March 2012 Newsletter

CAS Newsletter March 2012

Volume #14, Newsletter #14 March-2012
Chipkin Automation Systems Inc.
IN THIS ISSUE

XML, JSON, AJAX … Everything You Wanted to Know About Web Protocols

HTTP: Hypertext Transfer Protocol. Client / Server protocol. Uses TCP/IP. Transports text and binary data. Primary purpose is to carry payloads for hypermedia information systems (World Wide Web) Plebut in practice has been used for a variety of purposes. Is not involved in the use of the data/ rendering the data. Can be used to carry HTML, XML, JSON and many other forms of data including binary objects like files and images. This protocol is popular for diverse services because so many sites leave port 80 (default HTTP port) open. Although  it can carry a very diverse set of payloads it’s a pretty simple protocol.

XML: It’s a (markup) language that defines a set of rules for encoding data in both human/machine readable form. Is not a protocol. Think of it as payload. Most often the HTTP protocol is used to transport XML data. A number of XML utilities are used by programmers to parse/process XML data. A number of tools are used to render/validate/document XML data. Not everyone uses the language the same way. Vendor A may encode data one way and vendor B may encode the data another way – even if they are dealing with the same type of data – eg. Energy measurement. To use XML you need to be sure that the protocol used to serve the payload is the same as the protocol used to request it (eg HTTP for both client and server) and that both the client and the server expect the data schema to be similar. An XML schema is the structure of the data where as an XML payload is a set of XML data that conforms to the specified schema but contains a specific set/instance of data/measurement.

AJAX: Not a protocol. Not a markup language. A combo of tech’s used to achieve the goal of updating a web page without interrupting the display of data. Ie a method of reading data in the background of a web page and using that data to update the foreground of the web page without interrupting it. Originally  Protocol=HTTP Payload=XML  Parser=JavaScript. However XML data can also be replaced by JSON data and a number of other parser’s are used. It’s a very generic term that implies few details.

JSON: JavaScript Object Notation

It’s a language that defines a set of rules for encoding data in both human/machine readable form. Is not a protocol. The encoding method is different to XML in format.

JSON vs XML:

JSON representation:

{“menu”: {
  “id”: “file”,
  “value”: “File”,
  “popup”: {
    “menuitem”: [
      {“value”: “New”, “onclick”: “CreateNewDoc()”},
      {“value”: “Open”, “onclick”: “OpenDoc()”},
      {“value”: “Close”, “onclick”: “CloseDoc()”}
    ]
  }
}}

The same text expressed as XML

<menu id=”file” value=”File”>
  <popup>
    <menuitem value=”New” onclick=”CreateNewDoc()” />
    <menuitem value=”Open” onclick=”OpenDoc()” />
    <menuitem value=”Close” onclick=”CloseDoc()” />
  </popup>
</menu>

 

RPC: Remote Procedure Call. Not a Protocol but is similar. Some vendors implement an RPC for data exchange. The RPC messages are transported using a protocol like HTTP. Instead of using a particular protocol to read data from a device, such a Modbus, you send a RPC call with parameters (like the point address). The device responds with a payload of data which (vendors choice) may be encoded with JSON or XML. This is like a protocol within a protocol. The HTTP carries the messages. The messages are series of requests/responses which must meet that vendors RPC rules (or RCP protocol). RPC is open, vendors choose the protocol, the payload, the methods, the formats.

In this example: JSON is used as the markup language to define the request. The request is sent using HTTP Protocol. It is sent to the RPC url on the device. The device unpacks the HTTP, extracts the payload. Presents the payload to a JSON parser/analyser which extracts the JSON elements. These are presented as parameters to the Remote Procedure Call logic in the device. It processes it and forms a response. The response is marked up using JSON (in this example but it could have been simple text or XML) and then packed into a HTTP packet and sent back.

