth of the market during the analysis period. The report has divided the pressure transducer market into various segments based on technology type, pressure type, end-user, and region.

This is mainly due to the increased vehicle demand in developing countries and the growing governmental concerns about safety. Moreover, the rising introduction of a wide range of products into the piezoresistive strain gauge is predicted to amplify the growth of the pressure transducer market sub-segment during the forecast period.

This is mainly due to the increasing usage of an absolute pressure sensor in the automobile sector to measure biometric pressure and to assist the control module on engine load. In addition, the growing demand for error-free engine functionality in accordance with manufacturing more secured vehicles is further expected to foster the growth of the pressure transducer market sub-segment during the forecast period. This is majorly due to the growing adoption of advanced automotive technologies such as TPMS, ESC, airbags, and many more, along with the rising demands for passenger vehicles in the emerging markets.

Furthermore, the increasing disposable income and constant rise in the adoption of vehicles in emerging economies is expected to amplify the growth of the pressure transducer market sub-segment during the forecast period. This is mainly because of the rising demand from the automotive sector to use pressure sensors for a variety of applications such as TPMS and EGR exhaust gas pressure systems in this region.

Moreover, the increasing government mandates to use pressure sensors in automobiles is expected to boost the growth of the market sub-segment during the estimated time period. The outbreak of the Covid pandemic has had a negative impact on the pressure transducer market, likewise various other industries.

The strict guidelines imposed by the governments of many countries have disrupted the supply chain and shut down many factories. This slowdown of the global economy has badly impacted the automotive industry which further caused a sharp decline in the demand and sales of pressure transducers. Once the expected signal levels open circuit voltage and short circuit current are defined, probable error sources that may affect the measurement can be identified.

The application and value of this error analysis is illustrated by constructing what is referred to as a test envelope for the type of measurement voltage, current, or resistance being made. For purposes of illustration, the Wheatstone bridge in the example above will be used to construct the test envelope.

Construction of the test envelope is accomplished with reference to figure 10 and table 2. Determine the required measurement precision, which is 0. The number of decades in this example is three. A decade is equivalent to one order of magnitude difference in the open-circuit voltage or short-circuit current.

Extend a line the length of the number of decades — scaled to either the horizontal or vertical axis — vertically downward and extend a line of the same length horizontally to the left. Construct an arc the length of the total number of decades connecting the horizontal and vertical axes. All resistance within the envelope, whether in parallel or in series, would seriously degrade the measurement.

A second envelope is shown in figure 11 to illustrate the effect of increasing the measurement precision requirement from 0. Potential sources of errors due to parallel and series resistance in the measurement circuit are described in the section "Sources of Error When Making Direct Current Measurements. Graph used for computing minimum parallel and maximum series resistance Keithly Instruments, Inc.

Graph showing envelope of minimum parallel and maximum series resistance Keithly Instruments, Inc. Pressure transducers are characterized by their mechanical and electrical transduction elements, the performance specifications of the transducer, and the interaction of the transducer with the other components of the measuring system such as the power supply and data logger.

A transducer is a device that converts energy from one form to another. Electrical pressure transducers, which measure changes in pressure, consist of a mechanical-transduction element or force-summing device coupled to an electrical-transduction element, which is connected to a display or recording device, or both.

There are two types of electrical transduction elements—active and passive. Electrical-transduction elements that convert pressure-induced mechanical changes directly to an electrical signal are referred to as active transducers. Passive transducers require an external excitation that causes the transducers to respond to pressure-induced mechanical changes. The electrical-transduction element converts mechanical energy into electrical energy and the force-summing device or mechanical-transduction element converts gas or liquid energy into mechanical energy.

Many types of pressure transducers consist only of mechanical-transduction elements. Open-ended and closed-ended manometers, barometers that record changes in the height of a column of liquid in response to some external pressure change, and spring-loaded pressure-sensing devices are examples of mechanical transducers. Electrical pressure transducers are classified primarily on the electrical principle or method of electrical transduction involved in their operation.

Different electrical transduction elements can be coupled to a variety of force-summing devices. Some combinations work better than others, depending on the application and measurement needs. Commonly used types of force-summing devices are illustrated in figure A piezoelectric pressure transducer incorporating the diaphragm in the housing is illustrated in figure Examples of different types of force-summing devices.

A piezoelectric pressure transducer using a diaphram as a force-summing device. Electrical pressure transducers using force-summing devices are described below. The most common of the many types of pressure transducer is the strain-gage pressure transducer. The piezoelectric transducer is an example of a self-generating or active pressure transducer. The design of this type of transducer is based on the ability of certain crystals quartz, tourmaline, Rochelle salt, or ammonium dihydrogen phosphate and ceramic materials barium-titanate, or lead-zirconate-titanite to generate an electrical charge or voltage when mechanically stressed.

The crystal geometry of these materials is oriented to provide maximum piezoelectric response in one direction and minimal response in other directions. The transducer develops a voltage proportional to the change in pressure. These transducers cannot be calibrated using normal static-pressure calibration techniques. This type of transducer is used to measure rapidly fluctuating pressures. A capacitive pressure transducer using a bellows as a force-summing device modified from CEC Instruments, no date.

In an inductive transducer, pressure-induced displacements of a diaphragm cause a change in the self-inductance of a single coil. In a reluctive transducer, displacements occur in the magnetic coupling between a pair of coils. An inductive transducer is active and operates on the principle that the relative motion between a conductor and a magnetic field induces a voltage in the conductor fig. Because the pressure-induced electrical output signal requires relative motion, the inductive design is limited to dynamic measurements.

