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Wednesday, December 21, 2011

RTD - The Thermal Resistive Sensor

Resistance Temperature Detectors (RTD), as the name implies, are sensors used to measure temperature by correlating the resistance of the RTD element with temperature. Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass core. The element is usually quite fragile, so it is often placed inside a sheathed probe to protect it. The RTD element is made from a pure material whose resistance at various temperatures has been documented. The material has a predictable change in resistance as the temperature changes; it is this predictable change that is used to determine temperature.


Common Resistance Materials for RTDs:

·  Platinum (most popular and accurate)

·  Nickel

·  Copper

·  Balco (rare)

·  Tungsten (rare) 

The resistance is directly proportional to a metal wire's length, and inversely proportional to the cross-sectional area.
R = k L / A         (1)
R = resistance (ohm, Ω)
k = constant of proportionality or resistivity of the material (ohm, Ω)
L = length of conductor (m)
A = cross sectional area of conductor (m2)
Resistivity and temperature can be expressed as
kt = ko [1 + α (t - to)]         (2)
        kt = resistivity at temperature t (ohm, Ω)
        ko = resistivity at standard temperature to (ohm, Ω)
        α = temperature coefficient of resistance (1/oC)
         t = temperature (oC)
         to = standard temperature (oC)

Combining (1) and (2):

R / Ro = α t + 1         (3)
Theory of Operation:
A basic physical property of a metal is that its electrical resistivity changes with temperature. All RTD's are based on this principle. The heart of the RTD is the resistance element. Several varieties of semi-supported wire-wound fully supported bifilar wound glass, and thin film type elements are shown here.

Some metals have a very predictable change of resistance for a given change of temperature; these are the metals that are most commonly chosen for fabricating an RTD. A precision resistor is made from one of these metals to a nominal ohmic value at a specified temperature. By measuring its resistance at some unknown temperature and comparing this value to the resistor's nominal value, the change in resistance is determined. Because the temperature vs. resistance characteristics are also known, the change in temperature from the point initially specified can be calculated. We now have a practical temperature sensor, which in its bare form (the resistor) is commonly referred to as a resistance element.

RTD Specifications:

Eight salient parameters must be addressed for every RTD application to ensure the desired performance. Many will be specified by the manufacturer of the instrument to which the RTD will be connected. If it is a custom circuit or special OEM application, the designers must make all the decisions. The four specifications dictated by the instrumentation or circuitry are: sensor material, temperature coefficient, nominal resistance, and, to some extent, wiring configuration. Sensor Material Several metals are quite common for use in RTD's, and the purity of the metal as well as the element construction affects its characteristics. Platinum is by far the most popular due to its near linearity with temperature, wide temperature operating range, and superior long-term stability. Other materials are nickel, copper, balco (an iron-nickel alloy), tungsten, and iridium. Most of these are being replaced with platinum sensors, which are becoming more competitive in price through the wide use of thin film-type resistance elements that require only a very small amount of platinum as compared to a wire-wound element. 

Temperature Coefficient:

The temperature coefficient, or alpha of an RTD is a physical and electrical property of the metal alloy and the method by which the element was fabricated. The alpha describes the average resistance change per unit temperature from the ice point to the boiling point of water. Various organizations have adopted a number of different temperature coefficients as their standards . 

Temperature Coefficient of Resistance
- α -
0.002 to 0.006
+- 0.00002
-0.02 to -0.09
-0.068 to 0.14

Nominal Resistance:

Nominal resistance is the pre-specified resistance value at a given temperature. Most standards, including IEC-751, use as their reference point because it is easy to reproduce. The International Electrotechnical Commission (IEC) specifies the standard based on 100.00 Ohms at 0°C, but other nominal resistance's are quite common. Among the advantages that thin film technology has brought to the industry are small, economical elements with nominal resistance's of 500, 1000, and even 2000 ohms.

