Looking for High Accuracy in Temperature Measurement?

Industrial temperature measurement is a critical aspect of many processes, including manufacturing, processing, and energy production. The choice of temperature sensor is vital to ensure accuracy, reliability, and durability. Several types of temperature sensors are available, including Resistance Temperature Detectors (RTDs), Thermocouples, Thermistors, and Infrared Sensors. Each sensor type has its unique strengths and weaknesses. However, after a thorough analysis, RTDs generally offer the highest level of accuracy in industrial settings.

RTDs (Resistance Temperature Detectors) are temperature sensors that use the predictable increase or decrease in electrical resistance of some materials with rising or falling temperatures. The most common RTD type is platinum (Pt100 or Pt1000) due to its stability, repeatability, and nearly linear temperature-resistance relationship. RTDs have a typical accuracy within ±0.1°C, making them among the most accurate temperature sensors available.

Thermocouples are a type of temperature sensor made from two dissimilar metals joined together at one end, and changes in temperature cause a small voltage, which can be measured and interpreted. While they are robust and can handle extreme temperatures, their accuracy is lower than that of RTDs, generally within ±0.5°C to ±2°C.

Thermistors are temperature-sensitive resistors, typically made from ceramic or polymer. While they can offer high accuracy, they have a non-linear response and a limited temperature range, making them less suitable for broad industrial applications.

Infrared sensors measure temperature by capturing the infrared energy emitted by an object. They are non-contact sensors, which can be advantageous in certain situations, but they also require a clear line of sight and can be affected by dust, fog, or other environmental factors.

RTDs are the primary choice for high-accuracy industrial temperature sensing for several reasons:

1. Their accuracy is superior to most other types, typically within ±0.1°C.
2. They exhibit good long-term stability, making them reliable over the lifespan of many industrial processes.
3. Platinum RTDs are highly repeatable and have a nearly linear temperature-resistance relationship, making them easy to interpret and integrate into control systems.

However, it's important to note that the choice of the sensor should ultimately depend on the specifics of the application, including the temperature range, required accuracy, environmental conditions, and budget. Thermocouples, for instance, might be more suitable for high-temperature applications, and infrared sensors may be necessary when a non-contact measurement is required.

In conclusion, RTDs are recommended for a broad range of industrial applications requiring high accuracy, stability, and repeatability. Nevertheless, a careful evaluation of the specific requirements of each application should always be carried out before making a final decision.

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Head Mount Thermocouple and RTD Transmitters

A head mount transmitter is a device used in temperature sensing systems that converts the signal generated by a thermocouple or a Resistance Temperature Detector (RTD) into a standardized output signal, typically 4-20 mA or a digital protocol like HART or Foundation Fieldbus. The transmitter is usually installed in a connection head, which is mounted directly on the temperature sensor assembly, hence the term "head mount."

Thermocouples and RTDs are commonly used temperature sensors. Thermocouples work on the principle of the Seebeck effect, where a voltage generates due to the temperature difference between two dissimilar metals. On the other hand, RTDs work on the principle that the electrical resistance of material changes with temperature.

The value provided by a head mount transmitter in temperature sensing includes:

1. Signal conditioning: The raw signal from the temperature sensor (mV for thermocouples or Ohms for RTDs) can be weak and susceptible to noise, interference, and degradation over long distances. The transmitter amplifies and conditions the signal, ensuring a more robust and reliable transmission.
2. Linearization: The relationship between temperature and the sensor output (voltage or resistance) may not be linear. The transmitter linearizes the signal, providing a more accurate representation of the measured temperature.
3. Standardization: By converting the sensor output into a standardized signal, such as 4-20 mA, the transmitter allows easier integration with other process control equipment, like controllers, recorders, and display units.
4. Temperature compensation: The transmitter can compensate for temperature-related errors that may occur in the sensor, such as cold junction compensation in thermocouples, ensuring higher accuracy in the measurement.
5. Digital communication: Some transmitters have digital communication capabilities, allowing for remote configuration, calibration, and diagnostics, as well as providing additional information, such as sensor health and status.

