Thermocouple is a pair of dissimilar metal wires joined at one end, which generate a net thermoelectric voltage between open pair according to the temperature difference between the ends.
Resistance thermometer use metals that alter their electric resistance when heated. Platinum is most commonly used material for industrial RTD. However copper and Nickel are also used for some applications. The resistance at 0˚C is called R˳ and it is important parameter to be defined. The most commonly used RTD element is of Platinum with resistance of 100Ω at 0˚C. RTD has a positive temperature coefficient. Normally industrial RTD are used up to temperature range of 400˚C.
Thermowell provide protection for temperature probes against unfavorable operating conditions such as corrosive media, physical impact (e.g. clinker in furnaces) and high pressure gas or liquid. Their use also permits quick and easy probe interchanging without the need to “open-up” the process.
It is closed-end reentrant tube designed for insertion of a temperature-sensing element, and provided with means for a pressure-tight attachment to a vessel.
Thermowells are typically constructed of solid drilled-out bar stock and are designed to protect a temperature sensors from flow, high pressure and harsh environment.
A Pyrometer is a non-contacting device that intercepts and measures thermal radiations. Without making any contact with the radiating body and the process is known as pyrometry. This device is useful for determining the temperature of an object’s surface.
Pyrometer strictly works on the principle of black body radiation. Here emissivity of the target plays an important role, as it governs how bright the target appears to the pyrometer. Due to its high accuracy, speed, economy and specific advantages, it is widely being used as a standard procedure in many industrial applications.
Industrial Electric Heaters convert electric energy to heat based on Ohm’s Law. Heat generated is transferred to the load via conduction, convection or radiation. Based on direct resistive heating, this technology is the most beneficial, hence preferred over fuel based heating
Compensating cable is made of alloys which are different from those of thermocouples but have the same output over a limited temperature range. Compensating cable is a connector between thermocouple and measuring instruments, these cables are less precise, but cheaper. They harness quite different relatively low cost alloy conductor materials, whose net thermocouple in question. The combination develops similar output as those of the thermocouple, but the operating temperature range has to be restricted to keep miss-match error acceptably small.
MI cable is a sheathed Thermocouple Cable, having an outer sheath of metal with Two to Eight Cores where positive and negative thermo elements run around Circular Pattern, embedded in MgO. Mineral Insulated Cables are suitable to high Mechanical, Chemical, and Electrical stability. Due to good Flexibility, Excellent mechanical strength, and pressure resistance, mineral insulated Thermocouples/RTD’s can be installed in complex installations.
Industrial furnaces provide continuous heating to process materials. They are generally built from high temperature (refractory) materials so that they can maintain high temperatures without breaking down. Often, furnaces are set to function for months at a time to complete a processing set.
Calibration is the comparison of a measuring instrument against an accurate standard to determine any deviation. Calibration equipment is an accurate standard device which is used to check accuracy of the test equipment (Temperature Sensors like thermocouple, RTDs, Pyrometers). Calibration of equipment is done to control errors so that the equipment(Sensor) gives reliable and acceptable measurement.
Temperature Uniformity Surveys commonly referred to as a TUS, establish objective evidence of the heat distribution characteristics throughout an oven or furnace system. It also identifies the qualified work zone within the oven or furnace system. TUS's can also be an effective way of determining the efficiency of the furnace system and to monitor furnace performance over time.
System Accuracy Test, commonly referred to as an SAT, is another critically important and often required test. System Accuracy Tests are performed to determine the error within the temperature control system (temperature control device/lead wire/thermocouple). Regardless of the application, knowing the error that exists in your temperature control system allows for more consistent control of quality.
The purpose of the uniformity survey is to determine the range of temperatures present at different locations in the furnace under normal operating conditions standards. It will cater the needs for uniform heating inside the furnace to match the product quality as well as process parameters.
This method is used when the body (whose temperature is to be measured) and the sensor(which ismeasuring the temperature) can remain in contact with each other, in other words, we can say that if thebody and the sensor can remain in contact with each other during the measurement of temperature thancontact method is used.
This method is used when the body (whose temperature is to be measured) and the sensor (which is measuring the temperature) are not allowed to remain in contact with each other, in other words, we can say that if the body and the sensor are not allowed to remain in contact with each other during the measurement of temperature then non-contact method is used.
A practical industrial or laboratory Thermocouple consists of only a single (measuring) junction; the reference is always the terminal temperature.
Possible measures are:
Three alternative tip configurations are usually offered:
To form the hot junction, a suitable method has to be adopted to obtain a good electrical contact between the thermocouple wires.
For Chromal/Alumal and other combinations, for use in high temperature measurements, welding is the only method to obtain a suitable joint. For this purpose Tig welding & Laser beam welding is mostly used.
Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is an arc welding process that uses a non-consumable tungsten electrode to produce the weld. The weld area is protected from atmospheric contamination by a shielding gas.