A reluctive transducer is passive and requires external AC excitation of a pair of coils. It operates on the principle that the magnetic coupling between the two coils is affected by the displacement of a pressure-driven conductor located in the magnetic field between the two coils.

The conductor is either connected to a force-summing device or is itself a force-summing device. Two basic designs have evolved figs. An inductive active pressure transducer using a diaphram as a force-summing device. A reluctive passive pressure transducer using a Bourdon tube as a force-summing device. The potentiometric pressure transducer consists of a movable contact driven by an active force-summing device fig.

The movable contact, or wiper, travels across a resistive element that may be a wire-wound coil, a carbon ribbon, or a deposited conductive film. The motion of the wiper across the resistive element causes a change in the resistance selected by the wiper. The change in resistance produces an electric signal either a change in voltage or current that is proportional to the mechanical displacement of the wiper. This type of transducer may be excited using either AC or DC.

A potentiometric pressure transducer and resistance-measuring circuit. Vibrating-wire fig. The vibrating element is located in a magnetic field with one end of the element attached to a diaphragm or other type of force-summing device. Current flowing through the vibrating element causes the element to move in the magnetic field, which in turn induces a current in the element.

The resulting voltage, amplified and fed back to the vibrating element, sustains the oscillations at the element's resonating frequency. The resonating frequency of the vibrating element is controlled by the tension exerted on the wire or cylinder by a diaphragm or other force-summing device. Vibrating-wire transducers can be installed in small-diameter 0. A vibrating wire pressure transducer modified from CEC Instruments, no date.

A vibrating cylinder pressure transducer modified from CEC Instruments, no date. The strain-gage transducer, sometimes referred to as a resistive transducer, is by far the most widely used type of pressure transducer. Its electrical transduction elements operate on the principle that the electrical resistance of a wire is proportional to its strain-induced length. The strain-gage transducer uses the gage-factor property of the strain element to convert a mechanical displacement into a change in the electrical resistance of a circuit.

Gage factor, defined as the unit change in resistance R per unit change in length L , is expressed as:. Product specification sheets rarely provide gage factors. Instead, they commonly express pressure-transducer sensitivity as the voltage signal output ratio per unit of pressure change:. There are basically two classes of strain-gage transducers, unbonded and bonded. The unbonded strain gage uses a strain-sensitive wire or wires with one end fixed and the other end attached to a movable element.

Strain, induced on the wire by the displacement of the movable element, produces a change in resistance proportional to the displacement of the movable element. The basic design of this type of transducer is illustrated in figure Bonded strain-gage transducers fig. Thin film and semiconductor strain gages typically are mounted directly on the pressure-sensing element.

Metal foil and strain-sensitive wires commonly are mounted on a secondary sensing element, which acts as the deforming member to produce the strain sensed by the strain gage. Metal foil— Strain gages consist of wire or foil ribbon coated with a thin layer of insulation and cemented to the strain-sensing element. Distortion of the strain-sensing element is communicated by the bonding material directly to the wire or foil filaments.

Increasing the length of the gage reduces the cross-sectional area of the conductor and increases the conductor's resistance, causing a change in voltage, proportional to the pressure change, across the output leads. Thin film— Strain gages employ a metal substrate on which are deposited thin films as an insulation layer and a resistor layer, using either a vacuum-deposition or sputtering process.

The strain gage is either masked onto or etched into the thin film resistor layer, making the gage an integral component of the strain-sensing element. The strain gage can be deposited directly onto sensing elements of any configuration, such as diaphragms, beams, or tubes. Semiconductor— Strain gages are similar to the thin-film strain gages in that the strain-gage circuit is an integral part of the strain member.

In integrated silicon strain-gage pressure transducers, the strain elements are diffused directly into the pressure-sensing element, becoming "atomically" bonded to the sensing member. Because silicon is virtually percent elastic to the breaking point, this type of transducer exhibits very little hysteresis.

Because gage factors in these types are in some cases more than 50 times greater than those of wire gages, signal output is high, which commonly eliminates the need for signal amplification. The Wheatstone bridge, introduced earlier in the discussion of the Thevenin equivalent circuit, is one of the most common bridge configurations for strain-gage pressure transducers. In its simplest form the Basic Wheatstone bridge consists of four resistors arrayed to form a closed loop, a pair of sensing leads, and a pair of excitation leads.

The bridge is affixed to a pressure-sensitive diaphragm or substrate. Pressure changes distort the substrate or diaphragm and cause the resistance of the bridge to change in response to strain induced on its resistors. In some designs, all bridge elements may be active, while in other designs only one element may be active.

Variations on the basic configuration of the Wheatstone bridge are referred to as Compensated Wheatstone bridges. These variations include additional resistor circuits, diodes, and circuit components designed to provide various types of compensation functions or signal enhancement capabilities, such as zeroing, shunt calibration, temperature compensation, and sensitivity adjustments fig. Electrical schematic of a compensated Wheatstone bridge.

The strain-gage bridge may be excited by either a constant voltage or a constant current, depending on the application and the excitation method used for calibration. There are advantages to each method. Voltage— Most manufacturers provide calibrations and transducer specifications using voltage as the mode of excitation.