Wiring Configuration:

The wiring configuration is the last of those parameters typically specified by the instrument manufacturer, although the system designer does have some control based on the application. An RTD is inherently a 2-wire device, but lead wire resistance can drastically reduce the accuracy of the measurement by adding additional, uncompensated resistance into your system. Most applications therefore add a third wire to help the circuit compensate for lead wire resistance, and thus provide a truer indication of the measured temperature. 
2 wire

Four wire RTDs provide slightly better compensation, but are generally found only in laboratory equiptment and other areas where high accuracy is required. When used in conjunction with a 3-wire instrument, a 4 wire RTD will not provide any better accuracy. If the fourth wire is not connected, the device is only as good as the 3 wire RTD; if the fourth wire is connected, new errors will be introduced. Connecting a 3 wire RTD to a 4 wire RTD instument can cause serious errors or simply not work at all, depending on the instrument circuitry. A 2 wire RTD can be used with either a 3-4 wire instrument by jumping the appropriate terminals, although this defeats the purpose and reintroduces the uncompensated resistance of the leads. To get the optimum performance, it is generally best to specify the RTD according to the instrument manufacturer's recommendations. 

Temperature Range: 
According to the ASTM, platinum RTDs can measure temperatures from -200°C to 650°C. You must consider the temperature limitations of all the materials involved, where they are applied, and the temperatures to which each will be exposed. A few quick examples to illustrate this point:
·  TFE Teflon should not be used for wire insulation if it will be exposed to temperatures above 200°C.
·  Moisture proof seals are commonly made with various types of epoxy that generally have limits below that of the Teflon insulation.
·  Many wire insulating materials become brittle at subzero temperatures and therefore should not be used for cryogenic work.
·  So state the intended temperature range right up front and let the applications engineer assit you, espically since it may affectthe materials chosen for internal
construction of the probe.


You are probably wondering why accuracy was not the first topic covered, because RTDs are generally known for their high degree of accuracy and it is typically on of the first specifications laid out. Well, the subject is not quite that simple, and it requires a bit of discussion. First, we must establish the difference between accuracy, precision, and repeatability. In the case of temperature, accuracy is commonly defined as how closely the sensor indicates the true temperature being measured, or in a more practical sense, how closely the resistance of the RTD matches the tabulated or calculated resistance of that type RTD at that given temperature.

Precision, on the other hand, is not concerned with how well the RTDs resistance matched the resistance from a look up table, but rather with how well it matched the resistance of other RTDs subjected to that temperature. Precision generally refers to a group of sensors, and if the group has good precision at several temperatures, we can also say that they are well matched. This is important when interchangeability is a concern, as well as in the measurement of temperature gradients. Repeatability can best be described as the sensor's ability to reproduce its previous readings at a given temperature.

Here's an example. An ice point reading is done with an RTD that is then used to take reading at 100°C, 150°C, 37°C and again at 0°C. A comparison of the first and last ice point readings will give you an indication of the sensor's repeatability under those conditions. A note of caution, however: an RTD's repeatability is very application-dependent. So when you get right down to it, accuracy without repeatability is worthless. If you start with a sensor that is ±0.03°C at 0°C but is found to have repeatability only around ± 0. 5°C, what you have is a sensor whose readings are far less reliable than a standard-accuracy probe with good repeatability. A high-accuracy RTD installed in a field application also does not ensure that you will be getting a highly accurate signal back at the control room.

Most 4-20 mA transmitters and many display units and controllers have adjustable zero and span controls that if improperly adjusted will destory the high accuracy of the RTD signal. The best solution for applications of this type is to have noth the RTD and the transmitter, or display, calibrated as a unit by a certified calibration laboratory.

Dimensions and Size:

The physical dimensions and size requirements can be more complicated than you might think. On the low end, a resistance element to be used in the construction of a sheathed RTD generally requires only that the element is small enough to fit into the desired sheath ID. For cylindrical elements, such as wire-wound units, this is obvious-just don't forget to allow for the wall thickness of the sheath. For thin film-type elements, we must apply the Pythagorean theorem; we need to know the (W) width of the element, and the (T) thickness of the element at its largest point. Then the minimum ID of the sheath will be given by; ID > (w2 + t2).

When we begin to discuss RTD probes and assemblies, the subject becomes more demanding. We need to examine the mounting arrangement: will it be used for direct immersion or with a thermowell? Or will it be something special, like an exposed airflow probe or surface mount sensor? Probe designs are endless in their configurations, and it seems that most applications have some unique requirements that make this a rather creative field in itself.

In many applications, the probe is immersed in a small vessel or piping system. Dimensions here are generally limited to sensor diameter (which affects response time); immersion depth into the fluid; and the mounting arrangement, i.e., will the sensor be screwed into a threaded port, typically with ANSI tapered threads, or will it be used in con-junction with a fluid seal already in place? Or will some other special considerations need to be made? There may be other variables, such as pressure limitations or high flow, depending on the complexity of the application. It is always best to look at the whole picture. and then discuss it with your applications engineer.