A head mount transmitter is essential in temperature sensing systems, providing signal conditioning, linearization, standardization, temperature compensation, and digital communication capabilities. It improves the temperature measurement system's accuracy, reliability, and performance.

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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:

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Wire-wound and Thin Film Resistance Temperature Detectors

The wire-wound platinum resistance thermometer was patented in 1924 and has since become the most popular device for measuring laboratory and industrial process temperatures. RTDs provide high accuracy, long-term stability, repeatability, and integration with electronic controls. As the temperature rises, so does electrical resistance in a very predictable manner. PRTs measure all sorts of processes, from industry to laboratory work. Additionally, the platinum resistance sensor has a linear resistance vs. temperature relationship over a wide operating range of -196°C to 850°C. 

Wire-wound and thin film sensing elements are the two most common forms of RTD sensing elements. Wire-wound sensing components apply in applications requiring high accuracy and long-term stability. The RTD sensor utilizes platinum wire wrapped around a ceramic core and offers various housings designed to give the optimum heat transfer and contact with the process, regardless of whether it is gas, liquid, or solid.

The thin film RTD sensor style is made by depositing a thin layer of platinum on a ceramic plate, then trimming a path with a laser or other techniques to create a narrow ribbon of platinum with a resistance of 100 ohms at 0°C. After that, the leads are joined and protected with glass. As a result, the sensor is relatively small and available in rectangular shapes in various sizes. They are inexpensive and, if properly packaged, can last for years. Long-term stability and repeatability are not as excellent as with wire wound sensors. 

Each sensor type for industrial applications meets the specifications of ASTM 1137 or IEC 60751. The specifications include an ice point (0°C) resistance and a temperature coefficient. The ice point resistance is calculated in a container using an ice bath made of ice and water. There is a tolerance called "interchangeability" associated with this measurement. Various interchangeability bands have label assignations such as A, B, or C, with A being the strictest and C being less tight.

The temperature coefficient of resistance, or TCR, is the amount of resistance change per degree Celsius change in temperature. The TCR of an industrial grade PRT is 0.00385 ohms/ohm/°C. In other words,  there is an average of 0.385 ohms of resistance per degree C of temperature change between 0°C and 100°C. The coefficients used for lab standards are 0.003925 and 0.003902, but the more widely used standard is now 0.00385 for industrial applications. For an accurate reading, you need to match the temperature coefficient and resistance of your PRT with the input requirements of the instrumentation you are using. Not doing this will result in a significant error.

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

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Industrial Uses of Resistance Temperature Detectors (RTDs) Explained

Introduction: What is an RTD?

RTDs are sensors that measure the temperature of a material and provide an indication of its resistance to thermal changes.

An RTD is a sensor that measures the temperature of a material and provides an indication of its resistance to thermal changes. RTDs can be manufactured as either a wire or as a thin film on silicon.

The first RTD was developed in 1887 by German inventor Hermann von Helmholtz.

RTDs are typically used in industrial applications such as power plants, refineries, paper mills, and steel mills where they monitor temperatures of process fluids, gases, or equipment surfaces.

RTDs have also been used for years in home appliances like ovens and furnaces to control the temperature inside them.

What is a Typical Industrial Use of RTDs?

RTDs are used in industrial settings to measure the temperature of liquids and gases. This is done by measuring the resistance of a metal element which changes with temperature. RTDs have many applications in industry, such as controlling the temperature of devices, monitoring equipment, and testing for leaks.

Industrial use of RTDs can be found in a wide range of industries. For example, they are used to monitor the temperature of food processing plants and oil refineries. They are also used for quality control purposes in semiconductor manufacturing plants and petrochemical factories.

Other Industrial Uses of Resistance Temperature Detectors

Industrial use of RTDs is extremely common in the manufacturing industry. They are used in industrial processes to measure and control temperature, as well as to detect hot spots and cool spots.