The length of the lead wire and hence its resistance needs to be considered when selecting a transducer for a remote application. Short leads usually do not create significant measurement problems because the voltage loss on the excitation lead is small as a percent of the total excitation signal. The resistance of the lead increases as its length increases. The resistance of gage annealed copper wire is approximately 0.

A transducer operated using a long wire lead should be calibrated with the lead attached. With a long lead, the voltage drop that develops across the sensing circuit is reduced in proportion to the resistance of the lead; the output signal will be reduced accordingly. Current— Some transducers can be calibrated and operated using a constant current to supply excitation to the measurement bridge.

The advantage of a constant-current excitation is that the effects of lead-wire resistance can be eliminated and the necessity of calibrating each unit with the lead wires attached can be avoided. Delivering a constant current to the device is achieved by allowing the voltage across the output leads to seek the necessary potential within prescribed limits required to make the current equal to the calibrated current setting.

A current-generating source with the capability to regulate the voltage drop across the input leads is required to provide a constant current. Because current is controlled at the input end of the circuit, the same current will be present across the sensing circuit, provided there are no current leaks or shorts in the measuring circuit.

Selection of a pressure transducer requires careful review of the literature from prospective vendors. Comparing instrument specifications is a difficult and time-consuming process. Vendors commonly specify different sets of parameters and, typically, it is not clear which definitions are being applied to properly interpret a stated specification. Confidence levels are rarely specified and reporting specifications vary from one manufacturer to another.

When in doubt, the vendor or manufacturer should be consulted for clarification and additional information. When selecting a pressure transducer, carefully consider the specifications of the instruments that will be used to excite and measure the output of the pressure transducer. These components of the measuring system may be the limiting factor in meeting overall performance objectives.

An analysis of the circuits, using the principles in "An Overview of Direct Current Analysis," may be helpful in selecting these instruments. The input and output characteristics of the transducer must be compatible with the excitation and recording system used. Terms frequently used to describe the performance characteristics of pressure transducers appear in the glossary. The definitions of many of these terms are based on terminology adopted by the Instrument Society of America Terms such as drift, error, error band, resolution, hysteresis, and time constant require additional explanation as illustrated in the following figures.

Drift, an undesired change in output over a period of time that is not a function of the measurand, is normally specified as a change in zero zero drift over time and a change in sensitivity sensitivity drift over time. Zero drift is illustrated in figure 23 as the vertical shift in the intersection of the best fit straight line between the initial calibration and the later calibration.

The sensitivity drift is shown by the difference in slope of the best fit straight line between the initial calibration and the later calibration, where the initial calibration curve, adjusted for zero drift, is shown as the dashed line. Error, the algebraic difference between the indicated value and the true value of the measurand, can be zero, span, or nonlinear error. Zero error is constant throughout the range of true values of the measurand as shown in figure Span error is an error that changes linearly with the value of the measurand, as illustrated in figure Nonlinear errors change as a nonlinear function of the value of the measurand, as illustrated in figure The error band is the band of maximum deviation of output values from a specified calibration curve or reference line.

The sum of all the defined errors cause the measured values to differ from the true values. The measured values will fall within the error band, as shown in figure Hysteresis is the maximum difference in output, at any measurand value within the specified range, when the value is approached first with an increasing and then with a decreasing measurand.

Expressed in percent of full-scale output, it is usually described in terms of maximum hysteresis in the output, as illustrated in figure The resolution of a measurement system is the smallest change in the measurand that can be measured or detected in the output reading.

The ability to detect significant differences in output can be due to the construction of the transducer, noise in the system, or the numerical resolution of a digital data logger. The definition of resolution is illustrated in figure The time constant is the length of time required for the output of a transducer to rise to 63 percent of its final value as a result of a step change in the measurand, as shown in figure The time constant of a transducer is analogous to the time constant of an electrical circuit, in which the time constant is the time required for a capacitor charging through a resistor to reach 63 percent of the applied voltage.

The type and number of sensors and data recorders needed for automated collection of water-level data depend on the objectives of the study. Determine these objectives prior to selecting system components. Options are numerous, but once the study objectives and needs are clearly determined, the selection of appropriate system components will be simplified.

Some considerations for planning the installation of a water-level collection system are presented below. For many installations, submersible pressure transducers may not be needed, nor may they be the most suitable water-level sensors. In the following sections, however, submersible pressure transducers are assumed to be the preferred water-level sensors. Nearly all submersible pressure transducers are capable of providing accurate results for short-term studies such as aquifer tests or slug tests but as the study duration increases, the chance of sensor failure and the amount of zero drift increases.

Purchasing more expensive sensors, engineered to withstand the added demands of long-term deployment, may be necessary. Sensor maintenance and recalibration also becomes a consideration when designing a long-term data-collection effort. For long-term investigations, the data logger and power-supply systems need more attention and protection.

It may be necessary to recondition and recalibrate the data logger occasionally or to house it in a dry environment to prevent failure of components due to long-term exposure to moisture. Sensor cables may need to be protected with tubing or pipe to prevent long-term damage from ultraviolet radiation, physical weathering, exposure to ozone, or vandalism.

System reliability is among the most important considerations when designing a water-level monitoring system to be operated over a long duration. Redundancy, designed into the system so a partial failure will not result in complete loss of data, can range from multiple sensors in the same borehole connected to one data logger, to two or three completely separate systems logging water-level fluctuations in the same well.

If a high degree of reliability is important, the study should be budgeted to provide early warning of system problems and fast access to replacement components to minimize down time. Many manufacturers use terms such as mean time between failure and reliability to present durability information on their products. Mean time between failure is most commonly defined as the total time that a number of sensors operate, divided by the number of sensors that fail during the operational period.