Laser beam welding (LBW) is a welding technique used to join multiple pieces of metal through the use of a laser. The beam provides a concentrated heat source, allowing for narrow, deep welds and high welding rates. LBW is a versatile process, capable of welding carbon steels, HSLA steels, stainless steel, aluminum and titanium. The speed of welding is proportional to the amount of power supplied but also depends on the type and thickness of the workpieces.
Many combinations of materials have been used to produce acceptable thermocouples, each with its own particular application spectrum. However, very few specific types are now easily available, and covering by far the majority of the temperature and environmental applications.
The standard covers the eight specified and most commonly used thermocouples. These thermocouple types can be subdivided in 3 groups, base metal, Noble (rare) metal and Refractory metal thermocouple.
Base metal thermocouple types are composed of common, inexpensive metals such as nickel, iron and copper. The thermocouple types E, J, K, N and T are among this group and are the most commonly used type of thermocouple.
Noble metal thermocouples are manufactured with wire that is made with precious or “noble” metals like Platinum and Rhodium. The main types are R, S and B.
Refractory metal thermocouples are manufactured with the exotic metals tungsten and Rhenium. These metals are expensive, difficult to manufacture and wire made with these metals are very brittle.
|Thermocouple Type||Material + & -||Temperature Range (°C)||Application|
|E||Chromel & Constantan (Ni-Cr & Cu-Ni)||-200 to 900° C||Inert media, Oxidizing media|
|J||Iron & Constantan (Fe & Cu-Ni)||0 to 750°C||Inert media, Oxidizing media, Reducing media Vacuum|
|K||Chromel & Alumel (Ni-Cr & Ni-AI)||-200 to 1250°C||Inert media, Oxidizing media|
|N||Nicrosil & Nisil (Ni-Cr & Ni-Si)||-270 to 1300°C||Inert media, Oxidizing media|
|T||Copper & Constantan (Cu & Cu-Ni)||-200 to 350°C||Inert media, Oxidizing media, Reducing media Vacuum|
|R||87% Platinum/13% Rhodium &
(Pt & Pt-Rh)
|0 to 1450°C||Inert media, Oxidizing media.|
|S||90% Platinum/10% Rhodium &
(Pt & Pt-Rh)
|0 to 1450°C||Inert media, Oxidizing media,|
|B||70% Platinum/ 30% Rhodium & 94%
(Pt-Rh & Pt-Rh)
|0 to 1700°C||Inert media, Oxidizing media.|
|C||95% Tungsten/ 5% Rhenium & 74% Tungsten/ 26% Rhenium||0 to 2320°C||Vacuum inert and reducing|
|G||Tungsten & 74% Tungsten/ 26% Rhenium||0 to 2320°C||Vacuum inert and reducing|
|D||97% Tungsten 3% Rhenium & 75% Tungsten/ 25% Rhenium||0 to 2320°C||Vacuum inert and reducing|
Noble metal thermocouples are manufactured with wire that is made with precious or “noble” metals like Platinum and Rhodium. Noble metal thermocouples can used in oxidizing or inert applications and must be used with a ceramic protection tube surrounding the thermocouple element. These sensors are usually fragile and must not be used in applications that are reducing or in applications that contain metallic vapors.
Refractory metal thermocouples are manufactured with wire that is made from the exotic metals tungstenand Rhenium. These metals are expensive, difficult to manufacture and wire made with these metals arevery brittle. These thermocouples are intended to be used in vacuum furnaces at extremely hightemperatures and must never be used in the presence of oxygen at temperatures above 300°C. Thereare several differentcombinations of alloys that have been used in the past but only one generally used atthis time.
Thermocouples are suitable for measuring over a large temperature range, up to 2300 °C. They are less suitable for applications where smaller temperature differences need to be measured with high accuracy, for example the range 0–100 °C with 0.1 °C accuracy. For such applications thermistors and resistance temperature detectors are more suitable. Applications include temperature measurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes. Some other applications are as follows:
Resistance thermometers offer the greatest benefits relative to other thermometer types in these situations:
The major application areas of RTDs are:
Thermowells provide protection for temperature probes against unfavorable operating conditions such as corrosive media, physical impact (e.g. clinker in furnaces) and high pressure gas or liquid. Their use also permits quick and easy probe interchanging without the need to “open-up” the process. The main application areas are:
One-piece Thermowells are suited to the highest process loads, depending on their design. Thus in the petrochemical industry, one-piece thermocouples are now used almost exclusively.
Calibration equipment is used in calibrating different Temperature units like: RTD, Thermocouple, pyrometer etc. The major applications are in:
MI Cables cover up the wide area of applications. Important are listed below:
The various applications of industrial heaters are:
Pyrometers have wide range of diverse applications in various industries:
There are numerous applications of industrial furnaces viz.
There are two types of thermocouple construction used most commonly.These are Mineral Insulated (M.I.) Thermocouples & Non M.I. Thermocouples.
