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Wednesday, February 2, 2011

Thermocouples








               One of the most common industrial thermometer is the thermocouple. It was discovered by Thomas Seebeck's in 1822. He noted that a voltage difference appeared when the wire was heated at one end. Regardless of temperature, if both ends were at the same temperature there was no voltage difference. If the circuit were made with wire of the same material there was no current flow.


A thermocouple consists of two dissimilar metals, joined together at one end, and produce a small unique voltage at a given temperature. This voltage is measured and interpreted by a thermocouple thermometer.


The thermoelectric voltage resulting from the temperature difference from one end of the wire to the other is actually the sum of all the voltage differences along the wire from end to end


Thermocouples can be made from a variety of metals and cover a temperature range 200 oC to 2,600 oC. Comparing thermocouples to other types of sensors should be made in terms of the tolerance given in ASTM E 230.



Base metal thermocouples


Thermocouple
Maximum Temperature (oC)
Continuous
Spot
Copper-Constantan
400
500
Iron-Constantan
850
1,100
Chromel-Constantan
700
1,000
Chromel-Alumel
1,100
1,300
Nicrosil-Nisil
1,250
-
Tungsten-Molybdenum*
2,600
2,650

* Not used below 1250 oC.





Principle of Working of Thermocouple

The working principle of thermocouple is based on three effects, discovered by Seebeck, Peltier and Thomson. All these have been described in brief below.


1) Seebeck effect: The Seebeck effect states that when two different or unlike metals are joined together at two junctions, an electromotive force (emf) is generated at the two junctions. The amount of emf generated is different for different combinations of the metals.
2) Peltier effect: As per the Peltier effect when two dissimilar metals are joined together to form two junctions, the emf is generated within the circuit due to different temperatures of the two junctions of the circuit.
3) Thomson effect: As per Thomson effect, when two unlike metals are joined together forming two junctions, the potential exists within the circuit due to temperature gradient along the entire length of the conductors within the circuit.


In most of the cases the emf suggested by Thomson effect is very small and it can be neglected by making proper selection of the metals. The Peltier effect play prominent role in the working principle of the thermocouple.

How Thermocouple Works?

The general circuit for the working of thermocouple is shown in the figure 1 above. It comprises to two dissimilar metals A and B. These are joined together to form two junctions, p and q, which are maintained at the temperatures T1 and T2 respectively. Remember that the thermocouple cannot be formed if there are no two junctions. Since the two junctions are maintained at different temperatures the Peltier emf is generated within the circuit and it is the function of the temperatures of two junctions.

If the temperature of both the junctions is same, equal and opposite emf will be generated at both junctions and the net current flowing through the junction is zero. If the junctions are maintained at different temperature, the emf’s will not become zero and there will be net current flowing through the circuit. The total emf flowing through this circuit depends on the metals used within the circuit as well the temperature of the two junctions. The total emf or the current flowing through the circuit can be measured easily by the suitable devise.

For measurement of the temperature of the body, one junction of the thermocouple is connected to the body whose temperature is to be measured. This junction is called as hot junction or the measuring junction. The other junction is connected to the body whose temperature is known. This junction is called as cold or reference junction.

Within the circuit of the thermocouple the devise for measuring the current or emf flowing the circuit is connected. It measures the amount of emf flowing through the circuit due to the two junctions of the two dissimilar metals maintained at different temperatures. In the figure 2 above the two junctions of the thermocouple and the devise used for measurement of emf (potentiometer) are shown.

Now, the temperature of the reference junctions is already known, while the temperature of measuring junction is unknown. The output obtained from the thermocouple circuit is calibrated directly against the unknown temperature. Thus the voltage or current output obtained from thermocouple circuit gives the value of unknown temperature directly.

Devices Used for Measuring emf within the Thermocouple Circuit

The amount of emf developed within the thermocouple circuit is very small, which is usually in millivolts, hence some highly sensitive instruments should be used for measuring the emf generated in the thermocouple circuit. The two devices used commonly for measuring emf within the thermocouple circuit are ordinary galvanometer and voltage balancing potentiometer. The manually or automatically balancing potentiometer is used more commonly.