Reliability is the probability that an item will perform its intended function for a specified interval under stated conditions. Specified interval refers to the length of the study or test. Stated conditions refer to the operational environment—weather, humidity, temperature, and electromagnetic interference.

Most of the time, the specified interval or the stated conditions supplied by the manufacturer are not the same as those of the hydrologic investigation. Also, reliability specifications usually refer to a single component of what commonly is a multiple-component system. For example, a pressure transducer may have one stated reliability and a data logger may have a different stated reliability, and the reliability of the combination of the two components the system reliability will be different from either of the reliabilities of the individual components.

Most of the time, the overall system reliability will approximate the reliability of the least-reliable component. Systems of pressure transducers, data loggers, cables and other supporting equipment used for sensing and recording water levels in wells should be sufficiently accurate to meet the needs of most ground-water projects of the USGS. For the case of large changes in water level for example, during aquifer tests , this measurement error may not be achievable, and an accuracy of 0.

Where the depth to water is greater than ft, an accuracy of 0. In summary, the measurement error and accuracy standard for most situations are 0. Many hydrologic investigations in the USGS require the accuracy of the preceding suggested standard. While most sensor manufacturers produce devices that achieve this accuracy, the added complexities of the wiring, data logger, power source, and environmental variability may unacceptably degrade the overall system accuracy.

Investigators may want to test for themselves the overall accuracy by conducting a pilot project before investing in a system that may not meet data objectives. In some cases, the desired accuracy may not be achievable with current technology or within budgetary constraints. For example, it is difficult to achieve a high level of accuracy with long leads, when depths to water are large, or when water-level fluctuations are large.

Stringent accuracy constraints require frequent check measurements in the field. If the study site is near the office, then frequent visits to the site to download data, perform site maintenance, replace failed components, or make accuracy check measurements may be reasonable. If, however, the site is remote or difficult to access, then the system needs to be designed to be operated remotely, and contain greater redundancy to better ensure uninterrupted collection of data. Remote sites may need an enhanced power supply, more robust shelters, extra data-storage capacity, equipment to allow communication with the site and transmission of data from the site, two or more transducers in a well, and automated checks for sensor drift.

When designing a data-collection system, determine which components are necessary and ensure that all of the components can communicate properly. Because power can be supplied by some data loggers, a short-term study might require only a pressure transducer connected to a data logger. For a long-term study, however, additional components including a power supply batteries, solar panels, voltage regulator , additional data storage devices, a shelter or shelters, and a data-transmission system may be needed.

Ensuring compatibility between components becomes more difficult as the number of components increases. For example, some data loggers can not interpret a digital signal from a transducer that makes an analog-to-digital conversion at the sensor. Similarly, the type of analog signal needs to be compatible; if the sensor sends an amperage signal, the data logger needs to be able to receive an amperage signal.

Some pressure transducers require a separate measurement of temperature in order to correct the transducer output for changes in temperature in the well. If a data logger is not capable of receiving the temperature signal, the overall system accuracy is reduced. The data logger must also be able to supply the excitation voltage or current required by the sensor. When designing the installation, the number of sensors the data logger can simultaneously record needs to be considered.

Water quality must be considered when planning an installation. If the well will be used for water-quality sampling, the transducer and cable should be easy to clean before installing. Do not use lead or plastic-coated lead weights to apply tension to the cable.

If the well is at a contaminated site, consider the possible effects of contaminants in the water that may corrode or otherwise degrade transducer components. Select components that are corrosion resistant, and easily decontaminated. Some manufacturers make chemical-resistant transducers of stainless steel or titanium, and polyfluroethelene-coated cables. For several wells in close proximity—for example, a nest of piezometers or multiple wells for a pumping test—one data logger that can receive signals from several pressure transducers usually is much less expensive than dedicating a data logger to each sensor.

Data retrieval from one data logger also is much simpler. Instrumentation of many wells requires many pressure transducers, which can become cost-prohibitive for some studies. In some situations it may be possible to prioritize the need for continuous water-level data, and record water levels in key wells with pressure transducers and data loggers while manually measuring levels in other wells.

If the study design calls for single, isolated wells to be instrumented, many manufacturers offer water-level sensing systems that allow the pressure transducer, data logger, power supply and cabling to be installed inside the well bore, thus protecting the entire system from the weather, vandalism, or theft. The setting in which the transducer is installed needs to be evaluated prior to installation. Wells installed in areas subject to strong electromagnetic fields, such as near generators, motors, pumps, power supplies, or similar devices may not be suitable candidates for some types of pressure transducers and may require additional protection and signal conditioning.

Natural occurrences such as storms, precipitation, and lightning likewise can affect the transducer and data logger. Wells installed in remote locations commonly require provision for additional data storage. If site visits are infrequent, the data-collection system should be robust, may need to contain redundancies, and may need to have a data-transmission capability.

If well diameters are small less than 2 in. For example, many wells installed in peat deposits are of small diameter to reduce the time lag for pressure equilibrium due to the typically low hydraulic conductivity of peat. Although some transducers are as small as 0. The investigator may need to choose a different type of transducer, such as a vibrating-wire pressure transducer, remembering that smaller transducers are usually more fragile.