Magnesium Oxide insulated thermocouples, are used in many process and laboratory applications. They are rugged in nature and bendable, and their fairly high temperature ratings make MgO thermocouples a popular choice for a multitude of temperature measuring applications.
MgO sensors are constructed by placing an element or elements into a sheath of a suitable material and size, insulating the elements from themselves and the sheath with loose filled or crushable Magnesium Oxide powder or insulators, and then swaging or drawing the filled sheath down to its final reduced size. The swaging process produces an element with highly compacted MgO insulation and provides high dielectric strength insulation between the elements themselves and their sheath.
Mineral insulated Thermocouples consist of thermocouple wire embedded in a densely packed refractory oxide powder insulate all enclosed in a seamless, drawn metal sheath (usually stainless steel).
At one end cores and sheath are welded from a “hot” junction. At the other end, the thermocouple is connected to a “transition” of extension wires, connecting head or connector.
In Non-M.I. thermocouples, thermocouple wires are either insulated with ceramic beads or after insulation of ceramic, covered by a metal sheath (usually stainless steel) and some form of termination (extension lead, connecting head or connector for example) is provided. In this type of construction thermocouple wires are protected from the measuring environment when a sheath protection is provided. The sheath material is dependent on the measuring environment usually stainless steel is used. According to the corrosive environment sheath selection is changed.
This construction does not provide flexibility & not found in small sizes. Not too good mechanical strength.
In Non M.I. construction sheath may be of ceramic or metal as per suitability.
Exposed, Grounded and Ungrounded all types of junctions are formed in both the M.I, & Non M.I. construction.
The mineral oxides used for insulation are highly hygroscopic and open-ended cables must be effectively sealed (usually with epoxy resins) to prevent moisture take-up. A carefully prepared mineral insulated thermocouple will normally have a high value of insulation resistance (many hundreds of mega ohms).
The junction tip of Mineral insulated thermocouple can be of three types as described previously. The tip can be insulated, grounded and reduced type.
Tolerance denotes the maximum allowable value obtained by subtracting the temperature reading or the temperature at the hot junction from the standard temperature converted from the applicable temperature EMF table.
Operating temperature limit means the upper temperature where thermocouple can be used continuously in air. Maximum limit means the upper temperature where thermocouple can be used temporarily for short period of time owing to unavoidable circumstances.
Principal factors that affect the life of a thermocouple are:
The response time for a thermocouple is usually defined as the time taken for the thermal voltage (output) to reach 63% of maximum for the step change temperature. It is dependent on several parametersincluding the thermocouple dimension, construction, tip configuration and the nature of the medium inwhich the sensor is located.
Thermocouple assemblies are'' tip'' sensing devices which lend them to both surface and immersion applications depending on their construction. However immersion type must be used carefully to avoid error due to stema conduction from the process which can result in a high or low reading respectively. A general rule is to immerse into the medium to a minimum of 4 times the outside diameter of them sheath; no quantitative data applies but care must be exercised in order to obtain meaningful results.
Although thermocouple assemblies are primarily tip sensing devices, the use of protection tubes renders surface sensing impractical. Physically, the probe does not lend itself to surface presentation and steam conduction would cause reading errors. If thermocouple is to be used reliably for surface sensing, it must be either exposed, welded junction from with very small thermal mass or be housed in a construction, which permits true surface contact when attaching to the surface.
Thermocouples are having significant role in Steel industries. Area wise, various types of thermocouples are used in different units of steel industry:
Stove dome/hot blast stove is one of the most critical sections in the steel industry. The hot blast has 3 stage operations, either on gas, on blast or bottled up (ready and waiting to be put on blast).
Temperature is one of the major parameter for controlling this criticalprocess.The temperature inside the blast furnace is about 1100°C and high pressure. The high temperature and high pressure makes the assemblyvery critical.
Coke is the most important raw material fed into the blast furnace in terms of its effect on blast furnace operation and hot metal quality. The coke making process involves carbonization of coal to high temperatures (1100°C) in an oxygen deficient atmosphere in order to concentrate the carbon.
Refractory thermocouples are very important for the startup phase and running phase of the refractory in the steel plants. These are normally very long length thermocouples (up to 40 meters) in mineral insulated construction. Thermocouples of various lengths can be grouped together in a single flange, for a particulararea of the furnace.
Fast response thermocouple normally K type with miniature or Standard connector for DRI kiln application and normally used with a hand held indicator for checking immediate temperature.
Thermocouple type J, K, T, E, N, R and S are widely used in different units of cement plant:
In pharmaceutical industries, thermocouples are used for Validation and Process. Various areas of applications are:
At 0˚ C, A Platinum RTD has a resistance of 100 Ω & a temperature co-efficient of about 0.00385 Ω/ Ω / ˚C. These non-linearties are described in Callender-Van Duesenequation. This equation consists of both a linear portion & a non- linear portion.