Sample request:

{
“version”: “1.0”,
“proc”: “GetPlantOverview”,
“id”: “1”,
“format”: “JSON”
}

WSDL: Web Services Definition Language. Its like RPC except its not wider open – the technologies used are specified. Protocol used to transport messages is HTTP. The Markup language for the payload is XML.  A WSDL implementation defines the abstract (like a schema ) and the specific. Ie. Part of a WSDL implementation is the definition of the services and the data. The other part is the specific data being transferred for a specific request/response. A WSDL description of a web service provides a machine-readable description of how the service can be called, what parameters it expects and what data structures it returns. It is written is XML.

SOAP: Simple Object Access Model.  It is a protocol. Its adds almost no value. Has become obsolete. SOAP adds a header and body and a few rules to the XML payload inside an HTTP message. When a SOAP message is received, the HTTP driver unpacks the HTTP payload and presents it to the SOAP driver which removes its header and other items and presents the XML payload to the application which will process it.  Why use it ? Without it, if your XML payload cannot be used or made sense of then there would be no service to send an error response. That’s one use.

REST: Representational state transfer. Uses HTTP Protocol to carry messages. Is a protocol. Simple. Requests are made by using HTTP GET,POST to specific urls. Specific of the request are defined as HTTP parameters. The response is coded any way the vendor chose and are not necessarily XML.  Each programmer who develops REST services on a server may make different choices when it comes to the URL’s the parameters and the format of the response.

JAVASCRIPT: A programming language that has nothing to do with JAVA. Javascript is interpreted at run time as opposed to being compiled so it runs slower than most applications. It is used in web pages and  serviced by web browser’s.  Not all browser’s support the same javascript functions and they don’t all work exactly the same way in each javascript enabled browser/application. The Javascript in a web page is not seen by the user. It is used to implement methods and processes which site behind the visible page. For example JavaScript may be used to read data in the background and to update the visible page with the new data. Because it is run in a browser, the language has been somewhat crippled to give the illusion that it cannot access the files and other resources on you computer.

SQL: Not a protocol. Not a marksup language. Not related to the transfer of data.  Refers both to the programming language and to describe Database systems which support the SQL programming language. They are optimized for speed and database size.They are relational. A free implementation is known as SQL*lite. SQL databases need to be stored In one system so for large systems a front server handles the requests and distributes them to other SQL servers. New non-SQL databases can easily be spread out onto multiple sites.

JQUERY: A javascript library that is very widely used on the web. It contains a number of functions (subroutines) for things like animation. They are all built using jasvascript. Think of it as a collection of javascript shortcuts or higher level methods.

FLASH: by Adobe. Multimedia platform. Manipulates vector and raster graphics to provide animation of text, drawings, and still images. It supports bidirectional streaming of audio and video, and it can capture user input via mouse, keyboard, microphone, and camera. Flash contains an object-oriented language called ActionScript and supports automation via the JavaScript Flash language (JSFL). Flash content may be displayed on various computer systems and devices, using Adobe Flash Player, which is available free of charge for common web browsers, some mobile phones, and a few other electronic devices (using Flash Lite). Does not run on iphones.

 


Using The CAS Modbus Scanner to Read Modbus 6 Digit Address (Also Known As JBUS)

The older Modbus systems limited the number of objects of each type to 9999.

I.e. Max Input Register = 39999 and Max Holding = 49999

Whereas the protocol has space to read up to 65535 objects of each type, older Modbus systems imposed a stupid validation and choked the address space to the 9999 limit. Newer systems like the CAS Modbus Scanner do not choke the address space and you can read any registers.

Example:

To read address Holding Register = 512345  (also known as Holding Reg with offset 12345 in PDU terminology)

Display of the request – we show both the 5 and 6 digit address.

 


FieldServer – How to Delete and Remove A Configuration

1. Install the FieldServer Utilities.

2. Start a CMD session.

  • Click the Start button.
  • Type CMD and Click the OK button.

 

  • You get a screen that looks similar to this one.

3. Power down the gateway.

4. Delete the existing configuration.

  • Type the following in the CMD session.

“C:\FieldServer Technologies\FieldServer Utilities\Bin\ruinet”  –i192.168.1.168  -zconfig.csv

192.168.1.168  ==   IP address of FieldServer. This one is provided for an example only.

The quotation marks are required.

When you have completed the line push the Enter key.