Thermowells are generally used for larger vessels and systems so that the system will not have to be drained in the event the sensor requires calibration or changing. Assuming the thermowell has already been specified, we need only to specify the probe diameter (typically ¼ in. OD for a 0.260 in. bore well), the depth of the thermowell bore, and how the RTD will be secured into the well (typically spring-loaded through a ½ in. NPT nipple or hex-nipple).

Material Compatability:

Most people specifying RTD probes have to pay attention only to the chemical compatibility that will prevent corrosion. This is generally straightforward and guidelines can be taken from other materials used in the system in which the RTD will be installed. If the piping system is constructed of 316 S.S., then the probe probably should be also. But always check a corrosion guide for corrosion rates and material recommendations if you have the slightest doubt.

For applications involving thermowells, the thermowell will carry the burden of corrosion protection. However, be sure to protect the connecting wires and any terminals or plugs from possible corrosion caused by splash or corrosives in the atmosphere.

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PTC Thermistors

Positive Temperature Coefficient Thermistors:

Here below is a PDF file that explains in detail about PTC thermistors. It contains details about its characteristics, properties, materials, configuration, advantages, disadvantages and also its applications.
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Tuesday, December 20, 2011

NTC Thermistors

Negative Temperature Coefficient Thermistors :Characteristics, Materials, and Configurations

General Properties and Features:

NTC thermistors offer many desirable features for temperature measurement and control within their operating temperature range. Although the word thermistor is derived from THERMally sensitive resISTOR, the NTC thermistor can be more accurately classified as a ceramic semiconductor. The most prevalent types of thermistors are glass bead, disc, and chip configurations, and the following discussion focuses primarily on those technologies.

Temperature Ranges and Resistance Values. :
NTC thermistors exhibit a decrease in electrical resistance with increasing temperature. Depending on the materials and methods of fabrication, they are generally used in the temperature range of -50°C to 150°C, and up to 300°C for some glass-encapsulated units. The resistance value of a thermistor is typically referenced at 25°C (abbreviated as R25). For most applications, the R25 values are between 100  and 100 k . Other R25 values as low as 10  and as high as 40 M  can be produced, and resistance values at temperature points other than 25°C can be specified.

Accurate and Repeatable R/T Characteristic.:
The resistance vs. temperature (R/T) characteristic (also known as R/T curve) of the NTC thermistor forms the "scale" that allows its use as a temperature sensor. Although this characteristic is a nonlinear, negative exponential function, several interpolation equations are available that very accurately describe the R/T curve [1,2,3]. The most well known is the Steinhart-Hart equation: 1/T = A + B(lnR) + C(lnR)3
where: T = kelvin temperature R = resistance at temperature T

Coefficients A, B, and C are derived by calibrating at three temperature points and then solving the three simultaneous equations. The uncertainty associated with the use of the Steinhart-Hart equation is less than ±0.005°C for 50°C temperature spans within the 0°C-260°C range, so using the appropriate interpolation equation or lookup table in conjunction with a microprocessor can eliminate the potential nonlinearity problem.

Sensitivity to Changes in Temperature.:
  The NTC thermistor's relatively large change in resistance vs. temperature, typically on the order of -3%/°C to -6%/°C, provides an order of magnitude greater sensitivity or signal response than other temperature sensors such as thermocouples and RTDs. On the other hand, the less sensitive thermocouples and RTDs are a good choice for applications requiring temperature spans >260°C and/or operating temperatures beyond the limits for thermistors.
Figure 1. Over the range of -50°C to 150°C, NTC thermistors offer a distinct advantage in sensitivity to temperature changes compared to other temperature sensors. This graph illustrates the R/T characteristics of some typical NTC thermistors and a platinum RTD

Interchangeability. :
Another important feature of the NTC thermistor is the degree of interchangeability that can be offered at a relatively low cost, particularly for disc and chip devices. Interchangeability describes the degree of accuracy or tolerance to which a thermistor is specified and produced, and is normally expressed as a temperature tolerance over a temperature range. For example, disc and chip thermistors are commonly specified to tolerances of ±0.1°C and ±0.2°C over the temperature ranges of 0°C to 70°C and 0°C to 100°C. Interchangeability helps the systems manufacturer or thermistor user reduce labor costs by not having to calibrate each instrument/system with each thermistor during fabrication or while being used in the field. A health care professional, for instance, can use a thermistor temperature probe on one patient, discard it, and connect a new probe of the same specifications for use on another patient--without recalibration. The same holds true for other applications requiring reusable probes.