Range –200 to 0°C: R (t) [Ω] = R (1 + At + Bt² + C (t – 100°C) t³)
Range 0 to 850°C: R̥ (t) [Ω] = R (1 + At + Bt²)
With: R0 is resistance at 0 °C
A = 3, 9083 x 10ˉ³ °Cˉ¹
B = -5,775 x 10ˉ⁷ °Cˉ²
C = -4,183 x 10ˉ¹² °Cˉ⁴
The criterion for selecting a material to make an RTD is:
Some of the common RTD materials are platinum, copper, nickel, Balco (an alloy of 70% nickel and 30 % iron). These metals have the advantage that they can be manufactured to a very high degree of purity and are, consequently, available with highly reproducible temperature/resistance characteristics. These metals can also be drawn to a fine diameter wires required in resistance thermometry.
|Element Material||Temperature Range (°C)||Resistance
|Relative Cost||Linearity Deviation (0-100°C)|
|Platinum||-200 to 850||1.3925 to 1.385||11||1100||+0.12|
|Copper||-200 to 260||1.427||1.72||1||0|
|Nickel||-80 to 300||1.672||7.8||20||-1.61|
|Balco||-200 to 230||1.518||20||15||-1.17|
As shown in table, although copper is cheapest, it also has the lowest resistivity and therefore requires inconveniently large sensing elements. On the other hand, even as nickel and nickel alloy have high resistivity, their resistance versus temperature coefficients are non-linear. They are also sensitive to strain and their resistivity suffer from an inflexion around the Curie point (358ºC) that makes the deviation of their resistance/ temperature expressions more complicated.
This platinum which not only has a high resistivity (more than six times that of copper) but also a high degree of stability and a wide temperature range. Although platinum is expensive it can be drawn into fine wires or strips and we only require small amounts for manufacturing RTDs. As a noble metal, it has minimum susceptibility to contamination.
The presence of impurities is undesirable since diffusion, segregation and evaporation may occur in service, resulting in a lack of stability. The resistivity is also sensitive to internal strains. Thus, it is essential that the platinum should remain in a fully annealed condition i.e. it should be annealed at a temperature higher than the maximum temperature of service.
The maximum temperature rating for RTD's is based on two different factors. First is the element material. Platinum RTD's can be used as high as 650°C (1202°F). Other materials are much lower in temperature rating and vary from material to material. The other determining factor for temperature rating is probe construction. There are construction considerations used in each of these different styles making them ideal for use in each of those ranges. Noone style is good for all ranges.
Platinum RTDs typically are provided in two classes; Class A and Class B.
Class A is considered high accuracy and has an ice point tolerance of ±0.06 ohms.
Class B is standard accuracy and has an ice point tolerance of ±0.12 ohms. Class B is widely used by most industries.
The accuracy will decrease with temperature. Class A will have an accuracy of ±0.43 ohms (± 1.45°C) at 600°C and class B will be ± 1.06 ohms (± 3.3°C) at 600°C. The chart below shows the tolerance versus temperature (IEC 751).
Other accuracy classes like 1/3, 1/5, 1/10 of class B are available.
Platinum resistance temperature sensors (PRT) They offer excellent accuracy over a wide temperature range(from -200 to +850°C).
Other resistance value options
RTD elements are also available with resistances of 200, 500, and 1000 Ω at 0°C. Such type of RTDs is normally known as PT200, PT 500, and PT 1000 respectively. These RTDs have the same temperature coefficients as previously described, but because of their higher resistances at 0°C, they provide more resistance change per degree, allowing for greater resolution.
Standard Platinum RTDs (SPRTs)
|Temperature||1/10 Din (±°C)||1/5 Din (±°C)||1/3 Din (±°C)||Class A (±°C)||Class B <±°C)|
The ITS-90 (International Temperature Scale of 1990- used as a worldwide practical temperature scale in national metrology labs like NIST, NPL) is made up of a number of fixed reference points with various interpolating devices used to define the scale between points. A special set of PRTs, called SPRTs, are used to perform the interpolation in such labs over the ranges 13.8033 K (Triple point of Equilibrium Hydrogen) to the Freezing point of silver, 971.78 °C.
There are two standards for platinum RTDs: The European standard (also known as the DIN or IEC standard) and the American standard.
The European standard, also known as the DIN or IEC standard, is considered the world-wide standard for platinum RTDs. This standard, DIN/IEC 60751 (or simply IEC751), requires the RTD to have an electrical resistance of 100.00 Ω at 0°C and a temperature coefficient of resistance (TCR) of 0.00385Ω/Ω/°C between 0 and 100°C. There are three resistance tolerances for Thin Film.