Wells with a large depth to water present special problems for instrumentation. Unusually deep wells, such as those at the Nevada Test Site where water levels range from tens of feet to several hundreds of feet deep, pose many unsolved anomalous data problems—spikes, drift, inexplicable rises and recessions, lost data—as well as correlation problems between manual and continuous data O'Brien, Pressure transducers hung from long cables can be affected by cable stretch.

The cable also can expand and contract with temperature changes, thus raising and lowering the transducer in the well and introducing errors into the data. Sensors are susceptible to impact-shock damage from hitting the sides of the well or being rapidly submerged during installation or calibration.

Deep wells present more opportunities for this kind of mechanical damage. Signals can degrade in long cables. The voltage attenuation and interference in long cables can cause erroneous data to be stored in the data logger. In addition to the effects on surface equipment, great differences in temperature and humidity between the surface and the water level, coupled with dissimilar metals, may lead to galvanic effects inducing voltages and transient currents that may distort the signal.

The signal wires and their axially wound shields can act as inductive and capacitive circuits, which then may lead to ferromagnetic resonant effects, inducing transient currents into the signal-bearing channels. One solution is to convert the analog signal to a digital signal at the sensor and transmit the digital signal up the sensor cable to the data recorder.

Another solution is to transmit the signal using AC current from ma current-mode transducers. Current-mode signals are less susceptible to degradation than the more common DC voltage-mode transmissions. Vent tubes on long cables have an increased chance of becoming clogged simply due to their greater lengths. Because of the great depths in some wells, environmental conditions may cause a variety of problems with both water-level measurements and data recording.

The varying temperature and humidity and the atmospheric pressure gradients between the water level and land surface may cause vent tubes to become congested and ultimately allow moisture to be transported down into the sensitive electronic and mechanical portions of a pressure transducer.

Preventing moisture entry is discussed in "Desiccation Systems. Accurate check and calibration measurements are much more difficult when the depth to water is great. Manual wireline or tape measurement is more difficult because of the line stretch caused by weight-induced tension or temperature-induced expansion. These problems may call into question the accuracy of the data record when it is compared against the manual measurements.

For most water-level investigations, the frequency of data collection is limited by the data logger and memory-storage system. Frequent observations require more memory in the data logger or storage devices. Commonly, water-level data in wells are collected no more frequently than hourly. For aquifer-test applications, however, the pressure transducer may limit data-collection frequency.

In some situations, where recovery from an aquifer test is very rapid, observations should be made on the order of every 0. Some pressure transducers may take more time than that for the output to stabilize following excitation of the sensor. Some data loggers store the average of measurements taken over a period of several seconds, so recorded measurements of rapidly changing water levels may lag behind the true water levels.

Data commonly are stored in a data logger or attached storage device, or both. In order to transmit the data from the logger to a computer in the office, a direct datalogger download can be made during a site visit or the data can be accessed remotely. Remote access can include automated transmission by satellite Jones and others, , phone line, cell phone, or radio signal; or the data can be stored on an onsite computer.

The frequency of this transmission would depend on the timeliness of the data, ranging from the need for immediate transmissions to long transmission intervals designed to prevent exceeding the data storage capacity of the data logger or on-site storage device. Consult the manufacturers' manuals for data transmission techniques.

Most study objectives are compromised to some extent by budgetary constraints. Developing a priority of goals, and determining the cost of these goals, will provide the greatest value for the funding available. For example, if accuracy is the main goal, but maintaining an uninterrupted data record is a lesser priority, a study design could include very high quality pressure transducers but no system redundancies.

Conversely, if maintaining a continuous data record is the highest priority, the study may be designed to have more than one sensor in each well, multiple data- storage systems, and backup power supplies, but use less expensive pressure transducers.

The availability of personnel to service the data-collection sites commonly is the single most important decision that needs to be made when designing a study. If the study can afford frequent site visits, the accuracy of data and continuity of record nearly always are increased. The user must be familiar with the behavior of the transducer, its installation, calibration and sources of error in the calibration to ensure that the data collected are reliable and reproducible.

Depending on the accuracy requirements of the study as discussed previously, the user may choose to perform simple field checks of the transducer characteristics supplied by the manufacturer, or perform more detailed tests on individual transducers in the office. Studies requiring greater accuracy will require more extensive testing of the transducers. User-assembled transducer systems will require the most extensive testing. Housing for low-cost pressure transducer with a thermistor.

A rugged and inexpensive housing for these transducers can be assembled with a PVC-pipe coupling and two bushings figs. The bottom bushing is center-drilled with a letter "D" bit 0. Fit the top bushing with a strain relief fitting. Insert the cable through the strain relief and top bushing and attach it to the transducer. The usual wiring convention is red-excitation; black-analog ground; white-signal high; green-signal low.

The pressure port is pressed into the bottom bushing, while optionally sealing the contact between the PVC and pressure port with cyanoacrylic glue. Glue the bottom bushing into the coupling with PVC glue, fill the cavity with potting compound, and press the top plug into place. Purchase extra potting material and practice potting a few times on dummy housings without transducers to become familiar with the procedure.

Users should take time in the office to become familiar with new pressure transducers and data loggers, as well as with recalibrated transducers before taking them to the field. Failure to do so can result in much wasted time in the field. Before submerging the transducer, a series of performance tests should be done in the office to determine:. To test, connect the transducer to the data logger and set the output interval to about 5 to 30 seconds, depending on transducer response time, with output to a computer so that numbers, a graph, or both appear on the screen.