The figure 2 above shows potentiometer connected in the thermocouple circuit. The junction p is connected to the body whose temperature is to be measured. The junction q is the reference junction, whose temperature can be measured by the thermometer. In some cases the reference junctions can also be maintained at the ice temperature by connecting it to the ice bath (see figure 3). This devise can be calibrated in terms of the input temperature so that its scale can give the value directly in terms of temperature.


Advantages with thermocouples

  • Capable of being used to directly measure temperatures up to 2600 oC.
  • The thermocouple junction may be grounded and brought into direct contact with the material being measured.

Disadvantages with thermocouples

  • Temperature measurement with a thermocouple requires two temperatures be measured, the junction at the work end (the hot junction) and the junction where wires meet the instrumentation copper wires (cold junction). To avoid error the cold junction temperature is in general compensated in the electronic instruments by measuring the temperature at the terminal block using with a semiconductor, thermistor, or RTD.
  • Thermocouples operation are relatively complex with potential sources of error. The materials of which thermocouple wires are made are not inert and the thermoelectric voltage developed along the length of the thermocouple wire may be influenced by corrosion etc.
  • The relationship between the process temperature and the thermocouple signal (millivolt) is not linear.
  • The calibration of the thermocouple should be carried out while it is in use by comparing it to a nearby comparison thermocouple. If the thermocouple is removed and placed in a calibration bath, the output integrated over the length is not reproduced exactly.

Thermocouple Types

Thermocouples are available in different combinations of metals or calibrations. The four most common calibrations are J, K, T and E. Each calibration has a different temperature range and environment, although the maximum temperature varies with the diameter of the wire used in the thermocouple.


Some of the thermocouple types have standardized with calibration tables, color codes and assigned letter-designations. The ASTM Standard E230 provides all the specifications for most of the common industrial grades, including letter designation, color codes (USA only), suggested use limits and the complete voltage versus temperature tables for cold junctions maintained at 32 oF and 0 oC.


There are four "classes" of thermocouples:
  • The home body class (called base metal),
  • the upper crust class (called rare metal or precious metal),
  • the rarified class (refractory metals) and,
  • the exotic class (standards and developmental devices).
The home bodies are the Types E, J, K, N and T. The upper crust are types B, S, and R, platinum all to varying percentages. The exotic class includes several tungsten alloy thermocouples usually designated as Type W (something).



Instrument
Temperature
Range
Accuracy
Recommended (oF)
Maximum (oF)
Type J probes
32 to 1336
-310 to 1832
1.8 to 7.9oF or 0.4% of reading above 32oF, whichever is greater
Type K probes
32 to 2300
-418 to 2507
1.8 to 7.9oF or 0.4% of reading above 32oF, whichever is greater
Type T probes
-299 to 700
-418 to752
0.9 to 3.6oF or 0.4% of reading above 32oF, whichever is greater
Type E probes
32 to 1600
32 to 1650
1.8 to 7.9oF or 0.4% of reading above 32oF, whichever is greater
Type R probes
32 to 2700
32 to 3210
2.5oF or 0.25% of reading, whichever is greater
Type S probes
32 to 2700
32 to 3210
2.5oF or 0.25% of reading, whichever is greater

Aricle Source: www.engineeringtoolbox.com

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LVDT Basics






What Is An LVDT?