Small Size:
The small dimensions of most bead, disc, and chip thermistors used for resistance thermometry make for a very rapid response to temperature changes. This feature is particularly useful for temperature monitoring and control systems requiring quick feedback.

Remote Temperature Sensing Capability:
Thermistors are well suited for sensing temperature at remote locations via long, two-wire cable because the resistance of the long wires is insignificant compared to the relatively high resistance of the thermistor.

Ruggedness, Stability, and Reliability:
As a result of improvements in technology, NTC bead, disc, and chip thermistor configurations are typically more rugged and better able to handle mechanical and thermal shock and vibration than other temperature sensors.

Materials and Configurations:
Most NTC thermistors are made from various compositions of the metal oxides of manganese, nickel, cobalt, copper, and/or iron. A thermistor's R/T characteristic and R25 value are determined by the particular formulation of oxides. Over the past 10 years, better raw materials and advances in ceramics processing technology have contributed to overall improvements in the reliability, interchangeability, and cost-effectiveness of thermistors.

Of the thermistors shown in Figure 2, beads, discs, and chips are the most widely used for precise temperature measurements. Although each configuration is produced by a unique method, some general ceramics processing techniques apply to most thermistors: formulation and preparation of the metal oxide powders; milling and blending with a binder; forming into a "green" body; heat-treating to produce a ceramic material; addition of electrical contacts (for discs and chips); and, for discrete components, assembly into a usable device with wire leads and a protective coating.
Figure 2. A variety of manufacturing processes are used to make NTC thermistors configured as beads (A), chips (B), discs (C), rods (D), and washers (E).

Bead thermistors, which have lead wires that are embedded in the ceramic material, are made by combining the metal oxide powders with a suitable binder to form a slurry. A small amount of slurry is applied to a pair of platinum alloy wires held parallel in a fixture. Several beads can be spaced evenly along the wires, depending on wire length. After the beads have been dried, the strand is fired in a furnace at 1100°C-1400°C to initiate sintering. During sintering, the ceramic body becomes denser as the metal oxide particles bond together and shrink down around the platinum alloy leads to form an intimate physical and electrical bond. After sintering, the wires are cut to create individual devices. A glass coating is applied to provide strain relief to the lead-ceramic interface and to give the device a protective hermetic seal for long-term stability. Typical glass bead thermistors range from 0.01 in. to 0.06 in. (0.25 mm to 1.5 mm) in dia.

Disc thermistors are made by preparing the various metal oxide powders, blending them with a suitable binder, and then compressing small amounts of the mixture in a die under several tons of pressure. The discs are then fired at high temperatures to form solid ceramic bodies. A thick film electrode material, typically silver, is applied to the opposite sides of the disc to provide the contacts for the attachment of lead wires. A coating of epoxy, phenolic, or glass is applied to each device to provide protection from mechanical and environmental stresses. Typical uncoated disc sizes range from 0.05 in. to 0.10 in. (1.3 mm to 2.5 mm) in dia.; coated disc thermistors generally measure 0.10 in. to 0.15 in. (2.5 mm to 3.8 mm) in dia.

Chip thermistors are manufactured by tape casting, a more recent technique borrowed from the ceramic chip capacitor and ceramic substrate industries. An oxide-binder slurry similar to that used in making bead thermistors is poured into a fixture that allows a very tightly controlled thickness of material to be cast onto a belt or movable carrier. The cast material is allowed to dry into a flexible ceramic tape, which is cut into smaller sections and sintered at high temperatures into wafers 0.01 in. to 0.03 in. (0.25 mm to 0.80 mm) thick. After a thick film electrode material is applied, the wafers are diced into chips. The chips can be used as surface mount devices or made into discrete units by attaching leads and applying a protective coating of epoxy, phenolic, or glass. Typical chip sizes range from 0.04 in. by 0.04 in. (1 mm by 1 mm) to 0.10 in. by 0.10 in. (2.5 mm by 2.5 mm) in square or rectangular shapes. Coated chip thermistors commonly measure from 0.08 in. to 0.10 in. (2.0 mm to 2.5 mm) in diameter. Very small coated chip thermistors 0.02 in. to 0.06 in. (0.5 mm to 1.5 mm) in dia. are available for applications requiring small size, fast response, tight tolerance, and interchangeability.