RTDs specified in IEC60751:
Class AA (Formerly 1/3B) = ± (0.1+0.0017×t) °C or 100.00 ± 0.04Ω at 0°C
Class A = ± (0.15+0.002×t) °C or 100.00 ± 0.06Ω at 0°C
Class B = ± (0.3+0.005×t) °C or 100.00 ± 0.12Ω at 0°C
Also, one special class not included in DIN/IEC60751:
Class 1/10B = ±1/10 (0.3+0.005×t) °C or 100.00 ± 0.012Ω at 0°C
The RTD elements nearly always require insulated leads attached. At temperatures below about 250 °C PVC, silicon rubber or PTFE insulators are used. Above this, glass fiber or ceramic are used. The measuring point, and usually most of the leads, requires a housing or protective sleeve, often made of a metal alloy which is chemically inert to the process being monitored. Selecting and designing protection sheaths can require more care than the actual sensor, as the sheath must withstand chemical or physical attack and provide convenient attachment points.
The thin film style of RTD is probably the most popular design because of their rugged design and low cost. The thin film element is manufactured by coating a small ceramic chip with a very thin (.0001”) film of platinum and then laser cutting or chemical etching a resistance path in the platinum film. The element is then coated with a thin layer of glass to protect it from harmful chemicals and gases. Larger extension lead wires are spot welded to the chip and this junction is then covered with a drop of epoxy to help hold the wires to the element.
The outer wound RTD element is made by winding the sensing element wire around a center mandrill, which is usually made of ceramic. This winding is then coated with glass or some other insulating material to protect and secure the windings. The winding wires are then spot welded to extension leads and secured to the body with ceramic cement or epoxy.
Each of the types has their advantages. The thin film is the least expensive to manufacture and also the most rugged. They can also be manufactured in very small sizes. The inner coil wire wound style is the most accurate. It is however, more expensive to manufacture and does not perform well in high vibration applications. The outer wound element is similar in cost to the inner coil element. It is not as accurate as the inner coil style but is more rugged.
RTDs are available with three different lead wire configurations. The selection of lead wire configuration is based on desired accuracy and instrumentation to be used for the measurement.
The two wires RTD is the simplest wire configuration. One wire is attached to each side of the element. A measure can be taken by any device equipped to measure resistance, including basic Volt Ohm Meters (VOM).This is the least accurate way of measuring temperature, due to the fact that the lead wire resistance is in series with the sensing element. The lead wire is at a different temperature than the sensing element and also has different resistance verses temperature characteristics. The longer the lead wire greater will be the effect on the measurement.
The three wires RTD is the most popular configuration for use in industrial applications. In order to minimize the effects of the lead resistances, a three-wire configuration can be used. Using this method the two leads to the sensor are on adjoining arms. There is a lead resistance in each arm of the bridge so that the resistance is cancelled out, so long as the two lead resistances are accurately the same. This configuration allows up to 600 meters of cable.
When used correctly, the three wire configuration eliminates the series resistance. This permits an accurate measurement of the sensing element. Two of the leads are connected to one side of the sensing element and the single lead to the other side. The resistance in L1 and L3 should be matched as close as possible; this will cause the lead resistance to cancel them. The color code for a three wire RTD is two red wires and one white.
A four wire RTD is the most accurate method to measure an RTD. It is primarily used in laboratories and is seldom seen in an industrial application. The four-wire resistance thermometer configuration increases the accuracy and reliability of the resistance being measured: the resistance error due to lead wire resistance is zero.A four wire RTD circuit removes the effect of mismatched resistances on the lead wires. A constant current is passed through L1 and L4, L2 and L3 measure the voltage drop across the RTD element. The color code for a four wire RTD is usually two red wires and two white wires. The following diagram illustrates a typical four wire measurement.
Mineral-Insulated resistance thermometers (M.I.) are equipped in general with Platinum-measuring resistors Pt100 Ω to DIN IEC 751. The inner (Cu) conductors are embedded in a closely compacted, inert mineral powder (MgO); the measuring resistor will be connected to the inner conductors, is also embedded and is surrounded by the metal sheath to form a hermetically sealed assembly. Sometimes inner conductor of constantan and nickel are also used.
The sheath functions as a useful protective cover in many situations. They are applied in locations where fast response, reduced space and or vibration resistance is a need. They can be furnished with a fixed cable or with a special plug which allows rapid fitting or exchange.
Mineral-insulated RTD temperature probes consist of a flexible, thin-walled stainless steel mineral insulated cable, in which low ohmic conductor copper wires are embedded in pressed, heat resistant magnesium oxide.
The temperature probe is connected to the wires of the internal conductors and accommodated in a stainless steel sheath. Thermowell and mineral-insulated cable are welded to one another.
The good heat transition between the sheath and the temperature probe permits short response times and high measuring accuracy. The vibration resistant (shake proof) design guarantees a long operating life. Temperature measurements at measuring points difficult to access are possible thanks to the flexible mineral-insulated cable. The smallest bending radius is 5 times the outer diameter.
Resistance thermometer systems are susceptible to three types of errors: The inherent tolerances built into the thermometers, gradients between the thermometer and the medium to be sensed, and errors introduced along the path between the sensor and readout or control instrument. Some sources of error are electrical; others result from the mechanical construction of the thermometer.