With the sensor upside down, place a few drops of silicone oil in the transducer port. Compare transducer outputs with the transducer in normal vertical position as it would be in a well upside down, and horizontal. A fluid mass such as silicone oil below the transducer plane gives a highest reading with the transducer upside down, an intermediate reading sideways, and the lowest reading in normal vertical position. Warm and cool the transducer, noting changes in output.

Vary the excitation voltage over its specified range, noting changes in transducer output. Leave the transducer to record for a few days at a constant temperature, then leave it to record while the temperature fluctuates over a temperature range that includes the range of temperatures expected in the field. For gage or differential transducers, the output during these tests should not change. The constant-temperature test is a test of drift, exclusive of temperature effect. The variable-temperature test is a test of drift including temperature effect.

For the variable-temperature test, return the transducer temperature to the starting temperature and subtract the prorated drift to get residual temperature effect. For an absolute transducer, subtract the output of an absolute-pressure standard from the measurements to get drift and temperature effect. Sudden temperature changes applied to a transducer such as by holding the transducer in your hand, or lowering a transducer into a well may cause unusual time-varying changes in output.

These changes commonly are due to temperature gradients across the transducer element or among circuit board components. Inexpensive pressure transducers can be soldered to inexpensive cable, potted in waterproof housings, and submerged in water to measure water-level changes in wells, piezometers, and stream gages; pore-pressure changes in saturated sediments; and soil-moisture tension.

Measure submergence by using one transducer under water and another transducer as a barometer. Submergence is the calibrated output of the underwater transducer minus the calibrated output of the barometer. If multiple absolute transducers are used for water-level measurements, three transducers should be used for the barometer to ensure redundancy. Differential-pressure transducers require a vent tube from the reference port to the atmosphere but do not require a barometer for adjustment to submergence.

An ancillary transducer used as a barometer, however, determines the barometric effect on water levels in wells and piezometers. Specifications for the transducers, cable, fittings and potting compound are given in Carpenter, Pressure tests to measure hysteresis are relatively easy to perform. Apply a vacuum to the transducer and release, noting output; then apply positive pressure and release, noting output when the output stabilizes. The difference between the stabilized outputs is hysteresis.

Temperature tests for hysteresis can be done in a bath of ice and fresh water by warming the transducer and putting it in the bath, noting the output; then cooling the transducer in a bath of ice and salt water and returning it to the bath of ice and fresh water, noting the output. Temperature tests for hysteresis are difficult to perform on absolute transducers because air pressure may have changed by the time the transducer has come to thermal equilibrium at the three different temperatures.

In field installations where the water level or temperature fluctuates rapidly, attaining the desired accuracy may require correcting the transducer readings for hysteresis. An interesting test to perform on differential pressure transducers is to apply the full-scale pressure to both ports. If the transducer were truly differential, no change in output would occur.

In fact, the sensing element has undergone a volumetric strain from the pressure change, and a change in output will occur. In silicon strain-gage transducers, the change in output from application of full-scale pressure to both ports can be as much as one percent of the specified full-scale output. This shift is of little consequence in applications using high-range transducers. When using low-range differential transducers to obtain high resolution water-level fluctuations, however, the shift caused by barometric fluctuations, which can be as much as 0.

The next set of performance tests is done in the office during and after submerging the transducer. If the transducer has an obvious port, fill it with water. A tiny piece of fine screen with openings smaller than 0. Inject water through the screen using a disposable insulin syringe, which has a very fine needle. The first wet test should be the effect of hydration on zero shift.

For this test, note the pressure in a fixed orientation immediately after filling the port with water. The next day note the pressure after assuring that the port is filled to the same level in the same orientation. Hydration may be noticeable in some transducers that contain silicone oil or gel in the sensing element or in the port. Hydration does not cause continuing drift, but it is necessary to ensure that the transducer is hydrated before it is installed to avoid having to subtract this small effect from the field data.

Perform the right-side-up, upside-down, and horizontal tests again, comparing the wet outputs with the dry outputs. The wet output minus the dry output, in pressure units of feet of water, gives the length of the port which is used to determine the transducer plane. If the port points down, the difference should be positive with the transducer upside down and negative with the transducer right side up. Immersing the transducer in water should give the result of zero output when the transducer plane is even with the water surface.

Another approach to determining the negative-standpipe effect is to connect a small-diameter copper or stainless-steel tube to the port, fill the tube and port with water, and bend the tube around until the tip is even with the transducer plane. Determining the transducer plane allows the user to become familiar with the positive and negative standpipe effects of the port.

In field operations, if the port is not filled with water, it will capture an air bubble. The pressure inside the bubble does not increase and decrease linearly with water-level changes, because the bubble's air-water interface moves up and down. In addition, the bubble will gradually dissolve, producing drift. Pulling the transducer out of the water to check zero drift will give a negative reading equal to the column of water held in capillary tension below the transducer plane.

Without a screen or small tube, some or all of the water may be shaken out, giving irreproducible results for a zero-drift check and different subsequent drifts for bubble dissolution. Not all transducers exhibit these standpipe effects, and the effects may be negative or positive. Experimentation and testing, which will determine the effects for a particular transducer, should be done in the office before going to the field. A common calibration procedure for pressure transducers uses a standpipe to obtain different values of submergence and a linear regression or straight-line fit.