RTDs are also used in many engineering applications such as process control, instrumentation, and automation for a variety of purposes.

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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.

Duro-Sense 100 OHM Platinum RTD Temperature Sensors

RTDs (Resistance Temperature Detectors) are temperature detecting devices that vary their resistance value when surrounding temperature changes. RTD sensing elements use well-known materials that change resistance in a predictable and repeatable manner. Their popularity and general use are outcomes of the RTD's predictability and stability. 

The most common type and material of RTD is the 100-ohm platinum sensor. Its use is ubiquitous in the laboratory and industrial process applications going back many decades. The precision, reproducibility, and stability of 100-ohm platinum RTDs (PT100) are well known. 

For the most part, resistance temperature detectors (RTDs) fall into two main categories. Thin-film elements are one form of RTD, and wire-wound elements are the other. Each type provides advantages in certain situations and purposes. The more common design, wire-wound, is a length of tightly coiled wire wrapped around a ceramic or glass bobbin. Because the wire and wrapping are delicate, it is usually enclosed in an encased metallic tube to protect them from stress and vibration. 

The 100-ohm platinum RTD provides accurate temperature readings with excellent stability and repeatability. They are also very resistant to electrical noise, making them ideal for temperature monitoring in industrial facilities near motors, generators, and high voltage equipment. 

The American and European (known as the DIN or IEC standard) 100-ohm platinum RTD standards are the same, with the IEC standard considered the default for PT100. According to the IEC751 standard, the RTD must have:

•  The electrical resistance of 100.00 O at 0°C 
• A TCR (temperature coefficient of resistance) of 0.00385 O/O/°C between 0 and 100°C. 

Because resistance is used to measure temperature in 100-ohm platinum RTDs, the lead wires, connections, and measurement devices contribute extra resistance, requiring external compensation to offset the error. A solution is found by inserting a third or fourth lead wire inversely proportional to the external resistances.

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Fast Selection Criteria between Thermocouples and RTDs

Both RTD and thermocouple probes monitor temperature, but which one is right for your application?

What temperature range are you trying to monitor?

Generally, if the temperature is above a hundred and fifty degrees Celsius, a thermocouple would be used. For anything below a hundred and fifty degrees Celsius, choose an RTD.

What is the required sensor accuracy? 

RTDs provide more accurate readings with repeatable results. Choose RTDs when temperature accuracy and repeatability are critical. 

What is the purchase budget?

Thermocouples can be up to three times less expensive than RTD sensors, making thermocouples the right choice when purchasing large quantities or requirements on tight budgets.

Use these three criteria to narrow down your selection process. There are many other differences between thermocouples and RTDs you need to understand before selection and application.

Always consult a temperature sensor application expert before installing or specifying a thermocouple or RTD where failure can cause harm or personal injury.

100-Ohm Platinum RTD Temperature Sensors

RTDs (short for Resistance Temperature Detectors) are temperature sensing devices that change resistance value as its temperature changes. The most popular RTD is the 100-ohm platinum sensor, used for many years to measure temperature in laboratory and industrial process applications. 100-ohm platinum RTDs (PT100) have a reputation for accuracy, repeatability, and stability.

Most RTD element designs include a length of finely coiled wire wound around a ceramic or glass bobbin. The inherent system is fragile, so it is typically placed inside a sheathed metallic tube to protect from shock and vibration. RTD sensing elements are made from a material with a very predictable and repeatable change in resistance. This predictability and stability is the basis for its widespread application.

The 100-ohm platinum RTD provides accurate temperature readings with reasonable accuracy, excellent stability, and repeatability. They are also significantly immune to generated electrical noise, and as such, they are well suited for temperature measurement in industrial plants, near motors, generators, and high voltage equipment.

There are two 100-ohm platinum RTD standards, the American and the European (known as the DIN or IEC standard), with the IEC standard considered the default for PT100. The IEC751 standard requires the RTD to have an electrical resistance of 100.00 O at 0°C and a TCR (temperature coefficient of resistance) of 0.00385 O/O/°C between 0 and 100°C.