Conformity specifies the amount of resistance a thermometer is allowed to deviate from a standard curve (such as the curve produced by the Callendar-Van Dusen equation).
A tolerance at the reference temperature is usually 0°C, and a tolerance on the slope or TCR. Below shown figure states that a resistance thermometer conforms most closely to its curve at the reference temperature, while the resistance fans out above and below this reference.
Interchangeability between two thermometers is no more than twice the value of their Conformity. Commercial platinum resistance thermometer elements are available with extremely tight tolerances, to within 0.026°C in some cases. When interchangeability is an overriding consideration, the specified may consider other means to achieve it. For example, manufacturers may alter their calibration procedures to fix the reference temperature and tightest tolerance at a point other than 0°C.
The resistance change per degree change in temperature is a function of base resistance and TCR (Temperature Coefficient of Resistance).Although a thermometer with higher sensitivity is not necessarily more accurate, a larger signal simplifies output electronics and is less susceptible to lead wire effects and electrical noise. In addition, a larger resistance produces the same voltage output with less measuring current, which helps to limit self-heating of the thermometer element.
A resistance thermometer is a passive resistance sensor; it requires a measuring current to produce a useful signal. Because this measuring current heats the element wire above the true ambient temperature, errors will result unless the extra heat is dissipated. Self-heating is most often expressed in mW/°C, which is the power in mill watts (1000 I2 R) required to raise the thermometers internal temperature by 1°C. The higher the mW/°C figure, the lower the Self Heating.
As an example, assume a 5 mA measuring current is driven through 100 platinum RTD at 100°C.
Self-heating is specified as 50 mW/°C in water moving at 3 ft/sec. The amount of heat generated is:
1000 mW x (0.005 A)2 x (138.5) = 3.5 mW
The self-heating error is:
(3.5 mW) / (50 mW/°C) = 0.07°C
The generated heat increases with higher sensor element resistance (when a constant current measurement device is used), or with increasing measuring current.
The resulting error is inversely proportional to the ability of the thermometer to shed extra heat; which in turn depends on thermometer materials, construction, and environment.
The worst self-heating occurs when a high resistance is packed into a small body. Thin film elements, with little surface area to dissipate heat, are an example. Self-heating also depends on the medium in which the thermometer is immersed. Error in still air may be over 100 times greater than in moving water.
The degree of accord between two successive readings with a thermometer is its repeatability. Loss of repeatability results from permanent or temporary changes to the resistance characteristics of the element and may be caused by exposing the thermometer to temperatures at or beyond the endpoints of its specified range. A repeatability test cycles the thermometer between low and high temperatures; any change to R is noted. A typical repeatability rating for 0°C industrial platinum resistance thermometers is ±0.1°C.
Sheaths and other structures surrounding resistive elements should maximize heat transfer from the sensed medium, minimize heat transfer from ambient which can alter readings, and provide necessary protection of the elements.Proper materials and construction can dramatically improve reading accuracy. One strategy practicable only with wire-wound resistance thermometers versus thermistors, thermocouples, and solid-state devices are temperature averaging. An element may be wound to average temperature over lengths of up to 100feet.
The encased probe is the standard resistance thermometer configuration for industrial process control and machinery protection. Most probe cases are stainless steel or Inconel to withstand high temperatures, although other materials offer advantages at intermediate ranges.
For example, the tip-sensitive probe of Figure a copper-alloy tip which conducts heat 20 times better than stainless steel. This design improves thermal contact with sensed surfaces and reduces errors from conduction along the sheath.
Standard probe diameters range from 0.125. - 0.250. Smaller probes respond faster when directly immersed, but larger probes may fit more snugly in standard Thermowells. Probe lengths range from a few inches to ten feet or more.
Selection depends on the nature of the medium being sensed and cost requirements. Direct immersion of a probe into a liquid requires a fitting with a pipe thread, which may be adjustable or welded on the probe.
Figure shows a typical assembly, with one thread for mounting the probe and another for a connection head. Connection heads provide a transition between probe leads and external signal wires.
Mounting in a solid material is best accomplished with a spring-loaded holder, which may be fixed or adjustable. Spring loading provides good contact of the probe tip against the bottom of the hole and dampens potentially damaging vibration. When liquids are particularly corrosive, under high pressure, or fast-flowing, a thermowell may be necessary. A thermowell is a tube, closed at one end, which protects the probe and allows its removal without breaking the liquid seal.
Unlike thermocouple junctions, which can be welded directly to metal surfaces, resistance thermometers present a certain amount of bulk; and heat losses to ambient air may affect readings. Small flat elements, such as thin films, may mount on surfaces, but fragile element and lead wire connections make installation difficult.
Figure shows a flexible resistance thermometer with a wire-wound sensing element sandwiched between insulating layers. It conforms closely to sensed surfaces, and has thin insulation to readily transmit heat to the sensing element. The wire element may be wound to nearly any size to average out temperature gradients, and the flexible construction can withstand extreme shock and vibration.