Washer-shaped thermistors are essentially a variation of the disc type except for having a hole in the middle, and are usually leadless for use as surface mount devices or as part of an assembly. Rod-shaped thermistors are made by extruding a viscous oxide-binder mixture through a die, heat-treating it to form a ceramic material, applying electrodes, and attaching leads. Rod thermistors are used primarily for applications requiring very high resistance and/or high power dissipation.
NTC thermistors can be attached to extension leads or jacketed cable and assembled into various types of housings. The optimum materials, dimensions, and configuration for a probe assembly are determined by careful review of the application requirements.
Comparison of Thermistor Configurations:

One of the problems the thermistor industry has faced over the years is that some manufacturers have claimed their particular style or configuration of thermistor is better than other configurations made by their competitors, without regard to other, more pertinent factors. These thermistor "politics," more harmful than beneficial to the industry, can confuse engineers and purchasing agents who are looking for reliable information to help them choose the appropriate product for their application. Although some thermistor qualities or capabilities, including interchangeability, repeatability, size, responsiveness, and stability, can either be enhanced or limited by style or geometry, these characteristics are much more dependent on a manufacturer's ability to understand the ceramics technology being used and to maintain control of the manufacturing process.

Glass-coated beads feature excellent long-term stability and reliability for operation at temperatures up to 300°C. Studies at the National Institute of Standards and Technology (NIST) and other laboratories indicate that some special bead-in-glass probes have measurement uncertainties and stabilities (better than ± 0.003°C for temperatures between 0°C and 100°C) that approach those of some standard platinum resistance thermometers [3,4,5]. The relatively small size of glass bead thermistors gives them a quick response to temperature changes, but for some applications this small size can make the devices hard to handle during assembly and have the effect of limiting their power dissipation. It is also more difficult and more expensive to produce glass beads with close tolerances and interchangeability. Individual calibration and R/T characterization, resistor network padding, or use of matched pairs are among the methods used to achieve interchangeability.

Chip and disc thermistors
are noted for their tight tolerances and interchangeability at a relatively low cost compared to bead thermistors. These qualities are inherent in the manufacturing processes. The thermistors' larger size permits power dissipation higher than that of beads, although at some expense of response times. Larger size can be a disadvantage in some applications. Because of their geometry, disc thermistors normally have larger coated diameters and higher power dissipation capabilities than chip thermistors. On the other hand, chip thermistors typically can be produced to smaller coated diameters and are better suited for applications requiring smaller size and faster response times. More recent designs of chip thermistors allow the production of sizes and response times approaching those of glass beads. In some cases, chip and disc thermistors with equivalent physical and electrical characteristics can be used in the same applications without any noticeable difference in performance.

Thermistors, thermocouples, RTDs, and other sensors and electronic components exhibit a phenomenon called drift, a gradual, predictable change in certain properties over time. For a thermistor, drift results in a change in resistance from its initial value, typically after being continuously exposed to or cycled to an elevated temperature. Thermistor drift is expressed as a percent change in resistance and/or as a change in temperature that occurs at a given exposure temperature for a certain length of time. As the exposure temperature increases, so do the drift and the drift rate [4,5,6].

Chip and disc thermistors with soldered leads and an epoxy or phenolic coating have potential limitations in their maximum operating temperatures, typically 150° C for short-term exposures (1-24 hours) and 105°C for long-term exposures (1-12 months). When subjected to environmental conditions above their recommended maximum operating temperatures, epoxy- or phenolic-coated chips and discs can begin to exhibit an undesirable, excessive amount of drift. When such thermistors are used at temperatures below the specified maximum operating temperatures, drift is minimal, on the order of 0.02°C to 0.15°C after 12 months of continuous exposure to temperatures between 25°C and 100°C, respectively. Recent advances in the techniques used to manufacture chip and disc thermistors with a glass coating have produced devices that combine the interchangeability advantage of chips and discs with the stability of glass beads [5,6]. For applications that require operating temperatures up to 200°C, these new devices offer a lower cost alternative to the conventional glass bead thermistors.

These comparisons can help determine whether a thermistor supplier is objectively evaluating an application in terms of the appropriate thermistor, or simply promoting the configuration it manufactures. For an example of the latter approach, where a manufacturer of disc thermistors stated that "Loose-tolerance thermistors are usually mass-produced by tape casting," and that "These devices . . . are designed for applications requiring neither interchangeability nor a high degree of accuracy," implying that all chip thermistors are loose tolerance. On the contrary, millions of precision chip thermistors with superior long-term stability are produced annually to an interchangeable tolerance of ±0.1°C, and they are available with an interchangeability of ±0.05°C. In reality, broad-tolerance and tight-tolerance thermistors are available in each of the three major thermistor configurations discussed above.