The equation is:. Equation 6 is solved for P giving:. Linear calibration of submersible pressure transducers can be done in the office using a vertical standpipe capped on the bottom and kept filled to overflowing with water, or in the field using a well. Be sure that no air is captured in the pressure port because, as explained earlier, the pressure at the diaphragm does not increase linearly with an increase in submergence when a bubble is present.

Ensure that water temperature is constant throughout the water column, because density stratification in the standpipe will affect the pressure readings. Calibration in a standpipe is done by inserting and withdrawing the transducer while maintaining the water level at the top of the pipe. The cable is marked and measured from the transducer plane at a minimum of five increments over the desired range of submergence.

The transducer is submerged in the standpipe to a mark on the cable, and the distance and transducer output are noted. Repeat the procedure for all the marks. The linear regression coefficients are determined using a hand calculator or a spreadsheet program. Enter the coefficients into the data logger, and repeat the submergence procedure as a final calibration check. For the insertion and withdrawal phases transducer output may not be identical for a given submergence depth.

This difference arises from hysteresis and lack of repeatability in positioning the transducer at precisely the same depth. Some experimentation is needed to determine positioning errors. In a standpipe calibration, it may be difficult to maintain or vary the temperature of the transducer in a controlled way. The transducer may also be calibrated in an observation well, as described in "Transducer Field Calibration. The problem with linear regression for calibration of silicon strain-gage pressure transducers is that the procedure leaves significant residual errors while giving the false appearance of a good fit when the calibration points are plotted on the same graph as the straight-line fit.

In fact, the r 2 for linear regressions can exceed 0. Major sources of error in linear calibration are thermal effects on the transducer and non-linear response of the transducer. Different manufacturers' transducers exhibit different curvature and temperature effects in their calibrations. The critical test in the calibration procedure involves plotting residuals of the calibration equation minus the calibration points. When the range of that plot is less than the accuracy required by the user, the calibration is adequate.

Thermal effects in a transducer can arise when water temperatures change at the transducer. In wells and piezometers, geologic features such as faults, fractures, and joints can provide conduits of high hydraulic conductivity. If the head varies between these features with time, water can flow vertically in the well, producing temperature changes. Well screens or gravel packs spanning aquifers with intervening confining units can allow flow within the borehole in response to nearby pumping owing to differences in hydraulic conductivity and specific storage between the aquifers.

For example, a temperature change of 3. Inadequate or nonexistent temperature compensation produces thermal effects in pressure transducers. Bridge networks in transducers commonly are compensated to:. The compensating networks commonly consist of series and parallel resistors, some of which exhibit temperature effects opposite to those of the transducer. Because a compensated design is a solution for the average of a transducer model, and because individual transducers vary, a particular design will reduce some errors and, in the process, introduce other, smaller errors.

A compensated design may make it difficult to use a transducer below a manufacturer's error specifications and may make temperature correction by the user much more difficult than uncompensated designs. Some manufacturers apply a simple linear shift or offset for temperature compensation. Such compensation does not usually interfere with user-determined temperature correction of transducers to achieve higher accuracy.

The act of measuring the response of the pressure transducer can in itself introduce errors due to the effect of the meter on the circuit being measured. These errors are caused by current loading, shunt-resistance loading, magnetic coupling, and ground loops. Voltage measurement errors, other than those attributable to instrument calibration, can arise from the physical limitations of the voltmeter and from extraneous voltages generated in other parts of the measurement circuit.

An ideal voltmeter would have an infinite input resistance and would draw zero current from the circuit being tested. Because the voltmeter resistance is not infinite and is connected between two points of unequal potential in parallel with the circuit , a current will occur in the voltage measurement. This current draw may change the current magnitudes in the circuit enough to cause a considerable change in potential being measured.

These limitations usually can be minimized with proper instrument selection. Some of the most important sources of error associated with voltage measurements are discussed below. Input-current loading error, caused by the small amount of input current I in that is drawn by the voltmeter during measurement fig.

To reduce this error, a voltmeter with a small input current should be used. Shunt-resistance loading error is caused by any resistance R shunt that is in parallel with the test signal fig. Shunt-resistance sources include the resistance of the voltmeter and insulation leakage across the test leads and connectors.

Shunt-resistance loading errors can be minimized by:. Thermal electromotive force EMF errors result from small voltages generated at the junction of two dissimilar metals by the thermocouple effect. The size of these thermocouple-induced EMFs depends on the type of metals used and the temperature differences between junctions.

These thermocouple effects will either add to or subtract from the desired signal, producing an unpredictable offset because the EMFs generated by the junctions vary in response to temperature. Thermal EMF errors can be minimized by:. Magnetic coupling errors occur when a magnetic field of varying strength passes through a loop in the measuring circuit. Voltage induced by the magnetic field is proportional to the strength of the magnetic field, the rate of change of the magnetic field, and the loop area of the measuring circuit.

Magnetic field effects can be minimized by:. Ground loops may produce both noise and error voltages. Ground loops arise from grounding the measurement instruments and output source at two or more different points on a common ground bus. The separation between ground points can result in a voltage between the output source and measurement instruments, causing a current to flow around the loop.

Resistance in the loop produces an unwanted voltage in series with the output voltage of the transducer. To avoid ground loops, all test and measurement components should be grounded at one single point. Some pressure transducers may include a ground wire attached to the outer housing.

When submerged, the pressure transducer itself may become a ground point in the circuit. To avoid creating a ground loop, the ground wire should not be connected to the measurement instrument if the measurement instrument itself also is grounded elsewhere. Alternatively, it may be better not to ground the measurement instruments but to use the pressure transducer ground as the common ground point for the entire measurement circuit.