In summary, resistance thermometers offer the greatest benefits relative to other thermometer types in these situations:
Advantages of resistance thermometers are:
Sheath material ranges from mild and stainless steel to refractory oxides (Ceramics) and a variety of exotic material including rare metals. Sensor inserts are fabricated unit which is comprised by a sensor and a terminal base; the sensor is housed in a stainless steel insert tube, usually of 6 or 8mm diameter and, then it is inserted into the actual protection tube. Good sheathing with physical contact between the insert tip and sheath end is essential to ensure good heat transfer. Spring contact is used in the terminal base to maintain this contact. This arrangement facilitates easy replacement of the sensor whenever required.
In general, there are two types of protection tubes:
Metallic tube, most commonly stainless steel, has mechanical advantage and higher thermal conductivity; they are generally immune to thermal shock. Metallic sheath can be used at temperature up to 1150°C. Ceramics are superior when high purity is required to avoid sensor or product being contaminated at elevated temperature.
Ceramic sheath's main application ranges between 1000 to 1800 °C. They may be in direct contact with the medium or may be used as a gas-tight inner sheath to separate the thermocouple from the actual metal protection tube. They should be mounted in hanging position above 1200°C to prevent distortion or fracture due to bending stresses. Even hairline cracks can lead to contamination of the thermocouple resulting in drift or failure. The wall thickness of the material is also important; thin walled tube is preferable to larger wall thickness. Cracks frequently developed if they are given excessively rapid temperature changes when they are quickly removed from hot furnace.
In van stone thermowell, stem, tip and sub part of flange all these three prepared by using single bar or rod material. There is no need of welding for these three parts of thermowell. The flange sub-part serves as a gasket in such type of thermowell. On this sub-part flange is used according to requirement.
The outside diameter decreases gradually along the immersion length. Used for high velocity applications.
It one end has flat surface. Used in low pressure applications or where flow characteristics around the thermowell are not important.
Such thermowell has semi-spherical tip at one end of thermowell. Used in higher pressure applications or where flow characteristics around the thermowell are important. This ensures a high degree of mechanical strength without losing the sensitivity or accuracy of the indicator
For spherical tip, special drill is used with a tip angle of 118ºC for production of thermowell. To achieve a possible uniform wall thickness, the tip is ball shaped or spherical in shape. Used in higher pressure applications or where flow characteristics around the thermowell are important. This ensures a high degree of mechanical strength without losing the sensitivity or accuracy of the indicator.
The Raised Face type is the most applied flange type, and is easily to identify. It is referred to as a raised face because the gasket surfaces are raised above the bolting circle face.
Ring-Type Joint (RTJ)
RTJ flanges have grooves cut into their faces which steel ring gaskets. The flanges seal when tightened bolts compress the gasket between the flanges into the grooves, deforming (or Coining) the gasket to make intimate contact inside the grooves, creating a metal to metal seal. Ring Type Joint gaskets are designed to seal by "initial line contact" or wedging action between the mating flange and the gasket. Ring Type Joint gaskets* are metallic sealing rings, suitable for high-pressure and high-temperature applications.
Flat Face (FF) The flat face (full face) flange has a gasket surface in the same plane as the bolting circle face. Applications using flat face flanges are frequently those in which the mating flange or flanged fitting is made from a casting. Flat face flanges are never to be bolted to a raised face flange.
Welding is a process of joining two metals by heating the metals to a suitable temperature. It may be done with or without the application of pressure, and with or without filler.
Full Penetration Welding- This type of welding ensures a fully welded interface between the two parts and is generally the strongest joint.
Partial Penetration Welding- This type consists of a partially welded interface with filler metal being laid on the surface of the two metals.
A WPS is a document that describes how welding is to be carried out in production. They are recommended for all welding operations and many application codes and standards make them mandatory.
The purpose of the document is to guide welders to the accepted procedures so that repeatable and trusted welding techniques are used. A WPS is developed for each material alloy and for each welding type used.
A WPS is supported by a Procedure Qualification Record (PQR or WPQR). A PQR is a record of a test weld performed and tested (more rigorously) to ensure that the procedure will produce a good weld.
The variables required to be documented are typically such items as:
Different types of coating are carried out in Thermowells. Some are as follows: Metal Thermowells with Tungsten Carbide / Ceramic / PTFE / PVDF / PFA Coatings.
|0.5-1.0mm||Below 650°C||Tungsten carbide coating offers effective wear resistance coating, as it offers exceptionally high hardness level ( maximum 74 HRC hardness). It is resistance to high temperature and corrosion & high abrasive condition.|
|2||Teflon Coated||0.5-1.0mm||200°C||Teflon coating provides a dry film lubricant whitch provides excellent resistance against corrosion and chemicals.|
|0.5mm||900°C||Boron nitride protects surface witch comes in contact with molten metal (specially aluminum). Boron nitride inhabits corrosion and chemical attacks, and provide easier release and longer life.|
|4||Zirconia Coated||0.5-0.1mm||1800°C||Zirconia coating produces a hard and chemically resistant protective layer suitable for high temperature and corrosion.|
|0.5-1.0mm||1200°C||Alumina coating provides a high temperature abrasive protection.|
|6||Stellite Coating||0.5-1.0mm||1200°C||Stellite coating provides very good abrasion resistance and good chemical corrosion resistance.|
Chemical- By PMI(Positive Material Identification).