Because 100-ohm platinum RTDs use resistance to measure temperature, the lead wires, connectors, and measuring devices introduce additional resistance. These must be compensated for by configuring the RTD circuit to null out these outside resistances by incorporating a third or fourth lead wire to offset the introduced error.

For more information about 100-ohm platinum RTD temperature sensors, contact Duro-Sense Corporation. Call them at 310-533-6877 or visit their website at https://duro-sense.com.

Temperature Sensors Used for Power Generation

In an electrical generating plant, most temperature measurements are performed with RTDs (resistance temperature detectors) and thermocouples (T/Cs). 

RTD's are sensors that produce a measurable change in electrical resistance, while thermocouples have a change in mV signal in response to temperature change. 

RTD's consist of a thin conductor (nickel, platinum, copper) wrapped around a glass or ceramic bobbin, placed into a protective sheath, and backfilled with an electrically inert material but thermally conductive. 

Power plants use 100-ohm platinum, 100-ohm nickel, 120-ohm nickel, and 10-ohm copper RTDs. Though offering excellent accuracy and long-term reliability, RTDs are vulnerable to mechanical shock and vibration in a generating plant. They are more costly than thermocouples and are generally limited to about 1110 ° F. A very appealing feature for RTDs is their electrical noise immunity, a significant advantage over thermocouples. Finally, inexpensive instrument wire is all that is required to connect the RTD to the measuring instrumentation. 

A thermocouple consists of two wires made of dissimilar alloys, joined at both ends. One junction is coined the "hot junction," the other is the "cold junction" (or reference junction). When the hot junction experiences temperature change, a voltage is produced proportional to the temperature difference between hot and cold junctions. 

T/Cs are made of various alloy combinations and "calibrations" for different temperature ranges. Type J, K and N are the most common thermocouples for power generation applications below 1800 ° F; R and S types are common for applications above 1800 ° F. Besides the evident higher temperature capacity, thermocouples have a quicker response and greater endurance to shock and vibration. However, thermocouples are more susceptible to conducted and radiated electrical noise due to the minute signals generated. Another problem with thermocouples is their deterioration over time when used at high temperatures, hence being less stable than RTDs. One final concern is running an expensive thermocouple extension wire of the same type as the sensor-measuring instrument thermocouple.

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Resistance Temperature Detector (RTD) Catalog

Resistance Temperature Detectors (RTD’s) operate under the principle that the electrical resistance of certain metals increases or decreases in a repeatable and predictable manner with a temperature change. RTD’s may have a lower temperature range than some thermocouples and a slower response time, however, they are more stable and repeatable over long periods of time. RTD’s higher signal output makes them easier to interface with computers and data loggers and reduces the effects of radio frequency interference. RTD’s are used in the plastic processing industry, environmental test chambers, motor windings, pumps and bearings, ovens, kilns, waste treatment and the pulp and paper industry, as well as many other applications.

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Full Duro-Sense Product Catalog Now Available

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Duro-Sense Corporation designs and manufactures the finest quality thermocouples, RTDs, and custom temperature assemblies used in a wide variety of industries including aerospace, medical, power generation, food and beverage, pulp & paper, chemical processing, plastics, pharmaceutical, and life science. Duro-Sense products are known throughout these industries for their quality and reliability.

From simple wire and tube assemblies for the plastics industry, to mineral insulated and multipoint thermocouples, to custom sensors for specific applications, Duro-Sense can design, manufacture, and deliver solutions to virtually any temperature related application.

Precision Thermocouples and RTDs for the Most Demanding Applications

Duro-Sense Corporation provides the highest quality thermocouples and RTDs to the aerospace, aviation, offshore drilling, medical, R&D, power generation, alternative energy, process control,  primary metals, high-tech and OEM industries.

Contact Duro-Sense by calling 310-533-6877 or visit their web site at https://duro-sense.com.

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.