Current-measurement errors , other than those attributable to instrument calibration, can arise from the physical limitations of the current meter and from unknown or unwanted currents generated in other parts of the measurement circuit. An ideal current meter would have zero input resistance and no voltage drop across the input terminals. As in the case of the voltmeter, actual instruments do have some small resistance that can cause measurement inaccuracies.

Since a current meter has some internal resistance, its insertion into the circuit in series may decrease the current in the measured branch. Current meter errors can be minimized with proper instrument selection. Some of the most important error sources associated with current measurements are discussed below. Voltage burden errors , caused by the input resistance R in of the current meter, result in a small voltage drop across the input terminals fig.

To reduce this effect, Rin should be as small as possible. Current meters measure current either by measuring the voltage across a shunt resistor, or by using an active circuit such as a current to voltage I to V converter. Shunt-resistor current meters tend to have very high voltage burdens in some cases up to several hundred millivolts , whereas the voltage burden of current meters with I to V converters are typically less than microvolts. Generated currents or stray currents can add to the desired signal current and introduce errors into the measurement.

The effects of these can be minimized with careful attention to the measurement environment. Stray currents may be introduced into the measurement circuit from a number of different sources.

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Microphones, speakers, thermocouples. Antenna transmitters and receivers are also transducers. Current transducers are used when going from a primary current source to a secondary signal output that we can measure. Magnetic Field Transducers are used when going from a magnetic field source to a signal out that we can measure. Pressure Transducers or force transducers and convert physical force into a number or reading that can be measured and understood.

These are also referred to as a load cell. A Piezoelectric Transducer converts the electrical charges produced by some forms of solid materials into energy. Thermocouples which are like electronic thermometers measure voltage changes and prevent our phones, thermostats, and cars from overheating. An electromechanical Transducer is any type of device that either converts electrical signals into sound waves as in a loudspeaker or converts a sound wave into an electrical signal as in the microphone.

Mutual Induction Transducers rely on two coils for mutual induction. One for generating excitation and another for output. Strain Gauges are types of transducers that convert a physical quantity such as a load, pressure, or displacement into mechanical strain on the strain generating body.

The strain is converted into electrical output using mounted strain gauges. Learn More. Every phone has a microphone and speaker. There are accelerometers to measure the movement of the phone for those who like counting daily steps. Electronic compasses. Those are all input transducers sensors. As you can see, transducers are basically everywhere. Some more examples of transducers can be found in your car. Pretty much everything on your dashboard originates from a transducer.

Your speedometer converts the rotational speed of your wheels into miles per hour. Tachometers generally have a Hall effect sensor that sends an electronic signal for every rotation of the engine. Thermocouples measure engine temperature, cabin temperature, and outside temperature. Pressure sensors measure your oil pressure and detect changes in tire pressure.

Accelerometers to detect crashes and deploy airbags. The actuator motors for your seat adjustment are also transducers. A transducer is broader and includes both sensors and actuators. A sensor specifically reacts to something in the environment — mechanical, electrical, temperature, pressure, and so on, and converts it to an electronic signal to be recorded or analyzed elsewhere.

So generally, a sensor is a type of transducer. When talking about sensor transducers, you have the sensor head, the part that reacts to change in the environment. The environmental change could be magnetic field and remember, electric current moving through a conductor generates the surrounding magnetic field , temperature, pressure, and so on.

You then need to convert that into a usable electric signal, so there is some sort of electronics involved. That could be something as simple as a resistor converting induced voltage to current and vice versa. It could also be an amplifier to strengthen a very small signal to something useful.

Or even an integrator circuit to translate the rate of change in current into the actual underlying current waveform. Generally, you compare it to a reference. Testing a transducer is similar to calibrating but without the same rigor. You put it in a test environment that you know is changing, like moving a thermocouple from the shade to the sun, or even just holding it in your hand, to warm it up and see if it responds to how you act.

For a magnetic field transducer, you can change the magnetic field, by example bringing it close to a permanent magnet, and see if the transducer output changes. For a current transducer, raise or lower the current and see if the output changes as you expect. An active transducer requires external power to generate a signal, which could be some sort of excitation, signal processing, integrator circuit, amplifier or other electronics. Most current transducers are active transducers.

A passive transducer has a signal induced just by the inherent properties of whatever the transducer sensor detects. For example, a thermocouple generates a voltage based on a change in temperature, without requiring an external power source. A current transformer steps the current up or down from its source.

It can be used for measuring current, with a lot of turns, for example, to 1 turns ratio. If you have a primary current of amps, your transfer will give you your current output of 1 amp, which is low enough you can probably measure it in your measurement instrument, perhaps converting to a voltage signal with a resistor. A transformer can also be reversed, taking a 1 amp source into a amp output.

A transformer inherently is limited to AC current only, as it is detecting a change in the primary current. A current transducer on the other hand directly converts the energy into another type of energy. For example, electric current into a voltage output signal. That electrical output might then be converted back into a current, but there is some sort of intermediate stage where conversion takes place. The key point is some sort of conversion must take place.

One inherent advantage of a current transducer over a transformer is that a transducer can measure DC currents as well as AC currents. One type of current transducer is a Hall sensor, which converts the magnetic field that is given off by all current conductors into a voltage signal. Note: Warren Buffett is not only the most successful long-term investor of all time, but also one of the best sources of wisdom for your investment strategy.

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