The letters LVDT are an acronym for Linear Variable Differential Transformer, a common type of electromechanical transducer that can convert the rectilinear motion of an object to which it is coupled mechanically into a corresponding electrical signal. LVDT linear position sensors are readily available that can measure movements as small as a few millionths of an inch up to several inches, but are also capable of measuring positions up to ±20 inches (±0.5 m).
Figure 1 shows the components of a typical LVDT. The transformer's internal structure consists of a primary winding centered between a pair of identically wound secondary windings, symmetrically spaced about the primary. The coils are wound on a one-piece hollow form of thermally stable glass reinforced polymer, encapsulated against moisture, wrapped in a high permeability magnetic shield, and then secured in a cylindrical stainless steel housing. This coil assembly is usually the stationary element of the position sensor.
The moving element of an LVDT is a separate tubular armature of magnetically permeable material called the core, which is free to move axially within the coil's hollow bore, and mechanically coupled to the object whose position is being measured. This bore is typically large enough to provide substantial radial clearance between the core and bore, with no physical contact between it and the coil.
In operation, the LVDT's primary winding is energized by alternating current of appropriate amplitude and frequency, known as the primary excitation. The LVDT's electrical output signal is the differential AC voltage between the two secondary windings, which varies with the axial position of the core within the LVDT coil. Usually this AC output voltage is converted by suitable electronic circuitry to high level DC voltage or current that is more convenient to use.

How Does An LVDT Work?

Figure 2 illustrates what happens when the LVDT's core is in different axial positions. The LVDT's primary winding, P, is energized by a constant amplitude AC source. The magnetic flux thus developed is coupled by the core to the adjacent secondary windings, S1 and S2. If the core is located midway between S1 and S2, equal flux is coupled to each secondary so the voltages, E1 and E2, induced in windings S1 and S2 respectively, are equal. At this reference midway core position, known as the null point, the differential voltage output, (E1 - E2), is essentially zero.
Figure 2
As shown in Figure 2, if the core is moved closer to S1 than to S2, more flux is coupled to S1 and less to S2, so the induced voltage E1 is increased while E2 is decreased, resulting in the differential voltage (E1 - E2). Conversely, if the core is moved closer to S2, more flux is coupled to S2 and less to S1, so E2 is increased as E1 is decreased, resulting in the differential voltage (E2 - E1).
Figure 3A shows how the magnitude of the differential output voltage, EOUT, varies with core position. The value of EOUT at maximum core displacement from null depends upon the amplitude of the primary excitation voltage and the sensitivity factor of the particular LVDT, but is typically several volts RMS. The phase angle of this AC output voltage, EOUT, referenced to the primary excitation voltage, stays constant until the center of the core passes the null point, where the phase angle changes abruptly by 180 degrees, as shown graphically in Figure 3B.
This 180 degree phase shift can be used to determine the direction of the core from the null point by means of appropriate circuitry. This is shown in Figure 3C, where the polarity of the output signal represents the core's positional relationship to the null point. The figure shows also that the output of an LVDT is very linear over its specified range of core motion, but that the sensor can be used over an extended range with some reduction in output linearity.
Figure 3
The output characteristics of an LVDT vary with different positions of the core. Full range output is a large signal, typically a volt or more, and often requires no amplification. Note that an LVDT continues to operate beyond 100% of full range, but with degraded linearity.

LVDT Support Electronics
Although an LVDT is an electrical transformer, it requires AC power of an amplitude and frequency quite different from ordinary power lines to operate properly (typically 3 Vrms at 3 kHz). Supplying this excitation power for an LVDT is one of several functions of LVDT support electronics, which is also sometimes known as LVDT signal conditioning equipment.
Other functions include converting the LVDT's low level AC voltage output into high level DC signals that are more convenient to use, decoding directional information from the 180 degree output phase shift as an LVDT's core moves through the null point, and providing an electrically adjustable output zero level.
A variety of LVDT signal conditioning electronics is available, including chip-level and board-level products for OEM applications as well as modules and complete laboratory instruments for users.
Figure 4
The cross sectional view of the DC-LVDT at left shows the builtin signal conditioning electronics module. The module is secured with a potting compound that is not shown in this drawing.
The support electronics can also be self-contained, as in the DC-LVDT shown in Figure 4. These easy-to-use position transducers offer practically all of the LVDT's benefits with the simplicity of DC-in, DC-out operation. Of course, LVDTs with integral electronics may not be suitable for some applications, or might not be packaged appropriately for some installation environments.

