Advances in Thermocouple Technology

Thermocouples remain a fundamental tool for temperature measurement in industrial processes due to their simplicity, durability, and broad temperature range. While the core technology has remained relatively stable, incremental advancements have improved their performance, accuracy, and applicability.

One area of ongoing development is the exploration of novel materials to enhance thermocouple properties. Researchers continue to study new alloy combinations and ceramic materials to extend operational temperature ranges and improve stability. However, most commercially available thermocouples still rely on established materials like nickel alloys and platinum-rhodium combinations.

Wireless technology integration is a promising field for thermocouples, aiming to reduce the need for extensive wiring in installations. While some wireless thermocouple systems are available, challenges remain in ensuring reliable data transmission and power management in industrial environments. Ongoing research focuses on improving wireless protocols and battery technologies to make these systems more practical.

Enhancements in signal processing and calibration have led to gradual improvements in measurement accuracy. Modern thermocouple systems may incorporate digital processing to compensate for non-linearities and external noise. Regular calibration against standard references ensures maintained accuracy, though the fundamental methods have not drastically changed in recent years.

Environmental and sustainability considerations are increasingly important in manufacturing. Efforts to reduce hazardous substances and promote recyclability in thermocouple production align with broader industry trends. While not unique to thermocouples, these practices reflect a general movement toward more eco-friendly manufacturing processes.

In conclusion, thermocouple technology continues to evolve through incremental improvements rather than radical innovations. Ongoing research and development aim to enhance material properties, integration capabilities, and environmental sustainability. These efforts contribute to the reliability and effectiveness of thermocouples as essential tools in temperature measurement across various industries.

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The Significance of Temperature Control and Thermocouples in Plastics Manufacturing

Temperature control is critical in the plastics industry, impacting product quality, efficiency, and safety. The importance of temperature control and the role of thermocouples in producing quality plastic items cannot be understated.

The Significance of Temperature Control in Plastics Manufacturing

• Quality of End Products: The physical properties of plastic, like strength, flexibility, and color, are susceptible to temperature. Precise temperature control ensures these properties meet the specific requirements of each product.
• Efficiency in Production: Proper temperature regulation speeds up production cycles, reduces energy consumption, and minimizes material wastage, leading to cost-effective operations.
• Preventing Defects: Inappropriate temperatures can lead to defects such as warping, shrinkage, or burn marks, compromising the integrity and appearance of the plastic products.
• Safety: Overheating can cause degradation of plastic materials, potentially leading to hazardous situations like toxic emissions or fire hazards.

How Thermocouples Enhance Temperature Control in Plastics Manufacturing

• Accuracy and Responsiveness: Thermocouples provide rapid and accurate temperature readings, essential for the precise control required in plastics processing.
• Durability and Versatility: They can withstand harsh environments and high temperatures, making them ideal for various plastics manufacturing processes, including extrusion, injection molding, and blow molding.
• Cost-Effectiveness and Simplicity: Thermocouples are inexpensive and easy to install and maintain, making them a practical choice for many plastics manufacturers.
• Integration with Automation Systems: They can be easily integrated into automated control systems, enabling real-time monitoring and adjustments, vital for maintaining consistent product quality.

Practical Applications

• Extrusion: Thermocouples monitor and control the temperature of the extruder barrel, ensuring the plastic melts correctly for uniform flow and thickness.
• Injection Molding: They regulate the mold temperature, which is crucial for achieving the product's desired shape and surface finish.
• Thermal Forming: Thermocouples help maintain the temperature for shaping plastic sheets without causing damage.

In the plastics industry, precise temperature control is not just a matter of efficiency or quality; it's a cornerstone for successful operations. Thermocouples are indispensable in achieving this control, offering accuracy, reliability, and versatility. Their integration into plastics manufacturing processes ensures that the industry can meet the growing demand for high-quality plastic products safely, efficiently, and cost-effectively.