After determining the appropriate specifications, the engineer and purchasing agent need to evaluate which configuration and supplier will best meet the requirements for process control, quality, on-time delivery, and value at a reasonable price. An important part of the evaluation process is to perform some basic tests on the design and quality of the thermistor and, wherever possible, include simulation of the actual environmental conditions of the intended application. To achieve optimum performance, thermistors are usually mounted into protective housings or probe assemblies.

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Thermistors : Introduction

Thermistors are thermally sensitive resistors and have, according to type, a negative (NTC), or positive (PTC) resistance/temperature coefficient. 

Thermistors differ from resistance temperature detectors  (RTD) in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature response is also different; RTDs are useful over larger temperature ranges, while thermistors typically achieve a higher precision within a limited temperature range [usually −90 °C to 130 °C].

Assuming, as a first-order approximation, that the relationship between resistance and temperature is linear, then:

ΔR = k ΔT
ΔR = change in resistance
ΔT = change in temperature
k = first-order temperature coefficient of resistance

Manufactured from the oxides of the transition metals - manganese, cobalt, copper and nickel, NTC thermistors are temperature dependant semiconductor resistors. Operating over a range of -200°C to + 1000°C, they are supplied in glass bead, disc, chips and probe formats. NTCs should be chosen when a continuous change of resistance is required over a wide temperature range. They offer mechanical, thermal and electrical stability, together with a high degree of sensitivity.

The excellent combination of price and performance has led to the extensive use of NTCs in applications such as temperature measurement and control, temperature compensation, surge suppression and fluid flow measurement.

PTC thermistors are temperature dependent resistors manufactured from barium titanate and should be chosen when a drastic change in resistance is required at a specific temperature or current level. PTCs can operate in the following modes:
•    Temperature sensing, switching at temperatures ranging from 60°C to 180°C, e.g. protection of windings in electric motors and transformers.
•    Solid state fuse to protect against excess current levels, ranging from several mA to several A (25°C ambient) and continuous voltages up to 600V and higher, e.g. power supplies for a wide range of electrical equipment.
•    Liquid level sensor.

Detailed explanation about NTC and PTC thermistors can be found in the upcoming post.

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Saturday, December 17, 2011

Open Channel Flowmeters

A common method of measuring flow through an open channel is to measure the height of the liquid as it passes over an obstruction as a flume or weir in the channel.

Common used is the Sharp-Crested Weir, the V-Notch Weir, the Cipolletti weir, the Rectangular-Notch Weir, the Parshall Flume or Venturi Flume.

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Coriolis Flowmeter

Direct mass measurement sets Coriolis flowmeters apart from other technologies. Mass measurement is not sensitive to changes in pressure, temperature, viscosity and density. With the ability to measure liquids, slurries and gases, Coriolis flowmeters are universal meters.

Coriolis Mass Flowmeter uses the Coriolis effect to measure the amount of mass moving through the element. The fluid to be measured runs through a U-shaped tube that is caused to vibrate in an angular harmonic oscillation. Due to the Coriolis forces, the tubes will deform and an additional vibration component will be added to the oscillation. This additional component causes a phase shift on some places of the tubes which can be measured with sensors.

The Coriolis flow meters are in general very accurate, better than +/-0,1% with an turndown rate more than 100:1. The Coriolis meter can also be used to measure the fluids density.

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Thermal Flowmeter

The thermal mass flowmeter operates independent of density, pressure, and viscosity. Thermal meters use a heated sensing element isolated from the fluid flow path where the flow stream conducts heat from the sensing element. The conducted heat is directly proportional to the mass flow rate and the he temperature difference is calculated to mass flow.

The accuracy of the thermal mass flow device depends on the calibrations reliability of the actual process and variations in the temperature, pressure, flow rate, heat capacity and viscosity of the fluid.

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Positive Displacement Flowmeter

The positive displacement flowmeter measures process fluid flow by precision-fitted rotors as flow measuring elements. Known and fixed volumes are displaced between the rotors. The rotation of the rotors are proportional to the volume of the fluid being displaced.