Physical- By Tensile, Elongation, Hardness, Micro & Macro examination, IGC tests.
A hydrostatic pressure test is a way in which leaks can be found in pressure vessels such as pipelines and plumbing. The test involves placing water, which is often dyed for visibility, in the thermowell at the required pressure to ensure that it will not leak or be damaged. It is the most common method employed for testing pipes and vessels. Using this test helps maintain safety standards and durability of a vessel over time. Newly manufactured pieces are initially qualified using the hydrostatic test.
Dye penetrant inspection (DPI), also called liquid penetrant inspection (LPI) or penetrant testing (PT), is a widely applied and low-cost inspection method used to locate surface-breaking defects in all non-porous materials (metals, plastics, or ceramics). LPI is used to detect casting, forging and welding surface defects such as hairline cracks, surface porosity, leaks in new products, and fatigue cracks on in-service components.
Radiographic Testing (RT), or industrial radiography, is a nondestructive testing (NDT) method of inspecting materials for hidden flaws by using the ability of short wavelength electromagnetic radiation (high energy photons) to penetrate various materials. irregularities and flows that can be detected which includes - cracks, voids, thickness, lack of fusion, lack of penetration, porosity and misalignment.
The selection of material is the most important factor for thermowell life.The material selected is based on the temperature of the application and the process medium.
All threaded well are made by easily welded or brazed materials.Welding and brazing is important for the installation requiring seal. The pipe thread provides mechanical strength, and the weld or braze provides the seal. Flanged wells(other than van stone type) consist of a bar stocks well which is solidly welded to a top quality flange. Standard construction uses a primary "J" groove weld and a bevel groove clean fillet. This double welded construction eliminates the possibility of crevice corrosion since no open joint are exposed from either inside or outside the installation. Socket weld well are simple to install, simply weld them into place.
The distance from the tip of the well to the underside of the thread or other connection is defined as the insertion length (designated as "U"). For best accuracy this length should be greater enough to permit the entire temperature sensitive part of element to project into the medium being measured. A properly installed element: in liquid, the element should be immersed up to its sensitive length plus one inch, and in air or gas, the element should be immersed up to its sensitive length plus three inches.
Almost all installation uses several type of temperature measuring sensor. The selection of a standard bore diameter can produce extreme flexibility within the plant. The same well can accommodate thermocouple, resistance thermometer, and bimetal thermometer.
Tapered shank provides greater stiffness with same sensitivity. The higher strength to weight ratio give these wells higher natural frequency than the equivalent length straight shank well thus permitting operation at higher fluid velocity.
At low velocities, the risk of thermowell failure is minimal and does not usually require frequency calculations.
If the following criteria are met, the designer may elect to wave calculation requirements.
In low density gases with a Scruton Number (Nsc) of >2.5 Reynolds Number <105, the in-line resonance is suppressed and therefore the acceptable ratio will be:
If a thermowell passes cyclic stress conditions for operation at the in-line resonance condition, the acceptable ratio will be:
If a thermowell fails the cyclic stress condition for operation at the in-line resonance condition, the thermowell natural frequency will be high enough to limit excitation of the in-line resonance. Therefore, the acceptable ratio will be:
Adjustable compression fitting are used directly on probe to achieve the required insertion length in the process and to ensure the proper sheathing of probes into thermowell. Compression fittings for attaching tubing (piping) commonly have ferrules in them. Compression fittings are popular because they do not require soldering, so they are comparatively quick and easy to use.
Nipples are made up with a flange from the same family on each end of a tube section. (Fittings that are manufactured with different flange families on each end are called hybrid adapters.) Straight nipples are manufactured with the same size flange on each end of straight section of tubing. Reducer nipples have different size flanges (from the same family) on each end.
The three piece unions have to be used in hazardous areas, for the junction between conduits pipes and boxes or various appliances. The unions are made up of three independent pieces that can be screwed up by rotating the same pieces among them.
Following are the two types of termination style:
The link between the thermoelectric wires of the thermocouple and those of the extension cable is made by means of non - compensated male and female connectors. The metallic body and casing of these connectors ensure the screening continuity as well as good temperature.
Standard & Miniature connectors are ideal for connecting thermocouple sensors and extension or compensating cable to each other. The pins are polarized to avoid an incorrect connection and the connector body is additionally marked for polarity. These connectors have color coding according to special standard like: ANSI, IEC etc.