If  the command complete OK you will see a screen similar to this one. The folder name and IP address may be different. Note the DELETED message. That is what we want to see.

If it completed OK then:

  • Download the configuration provided using the normal method.

If not then:

 


Force and Its Effects


Definition

A quantity competent enough to modify the size, shape, or motion of an object is termed as force. In other words “A force is an influence (such as a push, gravity, or friction) that causes an object either to change its velocity or to store energy through deformation”.1

Since force is a vector quantity, it has got both direction and magnitude. In case, a body is in motion, the energy of its motion can be quantified as the momentum of the object i.e. the product of its mass and its velocity. When the body is free to move, its velocity will be changed by the action of a force.

Units of Measurement
The precise measurement of force is significant in many areas, like engine thrust determination, the weighing of large structures, and materials testing. The magnitude of a force is measured in:

  1. Newtons (In the SI system): One Newton is defined as the force required to accelerate a mass of one kilogram at a rate of one metre per second, per second
  2. Pounds (In the British/American system)

Basic Forces
Four basic forces found in nature are:

  1. Gravitational Force: It is the force of attraction between any two bodies in the universe. It is the weakest of all and also the easiest to observe. It is always attractive and has an infinite range.
  2. Magnetic Force: It can be the force between two magnets or force on a magnet placed in a magnetic field. It can be either attractive or repulsive.
  3. Strong nuclear Force: It is a strong force with a short range. It is a non-central force which acts within the nucleus. It is not directed along the line joining the centres of the interacting particles.
  4. Weak Nuclear Force: Its range is shorter than the strong nuclear force and this type of force is considerable only for certain nuclear processes like radioactive beta decay.

Pressure
It is the ratio between force acting on a surface and the area of that surface. It is measured in units of force divided by area:

    Pounds per square inch (psi)
    Newtons per square meter, or Pascals

Whenever an object is subjected to an external stress i.e. pressure with the aim to cause a reduction in its volume, this process is called compression. The majority of liquids and solids are practically incompressible, whereas gases are not.

Boyle’s Law
It is the first Gas Law which states that the pressure and volume of a gas are inversely proportional to one another i.e. PV = k, where P is pressure, V is volume and k is a constant of proportionality.

Charles’ Law
It is the Second Gas Law which states that the volume of an enclosed gas is directly proportional to its temperature i.e. V = kT, where T is its absolute temperature.

Third Gas Law
According to this law, the pressure of a gas is directly proportional to its absolute temperature i.e. P = kT. After combining the three relationships we get the ideal gas law i.e. PV = kT. This approximate relationship holds accurate for many gases at relatively low pressures and high temperatures.


Strain Gauge

Any external force applied to a stationary object produces stress and strain. The object’s internal resisting forces are referred to as stress while the displacement and deformation that occur is termed as strain. Strain can be either compressive or tensile and is usually measured by strain gauges. In general, strain gauges are devices used to measure displacement, force, load, pressure, torque or weight.

Main Features
Following are the key features of a strain gauge:

  • Strain-gauge sensor is one of the most commonly used means of load, weight, and force detection.
  • It is a device which is used for measuring the changes in distances between points in solid bodies that happen when the body is deformed.
  • Resistance strain gauge is a helpful tool in the field of experimental stress analysis. It operates on the principle that the electrical resistance of a copper or iron wire changes when the wire is either stretched or compressed.
  • Usual strain gauge resistances range from 30 Ohms to 3 kOhms (unstressed).
  • Size of strain gauges is normally smaller than a postage stamp.
  • An ideal strain gage is small in size and mass, low in cost, easily attached, and highly sensitive to strain but insensitive to ambient or process temperature variations.
  • The ideal strain gauge would undergo change in resistance only because of the deformations of the surface to which the sensor is coupled. However, in real applications, there are many factors which influence detected resistance such as temperature, material properties, the adhesive that bonds the gage to the surface, and the stability of the metal.