Why Use An LVDT?

LVDTs have certain significant features and benefits, most of which derive from its fundamental physical principles of operation or from the materials and techniques used in its construction.

Friction-Free Operation

One of the most important features of an LVDT is its friction-free operation. In normal use, there is no mechanical contact between the LVDT's core and coil assembly, so there is no rubbing, dragging or other source of friction. This feature is particularly useful in materials testing, vibration displacement measurements, and high resolution dimensional gaging systems.

Infinite Resolution

Since an LVDT operates on electromagnetic coupling principles in a friction-free structure, it can measure infinitesimally small changes in core position. This infinite resolution capability is limited only by the noise in an LVDT signal conditioner and the output display's resolution. These same factors also give an LVDT its outstanding repeatability.

Unlimited Mechanical Life

Because there is normally no contact between the LVDT's core and coil structure, no parts can rub together or wear out. This means that an LVDT features unlimited mechanical life. This factor is especially important in high reliability applications such as aircraft, satellites and space vehicles, and nuclear installations. It is also highly desirable in many industrial process control and factory automation systems.

Overtravel Damage Resistant

The internal bore of most LVDTs is open at both ends. In the event of unanticipated overtravel, the core is able to pass completely through the sensor coil assembly without causing damage. This invulnerability to position input overload makes an LVDT the ideal sensor for applications like extensometers that are attached to tensile test samples in destructive materials testing apparatus.

Single Axis Sensitivity

An LVDT responds to motion of the core along the coil's axis, but is generally insensitive to cross-axis motion of the core or to its radial position. Thus, an LVDT can usually function without adverse effect in applications involving misaligned or floating moving members, and in cases where the core doesn't travel in a precisely straight line.

Separable Coil And Core

Because the only interaction between an LVDT's core and coil is magnetic coupling, the coil assembly can be isolated from the core by inserting a non-magnetic tube between the core and the bore. By doing so, a pressurized fluid can be contained within the tube, in which the core is free to move, while the coil assembly is unpressurized. This feature is often utilized in LVDTs used for spool position feedback in hydraulic proportional and/or servo valves.

Environmentally Robust

The materials and construction techniques used in assembling an LVDT result in a rugged, durable sensor that is robust to a variety of environmental conditions. Bonding of the windings is followed by epoxy encapsulation into the case, resulting in superior moisture and humidity resistance, as well as the capability to take substantial shock loads and high vibration levels in all axes. And the internal high-permeability magnetic shield minimizes the effects of external AC fields.
Both the case and core are made of corrosion resistant metals, with the case also acting as a supplemental magnetic shield. And for those applications where the sensor must withstand exposure to flammable or corrosive vapors and liquids, or operate in pressurized fluid, the case and coil assembly can be hermetically sealed using a variety of welding processes.
Ordinary LVDTs can operate over a very wide temperature range, but, if required, they can be produced to operate down to cryogenic temperatures, or, using special materials, operate at the elevated temperatures and radiation levels found in many nuclear reactors.

Null Point Repeatibility

The location of an LVDT's intrinsic null point is extremely stable and repeatable, even over its very wide operating temperature range. This makes an LVDT perform well as a null position sensor in closed-loop control systems and highperformance servo balance instruments.

Fast Dynamic Response

The absence of friction during ordinary operation permits an LVDT to respond very fast to changes in core position. The dynamic response of an LVDT sensor itself is limited only by the inertial effects of the core's slight mass. More often, the response of an LVDT sensing system is determined by characteristics of the signal conditioner.

Absolute Output

An LVDT is an absolute output device, as opposed to an incremental output device. This means that in the event of loss of power, the position data being sent from the LVDT will not be lost. When the measuring system is restarted, the LVDT's output value will be the same as it was before the power failure occurred.

Article Source : www.macrosensors.com

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