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Exploring the Evolving Market for Industrial Thermocouples

Over the last decade, the industrial thermocouple market was heavily influenced by the following sectors:

• Oil & Gas: Thermocouples are widely used in the oil & gas industry for temperature measurement during exploration, drilling, production, refining, and transportation processes.
• Energy & Power: Thermocouples are extensively used in power plants, including nuclear, thermal, and renewable energy plants, to measure temperature at various stages of energy production.
• Chemicals & Petrochemicals: In chemical and petrochemical industries, thermocouples play an essential role in temperature measurement during the production and processing of chemicals, fertilizers, plastics, and more.
• Metallurgy & Steel Production: Thermocouples are vital in steel and other metal production processes, where accurate temperature measurement is crucial for quality control and efficient production.
• Automotive: Thermocouples are used in automotive manufacturing for various applications, including engine performance monitoring, exhaust system analysis, and brake system testing.
• Aerospace & Defense: Thermocouples are employed in aerospace and defense applications to monitor the temperature of aircraft engines, rocket propulsion systems, and other critical components.
• Semiconductor Manufacturing: Thermocouples are used to measure the temperature of semiconductor wafers during manufacturing, essential for producing high-quality integrated circuits.

In the next decade, considering the trends in these industries and emerging technological advancements, the most significant potential markets for industrial thermocouples are likely to be:

• Renewable Energy: As countries worldwide continue to shift towards renewable energy sources like solar, wind, and hydropower, there will likely be a growing demand for thermocouples to monitor temperature in various renewable energy systems.
• Electric Vehicles (EVs): The booming EV market is expected to drive the demand for thermocouples, as they monitor battery temperature, motor temperature, and other critical components in electric vehicles.
• Smart Manufacturing: Adopting Industry 4.0 practices, including automation, IoT, and digitalization, will increase the demand for advanced thermocouples integrated with wireless communication and intelligent features for predictive maintenance and real-time monitoring.
• Semiconductor Manufacturing: The ongoing advancements in semiconductor technology and the increasing demand for electronics and IoT devices are expected to drive the need for accurate temperature measurement in semiconductor manufacturing.
• Hydrogen Economy: As countries invest in developing hydrogen as a clean energy source, the hydrogen production, storage, and transportation sectors are expected to require accurate temperature monitoring, leading to increased demand for thermocouples.

Given the current trends and technological advancements, these sectors have the potential to shape the industrial thermocouple market in the next decade.

Duro-Sense

310-533-6877

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Thermocouples and RTDs: Understanding Their Uses in Industrial Temperature Sensing

Various industrial applications widely use thermocouples and RTDs for temperature measurement. The choice of which to use depends on the application's specific requirements. Here are some industrial applications where one may be better suited than the other:

Industrial Applications where Thermocouples are better suited:

• High-temperature measurements: Thermocouples can measure temperatures ranging from -270°C to 2700°C and are more suitable for high-temperature measurements than RTDs.
• Quick response: Thermocouples have a faster response time than RTDs and are suitable for measuring fast-changing temperature processes.
• Harsh environments: Thermocouples can withstand harsh environments, such as high-pressure environments, corrosive or abrasive materials, and vibration, making them more suitable for applications where the temperature probe becomes exposed to such environments.
• Low cost: Thermocouples are relatively inexpensive compared to RTDs, making them a preferred choice in cost-sensitive applications.

Industrial Applications where RTDs are better suited:

• High accuracy: RTDs have higher accuracy than thermocouples and are, therefore, more suitable for applications that require precise temperature measurements.
• Stable and repeatable: RTDs are stable over time and offer repeatable measurements, making them a better choice for applications where process control is critical.
• Wide temperature range: Although RTDs have a lower temperature range than thermocouples, they can still measure temperatures as low as -200°C, making them more suitable for low-temperature applications.
• Longer lifespan: RTDs have a longer lifespan than thermocouples and are a better choice for applications where longevity is critical.

Examples of industrial applications for thermocouples:

• Steel industry: For measuring temperature in furnaces and blast furnaces.
• Petrochemical industry: For measuring temperature in pipelines, storage tanks, and reactors.
• Power generation: For measuring temperature in turbines and boilers.
• Glass industry: For measuring temperature in glass furnaces.

Examples of industrial applications for RTDs:

• Pharmaceutical industry: For measuring temperature in bioreactors and other critical process equipment.
• Food industry: For measuring temperature in food processing equipment.
• Aerospace industry: For measuring temperature in aircraft engines and other high-precision applications.
• Laboratory and research applications: For measuring temperature in calibration and testing equipment.