Gauge Factor
Essentially, all strain gauges are designed to convert mechanical motion into an electronic signal. The strain experienced by the sensor is directly proportional to the change in capacitance, inductance, or resistance of the gauge used. For instance, if a wire is held under tension, it gets slightly longer and its cross-sectional area is reduced. This causes a change in its resistance proportional to the strain sensitivity of the wire’s resistance. When a strain is introduced, the strain sensitivity, which is also known as the gage factor (GF) of the sensor, is given by:

Where, RG is the resistance of the undeformed gauge,

 R is the change in resistance caused by strain, and

 

is strain.

 

Applications
Strain gauges are frequently used following areas

  1. In mechanical engineering research and development to measure the stresses generated by machinery
  2. Aircraft component testing: Tiny strain-gauge strips glued to structural members, linkages, and any other critical component of an airframe measure stress

Strain Gauge Characteristics
Every strain gage wire material has its own characteristic:

  • Gauge factor
  • Resistance
  • Temperature coefficient of gage factor
  • Thermal coefficient of resistivity
  • Stability

Strain Gauge Materials
Typical materials include:

  • Constantan (copper-nickel alloy)
  • Nichrome V (nickel-chrome alloy)
  • Platinum alloys (usually tungsten)
  • Isoelastic (nickel-iron alloy)
  • Karma-type alloy wires (nickel-chrome alloy)
  • Foils
  • Semiconductor materials

The most popular alloys employed for strain gages are copper-nickel alloys and nickel-chromium alloys.

Temperature Effects

  • High temperatures can affect the internal structure of strain-sensing materials like copper.
  • Temperature can influence not only the properties of a strain gage element, but also can amend the properties of the base material to which the strain gage is attached.
  • Variation 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 or the base material can result in errors which are extremely intricate to correct. For example, a change in the resistivity or temperature coefficient of resistance of the strain gage element can modify the zero reference used to calibrate the unit.

Measuring Circuits
For measurement of strain via a bonded resistance strain gage, it must be connected to an electrical measuring circuit which can measure even the minute changes in resistance corresponding to strain. Modern strain-gage transducers usually employ a grid of four strain elements electrically connected to form a Wheatstone bridge measuring circuit. A Wheatstone bridge is a divided bridge circuit employed for the measurement of static or dynamic electrical resistance. The output voltage of the Wheatstone bridge is expressed in millivolts output per volt input. Besides, this bridge circuit is appropriate for temperature compensation. A quarter bridge strain gauge circuit is shown in the figure below:

Installation Diagnostics
Following steps should be followed while checking strain gauge installations:

  • First of all measure the base resistance of the unstrained strain gage after its proper mounting but before complete wiring.
  • Check for surface contamination by measuring the isolation resistance between the gauge grid and the stressed force detector specimen by means of an ohmmeter, if the specimen is conductive. This should be done before connecting the lead wires to the instrumentation.
  • Also, check for irrelevant induced voltages in the circuit by reading the voltage when the power supply to the bridge is disconnected. Ensure that Bridge output voltage readings for each strain-gage channel are practically zero.
  • Now, connect the excitation power supply to the bridge and verify both the correct voltage level and its stability.
  • Test out the strain gage bond by applying pressure to the gage. The reading should not be affected.



Types of Strain Gauges

Various means like mechanical, optical, acoustical, pneumatic or electrical can be used to measure deformation (strain) of an object. Earlier mechanical devices such as extension meter (extensiometer) were used to measure strain by measuring the change in length and comparing it to the original length of the object. However, mechanical strain gauges offer certain limitations like low resolutions. Besides they are bulky and difficult to use.

Further, capacitance and inductance-based strain gages were introduced but these devices’s sensitivity to vibration, their mounting requirements, and circuit complexity restricted their usage.

Next are the photoelectric gauges. These gauges use a light beam, two fine gratings, and a photocell detector to generate an electrical current proportional to strain. A photoelectric gauge can be as short as 1/16 inch but its usage proves to be extremely costly and delicate.

In 1938, the first bonded, metallic wire-type strain gage was introduced. The metallic foil-type strain gage is constructed of a grid of wire filament of approximately 0.001 in thickness, bonded directly to the strained surface by a thin layer of epoxy resin. When a load is applied to the surface, it gets strained and experiences a change in length. This resulting change in length is conveyed to the resistor and the corresponding strain is measured in terms of the electrical resistance of the foil wire, which varies linearly with strain. Other types of Strain Gauges are described below.