The number of rotations of the rotor is counted by an integral electronic pulse transmitter and converted to volume and flow rate.

The positive displacement rotor construction can be done in several ways:
•    Reciprocating piston meters are of single and multiple-piston types.
•    Oval-gear meters have two rotating, oval-shaped gears with synchronized, close fitting teeth. A fixed quantity of liquid passes through the meter for each revolution. Shaft rotation can be monitored to obtain specific flow rates.
•    Nutating disk meters have moveable disks mounted on a concentric sphere located in spherical side-walled chambers. The pressure of the liquid passing through the measuring chamber causes the disk to rock in a circulating path without rotating about its own axis. It is the only moving part in the measuring chamber.
•    Rotary vane meters consists of equally divided, rotating impellers, two or more compartments, inside the meter's housings. The impellers are in continuous contact with the casing. A fixed volume of liquid is swept to the meter's outlet from each compartment as the impeller rotates. The revolutions of the impeller are counted and registered in volumetric units.

The positive displacement flowmeter may be used for all relatively nonabrasive fluids such as heating oils, lubrication oils, polymer additives, animal and vegetable fat, printing ink, freon, and many more.
Accuracy may be up to 0.1% of full rate with a TurnDown of 70:1 or more.

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Ultrasonic Doppler Flowmeter

The effect of motion of a sound source and its effect on the frequency of the sound was observed and described by Christian Johann Doppler.

The frequency of the reflected signal is modified by the velocity and direction of the fluid flow.

If a fluid is moving towards a transducer, the frequency of the returning signal will increase. As fluid moves away from a transducer, the frequency of the returning signal decrease.

The frequency difference is equal to the reflected frequency minus the originating frequency and can be use to calculate the fluid flow speed.

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Electromagnetic Flowmeter

An electromagnetic flowmeter operate on Faraday's law of electromagnetic induction that states that a voltage will be induced when a conductor moves through a magnetic field. The liquid serves as the conductor and the magnetic field is created by energized coils outside the flow tube.

The voltage produced is directly proportional to the flow rate. Two electrodes mounted in the pipe wall detect the voltage which is measured by a secondary element.

Electromagnetic flowmeters can measure difficult and corrosive liquids and slurries, and they can measure flow in both directions with equal accuracy.

Electromagnetic flowmeters have a relatively high power consumption and can only be used for electrical conductive fluids as water.

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Vortex Flow Meter

An obstruction in a fluid flow creates vortices in a downstream flow. Every obstruction has a critical fluid flow speed at which vortex shedding occurs. Vortex shedding is the instance where alternating low pressure zones are generated in the downstream.

These alternating low pressure zones cause the obstruction to move towards the low pressure zone. With sensors gauging the vortices the strength of the flow can be measured.

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Turbine Flowmeter

There is many different manufacturing design of turbine flow meters, but in general they are all based on the same simple principle:

If a fluid moves through a pipe and acts on the vanes of a turbine, the turbine will start to spin and rotate. The rate of spin is measured to calculate the flow.

The turndown ratios may be more than 100:1 if the turbine meter is calibrated for a single fluid and used at constant conditions. Accuracy may be better than +/-0,1%.

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Calorimetric Flowmeter

The calorimetric principle for fluid flow measurement is based on two temperature sensors in close contact with the fluid but thermal insulated from each other.

One of the two sensors is constantly heated and the cooling effect of the flowing fluid is used to monitor the flowrate. In a stationary (no flow) fluid condition there is a constant temperature difference between the two temperature sensors. When the fluid flow increases, heat energy is drawn from the heated sensor and the temperature difference between the sensors are reduced. The reduction is proportional to the flow rate of the fluid.

Response times will vary due the thermal conductivity of the fluid. In general lower thermal conductivity require higher velocity for proper measurement.
The calorimetric flowmeter can achieve relatively high accuracy at low flow rates.

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Basics of Instrumentation & Control

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About This Blog

Hai friends…welcome to my blog. This blog is exclusively for instrumentation engineering students which will provide sources for their reference and studies. As you all know Instrumentation is now a fast emerging and developing field in Engineering. This blog has different categories like PLC, SCADA, DCS, Sensors and Transducers, Computer control of process, Industrial Instrumentation, etc.

This blog will also provide an exclusive ‘ASK ME’ section where you can make any queries and share your ideas about instrumentation. The solution for your queries will be given to you by mail from best of my knowledge and reference.

So I wish this blog may be very useful for your studies and reference.

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