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What's The Difference Between Thermocouples and RTDs?

Thermocouples and resistance temperature detectors (RTDs) are both types of temperature sensors that are used to measure the temperature of a system. While they both operate on the same basic principle of using a physical property to determine temperature, they differ in the specific way they do this and the accuracy and precision of their measurements.

A thermocouple is a type of temperature sensor that is made up of two different metals that are joined together at one end. When a temperature difference is applied to the other end of the thermocouple, a small electrical voltage is generated. This voltage is proportional to the temperature difference, and can be measured and used to determine the temperature of the system. Thermocouples are relatively simple and inexpensive, but they are not very accurate or precise and are only capable of measuring a limited range of temperatures.

On the other hand, a resistance temperature detector (RTD) is a type of temperature sensor that uses the principle of electrical resistance to measure temperature. RTDs consist of a coil of fine wire that is wrapped around a core material, typically made of a metal with a high electrical resistance such as platinum, nickel, or copper. When the temperature of the RTD changes, the electrical resistance of the wire also changes, and this change can be measured and used to determine the temperature of the system. RTDs are generally more accurate and precise than thermocouples, and can be used to measure a wider range of temperatures. However, they are also more complex and expensive than thermocouples.

In summary, the key differences between thermocouples and RTDs are the way they measure temperature, the accuracy and precision of their measurements, and the range of temperatures they can measure. Thermocouples use the voltage generated by two different metals to measure temperature, while RTDs use the change in electrical resistance of a wire to measure temperature. Thermocouples are relatively simple and inexpensive, but not very accurate or precise, while RTDs are more complex and expensive, but can provide more accurate and precise measurements over a wider range of temperatures.

For expert guidance specifying or applying thermocouples or RTD's in your application, contact:

Duro-Sense Corporation
310-533-6877
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Industry Leading Delivery Times for Thermocouples and RTDs

Customers dislike having to wait. Customer happiness and loyalty are directly related to how soon items are delivered. 

Lead time is a critical component of any effective supply chain. It is vital to a company's success to reduce lead times as much as feasible. Production lead time may be a significant factor in your company's success. Whether it's a poor movie or a dull and tiresome speech, shorter is usually better. Selecting the correct manufacturer is critical when determining the lead time.

Duro-Sense has over four decades of manufacturing experience and has developed essential solutions for reducing lead times without sacrificing quality. Allow us to show you.

Three Simple Questions for Choosing Thermocouples or RTDs

What's the best choice for your industrial temperature sensing requirement, a thermocouple or RTD? In industrial installations, both types of sensor can be specified with similar mounting accessories, dimensional specifications, and instrument interfaces. However, there are three criteria you need to consider before choosing between an RTD or thermocouple probe.

First, what is the temperature range you are trying to monitor?

Generally, if the temperature exceeds five hundred degrees Celsius (500 deg C), thermocouples are for you. RTD's are best between -200 and 500 °C, while thermocouples have a range of -180 to 2,320 °C. For anything above 500 Celsius, you should select the appropriate thermocouple calibration for the sensing range you're working in.

Second, what type of sensor accuracy do you need? 

RTDs are more accurate temperature sensors, offer highly repeatable readings, drift less over time, and are suitable for high precision requirements. Thermocouples are generally less accurate and are subject to drift. Typical thermocouple accuracy is 2 deg. C.

Third, how about the budget you're working under? 

Thermocouples can be up to three times less expensive than RTD probes, making thermocouples a good choice if high accuracy and repeatability are not critical. One caveat though. Make sure you consider any additional cost incurred with long runs of thermocouple extension wire. For installations requiring dozens or even hundreds of temperature sensors, the significant difference in basic sensor cost is an important consideration.

These three criteria are VERY basic, and intended just to point you in the right direction. There are many other differences between thermocouples and RTDs that need to be understood before application.  Consult a temperature sensor expert prior to installing or specifying a thermocouple or RTD wherever or whenever failure can cause harm.

Quick Comparison of Temperature Sensors

Thermocouples are commonly used because of their simplicity, reliability, and relative low cost. They are self-powered and eliminate the need for a separate sensor power supply. Thermocouples are quite durable when selected for a given application appropriately. Thermocouples can also be used for applications with high temperatures.