Semiconductor Strain Gauges

In the year 1970, the first semiconductor strain gages were developed for the use in automotive industry. Semiconductor strain gauges exhibit following key features:

  • Unlike other strain gauges, semiconductor strain gages are based upon the piezoresistive effects of silicon or germanium and measure the change in resistance with stress as opposed to strain.
  • The semiconductor bonded strain gage is a wafer with the resistance element diffused into a substrate of silicon.
  • No backing is provided for the wafer element and bonding it to the strained surface needs extra care since only a thin layer of epoxy is used to attach it.
  • Size of a semiconductor strain gauge is much smaller and the cost much lower than for a metallic foil sensor.
  • Advantages include higher unit resistance and sensitivity.
  • Disadvantages: Greater sensitivity to temperature variations and tendency to drift as compared to metallic foil sensors. Also the resistance-to-strain relationship is nonlinear, varying 10-20% from a straight-line equation. However, by means of computer-controlled instrumentation, these limitations can be overcome via software compensation.

Thin-film Strain Gauges

Thin-film strain gage is more advanced form of strain gauge as it doesn’t necessitate adhesive bonding. A thin film strain gauge is constructed by first depositing an electrical insulation, usually a ceramic onto the stressed metal surface, and then depositing the strain gage onto this insulation layer. Techniques used to bond the materials molecularly are:

  • Vacuum deposition
  • Sputtering method

Advantages

  1. Since the thin-film gage is molecularly bonded to the specimen, the installation is very stable and the resistance values experience less drift.
  2. The stressed force detector can be a metallic diaphragm or beam with a deposited layer of ceramic insulation.

Diffused Semiconductor Strain Gauges

A further improvement in strain gage technology was introduced with the advent of diffused semiconductor strain gages since they purge the need for bonding agents. Main features are listed below:

  1. By eliminating bonding agents, errors due to creep and hysteresis also are eliminated.
  2. The diffused semiconductor strain gage employs photolithography masking techniques and solid-state diffusion of boron to molecularly bond the resistance elements.
  3. Diffused semiconductors are frequently used as sensing elements in pressure transducers.
  4. Limitations include sensitivity to ambient temperature variations, which can be compensated by intelligent transmitter designs.

Advantages

  • Small size
  • Inexpensive
  • Accurate and repeatable
  • Available wide pressure range
  • Generate a strong output signal

Bonded Resistance Gauges

Following are the chief characteristics of bonded resistance strain gauges:

  • They are reasonably inexpensive.
  • They can pull off overall accuracy of better than +/-0.10%.
  • They are available in a short gauge length and have small physical size.
  • These strain gauges are only moderately affected by temperature changes.
  • They are extremely sensitive and have low mass.
  • Bonded resistance strain gages can be employed to measure both static and dynamic strain.
  • These types of strain gauges are appropriate for a wide variety of environmental conditions. They can measure strain in jet engine turbines operating at very high temperatures and in cryogenic fluid applications at temperatures as low as -452*F (-269*C).

Construction of a Bonded resistance strain gauge is shown in the figure below:

Selection of a Proper Gauge
Three primary considerations in strain gauge selection are mentioned below:

  1. Operating temperature
  2. Nature of the strain to be detected
  3. Stability requirements

In addition, choosing the right carrier material, grid alloy, adhesive, and protective coating plays an important role in the success of the application.



Load Cell

Load Cell is a specific type of transducer or sensor capable of transforming force (load) into a measurable electrical output. Load cells measure tension, compression, or shear. A typical load cell device comprises of four strain gauges in a Wheatstone bridge configuration.

Working
Force conversion into an electrical signal by a load cell is usually carried out indirectly and in two stages. The first stage conversion is done through a mechanical arrangement in which the force being sensed deforms the strain gauge. This strain gauge then converts the deformation (strain) to electrical signals. One can also get load cells with one or two strain gauges. The electrical signal generated by strain gauge is in the order of a few millivolts which needs further amplification by an instrumentation amplifier. The output of the transducer is fed to an algorithm calculator to calculate the force applied to the transducer.