Resistance temperature detectors (RTDs) are attractive alternatives to thermocouples when the output is desired to be highly accurate, stable and linear (i.e. just how close the calibration curve looks a straight line). The superior linearity of relative temperature resistance enables simpler signal processing devices for RTDs than thermocouples.

Thermistors are similar to RTD because they're a resistance measurement device, but thermistors use a very cheap polymer or ceramic material as the element in lieu of the use of pure metal.

For more information on any type of industrial or OEM temperature sensor, contact Duro-Sense by calling 310-533-6877 or by visiting https://duro-sense.com.

Thermocouples: Proper Use, Recommended Practices, and Avoiding Problems

Proper use and maintenance of thermocouple systems begin with good system design based on the strengths and weaknesses of various thermocouple types. Because these sensors contain sensitive electronics, general good practice includes use of shielded cases and twisted- pair wire, use of proper sheathing, avoidance of steep temperature gradients, use of large-gauge extension wire, and use of guarded integrating voltmeters or ohmmeters, which electronically filter out unwanted signals. The signal conditioner should be located as close as possible to the sensor, and twisted copper-wire pairs should be used to transmit the signal to the control station. To minimize electromagnetic field interference, sensor system wires should not be located parallel to power supply cables. The primary causes of loss of calibration in thermocouples include the following:

1. Electric “noise” from nearby motors, electric furnaces, or other such electrically noisy equipment;
2. Radio frequency interference from the use of hand-held radios near the instrument.
3. “Ground loops” that result when condensation and corrosion ground the thermocouple and create a ground loop circuit with another ground connection in the sensing circuit.

Most problems with thermocouples are aggravated by use of the thermocouple to measure temperatures that approach or exceed their upper temperature limits. Careful recording of events that could affect measurements should be kept in a logbook. Any adjustments or calibrations should also be recorded. The logbook should contain the names of individuals performing maintenance and calibrations as well as defined procedures. In systems monitoring many locations, such a log is especially useful for fault diagnosis.

Thermocouples sometimes experience catastrophic failures, which may be preceded by extreme oscillations or erratic readings. In such cases, all connections associated with the thermocouple should be checked for loose screws, oxidation, and galvanic corrosion. In many cases, drift may be a more serious problem because it can go unnoticed for long periods of time. The most common causes of loss of calibration are excessive heat, work hardening, and contamination. Work hardening generally is due to excessive bending or vibration and can be prevented with properly designed thermowells, insertion lengths, and materials. Contamination is caused by chemicals and moisture, which sometimes attack wiring by penetrating sheaths, and can result in short-circuiting. A simple test to check for this problem is to disconnect the sensor at its closest connection and check for electrical continuity between the wires and the sheath using a multimeter. If the meter indicates continuity, the sensor should be replaced. Because the electromotive force (EMF) produced by thermocouples is so small, electrical noise can severely affect thermocouple performance. For that reason, it also is very important that transmitters be isolated. Thermocouples used in the vicinity of electrostatic precipitators must be shielded to avoid electrical interference. If the potential electrical interference is high, an RTD or other type of sensor may be preferred to thermocouples. With respect to thermocouple and protection tube selection, the following should be noted:

1. Type J thermocouples particularly should not be used in applications in which they might be exposed to moisture because the iron in the thermocouple will rust and deteriorate quickly;
2. Type K thermocouples should not be used in the presence of sulfur, which causes the element to corrode; because cutting oils often contain sulfur, protection tubes should be degreased before being used; stainless steel sheaths should be used to protect Type K thermocouples in stacks where SO2 emissions are significant;
3. Platinum thermocouple elements (Types R, S, or B) should not be used with metal protection tubes unless the tubes have a ceramic lining because the metal will contaminate the platinum;
4. Ceramic, silicon carbide, and composite (metal ceramic, Cerite-II, Cerite-III) protection tubes are subject to thermal shock and should be preheated prior to inserting in high temperature process environments; and
5. Molybdenum - or tantalum-sheathed thermocouples will fail rapidly if placed in oxidizing atmospheres.

During one study of thermocouple performance, 24 combinations of thermocouple and sheath material types were tested at temperatures up to 1200 C (2200 F). The results indicated that above 600 C (1110 F) thermocouples are affected by complex chemical interactions between their components; even though wires and sheaths were physically separated, exchange of constituents occurred. The study concluded that thermocouples maintain calibration better if sheath material is similar in composition to thermocouple alloys. By using similar alloys longer performance can be expected for sensors subjected to temperatures above 600 C (1110 F), and the use of similar alloys is essential for temperatures above 1000 C (1830 F).

For more information on the proper use of thermocouples, contact Duro-Sense Corporation by calling (310) 533-6877 or visit their website at https://duro-sense.com.

Reprinted from CAM Technical Guidance Document courtesy of EPA.gov.

Overview of Thermocouple Types and Ranges

Thermocouples have been classified by the International Society of Automation (formerly Instrument Society of America) and the American National Standards Institute (ANSI), and are available for temperatures ranging from -200 deg. to 1700 deg.C (-330 deg. to 3100 deg.F). These standard tolerance thermocouples range in tolerance from ±0.5 percent to ±2 percent of true temperature. The table below presents commonly available thermocouple types and operating ranges.

Commonly Available Thermocouple Types and Operating Ranges

Thermocouples must be selected to meet the conditions of the application. Thermocouple and extension wires (used to transmit the voltage from the thermocouple to the monitoring point) are generally specified and ordered by their ANSI letter designations for wire types. Positive and negative legs are identified by the letter suffixes P and N, respectively. General size and type recommendations are based on length of service, temperature, type of atmosphere (gas or liquid constituents), and desired response times. Smaller wire gauges provide faster response but do not last as long under adverse conditions. Conversely, larger gauges provide longer service life but with longer response times. Thermowells and sheaths are recommended by thermocouple manufacturers for the extension of thermocouple life. Instruments used to convert thermocouple voltage to temperature scales are coded using the same letter designations. Failure to use matching thermocouples and instruments will result in erroneous readings.

Type J thermocouples use iron for the positive leg and copper-nickel (constantin) alloys for the negative leg. They may be used unprotected where there is an oxygen-deficient atmosphere, but a thermowell is recommended for cleanliness and generally longer life. Because the iron (positive leg) wire oxidizes rapidly at temperatures over 1000 deg.F, manufacturers recommend using larger gauge wires to extend the life of the thermocouple when temperatures approach the maximum operating temperature.

Type K thermocouples use chromium-nickel alloys for the positive leg and copper alloys for the negative leg. They are reliable and relatively accurate over a wide temperature range. It is a good practice to protect Type K thermocouples with a suitable ceramic tube, especially in reducing atmospheres. In oxidizing atmospheres, such as electric arc furnaces, tube protection may not be necessary as long as other conditions are suitable; however, manufacturers still recommend protection for cleanliness and prevention of mechanical damage. Type K thermocouples generally outlast Type J, because the iron wire in a Type J thermocouple oxidizes rapidly at higher temperatures.

Type N thermocouples use nickel alloys for both the positive and negative legs to achieve operation at higher temperatures, especially where sulfur compounds are present. They provide better resistance to oxidation, leading to longer service life overall.

Type T thermocouples use copper for the positive leg and copper-nickel alloys for the negative leg. They can be used in either oxidizing or reducing atmospheres, but, again, manufacturers recommend the use of thermowells. These are good stable thermocouples for lower temperatures.

Types S, R, and B thermocouples use noble metals for the leg wires and are able to perform at higher temperatures than the common Types J and K. They are, however, easily contaminated, and reducing atmospheres are particularly detrimental to their accuracy. Manufacturers of such thermocouples recommend gas-tight ceramic tubes, secondary porcelain protective tubes, and a silicon carbide or metal outer protective tube depending on service locations.

For more information about thermocouples, contact Duro-Sense Corporation by visiting https://duro-sense.com or calling 310-533-6877.

Temperature Sensors for the Toughest Applications

Click for larger view.

Duro-Sense Corporation manufactures the finest quality temperature sensors available. The company has a long history of supplying thermocouples and RTDs to the top manufacturers in aerospace, medical equipment, semiconductor processing, plastics processing, and heavy industry.  From large industrial thermocouples used in primary metal production, to miniature, discreet sensors used in military aircraft, Duro-Sense products are proven to be ultra-reliable, accurate, and of extremely high value.