Classification of Load Cells
Load cell designs are classified on various bases. They are classified according to the

Type of output signal generated

  • Pneumatic
  • Hydraulic
  • Electric

Way they detect weight

  • Compression: Compression load cells measure a pushing together force along a single axis
  • Tension: Tension load cells measure a pulling apart force along a single axis
  • Shear: Shear load cells measure the displacement of a structural element to find out force. Shear cell types for load sensors include shear beam, bending beam, or single point bending beam.

Hydraulic Load Cells
Hydraulic load cells are also referred to as hydrostatic load cells. These are mainly used in industrial applications. These are force -balance devices, measuring weight as a change in pressure of the internal filling fluid. They give linear output reasonably unaffected by the amount of the filling fluid or by its temperature. Accuracy can be achieved within 0.25% full scale or better, provided the load cells have been correctly installed and calibrated. This range of accuracy is suitable for most process weighing applications. Since this sensor has no electric components, it is perfect for use in hazardous areas. Typical hydraulic load cell applications include tank, bin, and hopper weighing. These load cells can eliminate troubles encountered with strain gauge load cells. Hydraulic load cell is probably the best load cell to use in an outdoor environment because it is immune to transient voltage (lighting).

Pneumatic Load Cells
Pneumatic load cells also work on the force-balance operating principle. These devices make use of multiple dampener chambers to offer higher accuracy as compared to a hydraulic device. In several designs, the first dampener chamber is utilized as a tare weight chamber. Pneumatic load cells are frequently employed to measure moderately small weights in industries where cleanliness and safety are principal concerns.

Advantages

  • Inherently explosion proof
  • Insensitive to temperature variations
  • They contain no fluids hence there is no chance of contamination of the process if the diaphragm ruptures.

Disadvantages

  • Comparatively slow speed of response
  • Need for clean, dry, regulated air or nitrogen

Strain-gauge Load Cells
Strain-gauge load cells are bonded onto a beam or structural member which gets deformed when weight is applied. In majority of the cases, four strain gages are employed to achieve maximum sensitivity and temperature compensation. Two of the gauges remain generally in tension, and two in compression, and are wired with compensation adjustments. When weight is applied, the strain changes the electrical resistance of the gauges in proportion to the load.

Advantages: As compared to other load cells strain gage load cells offers more accuracy and lower unit costs. Although there are several varieties of load cells available, strain gauge based load cells are the most widely used type.

Piezo-electric Load Cells
In these types of load cells a piezoelectric material is compressed which then generates a charge that is conditioned by a charge amplifier. It is valuable for dynamically measuring force. Geomechanical applications entail vibrating wire load cells owing to their low amounts of drift. All load cell rings when they are subjected to an abrupt load change and this is caused by the spring like behavior of the load cell.

Selection of a Load Cell
Following points must be kept in mind while selecting a proper load cell:

  • It is crucial to decide whether one wants a tension or a compression load cell. A tension load cell measures the amount of weight pulling it, whereas a compression load cell measures the weight by pushing against it. A compression load cell is generally placed beneath the object that needs to be weighed. Keeping all this in mind, one can easily choose an appropriate load cell.
  • The number of cells should be decided based upon the number of supports used. In ideal situations, each support should have a load cell and this is more essential if the weight is not equally distributed between the supports.



Pressure Transducer

The demand for pressure measuring instruments arises with the advent of steam age. Mechanical methods of measuring pressure such as Bourdon tubes or bellows, where mechanical displacements were transferred to an indicating pointer were the first pressure instruments. Initially, these tubes were constructed of glass, and scales were added to them as per requirements. However, these mechanical motion balance pressure measuring arrangements were large, cumbersome, and not well suited for integration into automatic control loops. Consequently, as control systems evolve to become more centralized and computerized, these devices were replaced by analog electronic and, more lately, digital electronic pressure transmitters. Pressure transmitters or transducers are ready to use instruments employed for measurement of pressure. These are OEM transducers with

  • pressure port
  • integrated compensation resistors
  • a cable or connector

The terms pressure gauge, sensor, transducer, and transmitter can be used interchangeably. Majority of modern pressure sensors operates on piezoresistance principle. Due to pressure, a material generates electricity at a certain rate, which leads to a specific level of charge flow related with a specific level of pressure. This charge is supplied to a wire which leads to a control panel and display for human analysis.