All Duro-Sense customers benefit from years of tackling difficult applications. By implementing stringent quality practices and advanced manufacturing processes, Duro-Sense continues to solve the most challenging temperature sensing applications.

Duro-Sense is a one-stop, full service provider of anything related to temperature sensing. Service. Quality. On-time delivery.

Rely on the Duro-Sense difference.
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Thermocouple Types, Materials of Composition, and Temperature Ranges

Nickel-Alloy Thermocouples

Type E: 
(pos) 90% Nickel / 10% Chromium;
(neg) 55% Copper / 45% Nickel (Constantan)
32 to 1600°F, 0 to 870°C

Type J:
(pos) 100% Iron
(neg) 55% Copper / 45% Nickel (Constantan)
32 to 1400°F, 0 to 760°C

Type K:
(pos) 90% Nickel / 10% Chromium
(neg) 95% Nickel / 2% Aluminum / 2% Manganese / 1% Silicon
32 to 2300°F, 0 to 1260°C


Type M:
(pos) 82% Nickel / 18% Molybdenum
(neg) 99.2% Nickel / 0.8% Cobalt
-58 to 2570°F, -50 to 1410°C

Type N:
(pos) 84.5% Nickel / 14% Chromium / 1.5% Silicon
(neg) 95.4% Nickel / 4.5% Silicon / 0.1% Magnesium
32 to 2300°F, 0 to 1260°C

Type T:
(pos) 100% Copper
(neg) 55% Copper / 45% Nickel (Constantan)
-328 to 700°F, -200 to 370°C

Platinum/rhodium-Alloy thermocouples

Type B:
(pos) 70% Platinum / 30% Rhodium
(neg) 94% Platinum  / 6% Rhodium
1600 to 3100°F, 871 to 1704°C

Type R:
(pos) 87% Platinum / 13% Rhodium
(neg) 100% Platinum
1000 to 2700°F, 538 to 1482°C

Type S:
(pos) 90% Platinum / 10% Rhodium
(neg) 100% Platinum
1000 to 2700°F, 538 to 1482°C

Tungsten/Rhenium-Alloy Thermocouples

Type C:
(pos) 95% Tungsten / 5% Rhenium
(neg) 74% Tungsten / 26% Rhenium
32 to 4200°F, 0 to 2315°C

Chromel–Gold/Iron-Alloy Thermocouples

Type P:
(pos) 55% Palladium / 31% Platinum / 14% Gold
(neg) 65% Gold / 35% Palladium
32 to 2543°F, 0 to 1395°C

Power Plant Temperature Sensors

Power plants (generating facilities) transform the mechanical energy of a spinning generator into electrical energy. Heat (from flame, nuclear reaction, or chemical reaction) is used to create steam that, in turn, produces the mechanical energy to drive turbines. There are many areas where precision temperature measurement and monitoring is critical to keep power plant systems running. Thermocouples and RTD sensors provide accurate, repeatable, and reliable  measurement.

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RTD and Thermocouple Selection and Location for Optimal Control

Loop diagram *

A perfectly designed temperature loop would precisely balance the power required to heat the media to it's desired temperature while compensating for system losses. In the real world however, there are many external variables that upset the balance between energy input and desired temperature. To offset these external variables and ensure adequate power is available to do the work, energy calculations with liberal safety factors are coupled with temperature controllers that throttle or proportion the amount of energy added to the process.

Most temperature control loops have (5) four major components:

1) The media to be heated (e.g. metal platen, a tank of liquid, a stream of gas)
2) A energy source (e.g. electric heater, steam, hot oil, flame)
3) A temperature sensor (thermocouple, RTD)
4) A controller (e.g. electronic thermostat, PID controller)
5) Control element (e.g. control valve, SCR, SSR)

Temperature controllers provide sophisticated functions that "learn" or understand the relationship between available power and sensor temperature. They then adjust the amount of energy (heat) added, based on the current reading of the sensor and the desired temperature setpoint.  Unfortunately, temperature controllers are often relied upon to overcome the oversights and inadequacies of poor control loop design.

Lag time *

In poor control situations the controller usually takes the blame, when in actuality,  the problem lies in the system design. The controller's actions are a function of the difference between setpoint and sensor reading, the availability of energy to eliminate that error, and the sensor lag time. The further the sensor or energy source is located from the process media, the wider the swings in energy input will be, and therefore produce a more difficult loop to balance. Considering this, it is important to optimize the sensor (thermocouple or RTD) placement. Any distance or barrier between sensor and process media introduces lag which is an impediment to close control.

In the most ideal situation, temperature sensors, the energy source and process media would all be at the same physical location. Since it's virtually impossible to accomplish this, compromises have to be made to allow for the mechanical, physical, and electrical realities of the application. Here are some practical recommendations for sensor selection and placement to improve temperature loop performance:

• Thermocouples, because of their low mass generally have a response advantage over RTDs.
• Exposed junction thermocouples are the fastest responding (least lag) sensor choice, but they are also the most prone to physical and chemical damage.
• Narrow, sheathed, grounded junction magnesium oxide insulated thermocouples are nearly as fast as exposed junctions, and provide protection from the process media.
• Applications that require protection sheaths and thermowells for RTDs and thermocouples increase sensor lag time.
• An immersion length of at least 10 times the diameter of the thermowell or sensor sheath should be used to minimize heat loss along the sensor sheath or thermowell wall from tip to process connection.
• Where possible, insert the temperature sensor in a pipe elbow into the oncoming flow.
• Tapered, swaged, or stepped thermowells are  faster responding
• Always make sure the fit between sensor outer diameter and thermowell inner diameter is tight, and that the tip of the sensor is in direct contact with the thermowell. 

* Images courtesy of Tony Kuphaldt and his book "Lessons In Industrial Instrumentation"

Advantages and Disadvantages of Thermocouples

Industrial thermocouples in ceramic
protection tubes (Duro-Sense)

Temperature sensors, inherent to their design or operating principle, are often the weakest link in the control loop. Fragility, noise sensitivity, and material deterioration have to be considered when applying temperature sensors. Generally speaking, when it comes to temperature sensing technology, RTDs are more accurate and stable, but are also fragile; thermistors have greater temperature sensitivity, but their electrical properties can change overtime; thermocouples (TC) are rugged, inexpensive, and operate over a wide temperature range, but they do have drawbacks that need to be understood for successful implementation.

The thermocouple's primary disadvantage is their weak output signal and their susceptibility to electrical noise. The mV signal generated is so small it requires conditioning, namely amplification and linearization.  The good news is this conditioning is built-in to and provided by the TC's corresponding controller, indicator or transmitter.  Calibration drift due to oxidation, contamination, or other physical changes to the alloys is another problem associated with thermocouples, requiring a facility to implement periodic recalibration procedures. Lastly, thermocouples require alloy-matching thermocouple extension wire when running lead wires any distance from sensor to instrument. This adds cost, as thermocouple extension wire is more expensive than standard wire.

Despite these concerns, thermocouples are widely used in industry because of their relative low cost, ruggedness, wide selection of operating ranges, and versatility in size, shape, and configuration. They have been used in millions of process control applications for over a generation, and provide an excellent balance of performance, cost, and simplicity. Successful application depends on knowing thermocouple's strengths and weaknesses, and consulting with an applications expert prior to specifying or installing any temperature sensor is always recommended to ensure the sensor's longest life and best performance.

Industrial Thermocouples

Industrial thermocouples are used to sense temperature in industrial processes. They come in a wide variety of types. The type of thermocouple, defined by the combination of dissimilar metals used, determines the functional temperature range and should be matched to the requirements of the application.

Magnesium Oxide (MgO) thermocouples consists of a thermocouple junction encased in a metal sheath, surrounded by compressed magnesium oxide insulation. The thermocouple junction, also known as the sensing junction, is the point where the two dissimilar metals meet and are usually welded together. Since the sensing junction is sealed from the environment, there is reduced potential for contamination and corrosion. MgO thermocouple sheaths are annealed and can be formed into different shapes and diameters.

Industrial thermocouple usually are constructed by inserting the thermocouple element into a metallic thermowell or ceramic protection tube. Not only does this protect the sensing element from the process, it accommodates easy removal and replacement.  Industrial thermocouples can be designed with virtually endless combinations of elements, wells, protection tubes, junction boxes, wiring terminals, and process connections.