Pressure Transmitter
It is a standardized pressure measurement package which includes following three fundamental components:

  • a pressure transducer
  • its power supply,
  • a signal conditioner/retransmitter used to transform the transducer signal into a standardized output
  • In pressure transmitters, process pressures can be transmitted 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 positioned at some distance from the data acquisition hardware, high output voltage signals are preferred and these signals must be guarded against both electromagnetic and radio frequency interference (EMI/RFI) when traveling longer distances.

Types of Pressure Sensors
Pressure sensing based upon diaphragm technology measures the difference in pressure of the two sides of the diaphragm. Depending upon the relevant pressure, pressure sensors have been classified into following three categories:

  1. Absolute pressure sensor: It measures absolute pressure using a vacuum as a reference point.
  2. Gauge sensor: It measures pressure by reference to the ambient atmospheric pressure. “Gauge” pressure is defined relative to atmospheric conditions.
  3. Differential pressure sensor: It measures the pressure difference between two contacts or ports. Differential pressure transducers are frequently employed in flow measurement where they are used to measure the pressure difference across a venturi, orifice, or other type of primary element. The detected pressure differential is related to flowing velocity and as a result to volumetric flow. Many features of modern pressure transmitters have appeared from the differential pressure transducer.

Pressure Gauge
It is normally referred to as a self-contained indicator which can transform the detected process pressure into the mechanical motion of a pointer. A pressure transducer might join the sensor element of a gauge with a mechanical-to-electrical or mechanical-to-pneumatic converter and a power supply.

Performance Characteristics

  1. Accuracy: “It 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”.
  2. Repeatability: It refers to the closeness of agreement among a number of successive measurements of the same variable.
  3. Linearity: It is a measure of how appropriate the output of transducer increases linearly with increasing pressure.
  4. Hysteresis error: This characteristic explains the phenomenon according to which the same process pressure results in different output signals depending upon whether the pressure is approached from a lower or higher pressure.
  5. Sensitivity: It determines the amount of variation that occurs in the output voltage as and when the input voltage varies, keeping in view that the measured pressure and the rated pressure of the transducer remains constant.

In industrial applications, good repeatability is considered more significant as compared to absolute accuracy. For pressure variations over a wide range, transducers with good linearity and low hysteresis are the ideal choice.

Limitations

  • Ambient and process temperature variations cause errors in pressure measurements, mainly in detecting low pressures and small differential pressures. For these applications, temperature compensators can be employed.
  • Power supply variations also affect the performance of pressure transducers.

Maintenance and Calibration
Pressure sensors call for scheduled, periodic maintenance and recalibration without any exemption. It is obligatory to periodically remove the transducer from the process for the maintenance purpose. It should be ensured that this procedure does not require shutting down the process and does not cause any injury or damage. Since the process fluid may be toxic, corrosive, or otherwise noxious to personnel or the environment, it is essential to guard against the release of such fluids during maintenance.

Pressure transducers can be recalibrated on-line or in a calibration laboratory. Laboratory recalibration is usually chosen over the other one. In the laboratory, there are generally two types of calibration devices:

  1. Deadweight testers: They provide primary, base-line standards,
  2. “Laboratory” or “Field” standard calibration devices: These are periodically recalibrated against the primary. These secondary standards are less accurate than the primary, but they provide a more convenient means of testing other instruments.



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Resources
Britannica
Engineers Edge
Interweigh
Omega – Load Cell
Omega – Pressure Gauges & Switches
Omega – The Strain Gauge
Sensorland – Load / Force Cell
Sensorland – The Pressure Sensor
Sensorland – The Strain Gauge
WiseGeek

 
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