Duro-Sense Corporation: Celebrating Our 40th Year in Business

Duro-Sense designs and manufactures temperature sensors and assemblies used in power generation, plastics production, semiconductor processing, environmental control, packaging, aerospace, medical equipment, foodservice equipment and a myriad of other industries. Duro-Sense partners with customers to optimize their temperature sensing processes, thereby assisting in improving their customers efficiency and profitability.

Since 1979, Duro-Sense has grown exponentially in product capability and market experience. The company today continues to succeed by operating under a simple core value – providing customers with superior products, meticulously engineered for their individual requirements.

Duro-Sense: A Long History of Solving Tough Temperature Sensing Problems

With engineering, design, and support resources available at their headquarters in Southern California, Duro-Sense delivers value-added temperature sensing solutions that improve operations and increase profitability for customers in the power generation, alternative energy, plastics, medical, gas & oil, chemical, refining, mining, agricultural, food service, pharmaceutical, and aerospace industries.

Duro-Sense offers their customers products and services designed to provide outstanding value and cost savings throughout the customer's equipment life span. By integrating the highest quality standards, state-of-the-art machinery, and decades of application experience, Duro-Sense assists customers through:

• Improved product quality
• Optimize asset uptime and performance 
• Lower total cost of operation and maintenance
• Increase equipment reliability
• Improve plant and personnel safety

ENGINEERING AND TECHNICAL SERVICES

Technical Analysis — Duro-Sense can identify temperature sensor operational issues that may be constraining output or elevating operating costs, and then recommend laser-focused solutions.

Reliability and Efficiency Services — Duro-Sense lends their decades of hands-on experience to offer practical temperature sensing solutions that improve the performance, efficiency, and reliability of your process control equipment - all while lowering your total cost of ownership.

Loop Design, Integration and Engineering Support — Duro-Sense engineers engage with their customers, providing support for grassroots project planning, system design, or project management requirements.

Equipment Life Cycle Optimization — Through a combination of assessments and technology, Duro-Sense experts help customers benchmark operational performance, define key metrics, and implement precise sensor solutions to achieve long-term operational goals.

Intelligent Product Design — By employing an array of sophisticated products, services, and software that collects, examines and understands data, Duro-Sense helps customers use predictive analytics to take action and improve asset reliability and reduce downtime.

Have a challenging temperature sensing requirement? Call Duro-Sense.

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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.

A Pro and Con Comparison of Thermocouples and RTDs

Thermocouple Advantages 

• Inexpensive
• Wide temperature range
• Various types, sizes and application methods
• Remote read back
• Read back electronics can be simple
• Usable in virtually any environment

Thermocouple Disadvantages

• Requires cold junction compensation
• Slow response time
• Not as accurate as many other devices without good CJC and calibration
• Susceptible to noise
• Connection cable/wire is expensive compared to copper conductors
• Cable/wire length is limited

RTD Advantages

• More linear than thermocouples
• Cold junction not an issue
• Special cable/wire not needed
• Cable/wire length can be much longer than TC’s
• Better noise immunity
• More stable over time than thermocouples
• Remote read back
• Usable in virtually any environment

RTD Disadvantages

• More expensive than thermocouples
• More delicate than thermocouples unless encased
• Not as wide of temperature range as thermocouples
• Requires more conductors per device
• Read back electronics more complex

Theory of RTD Operation

An RTD is a temperature measuring device that changes resistance with temperature change, rather than changing voltage, as with a thermocouple.

Most commonly used is the platinum 100 ohm RTD because of their stability in air and linearity. Their resistance is 100 ohms @ 0 Deg.C and increases with temperature.

Common terms associated with RTD’s are temperature coefficient or alpha, and tolerance class.

Alpha is ohms per ohm per Deg.C.
The average resistance change per unit of temperature from boiling point to ice point of water:

• R_boiling – R_{ice point}/100deg/100ohms
• 138.5 – 100.0/100/100 = .00385

Tolerance class is the amount an RTD will differ from the standard resistance curve per Deg.C.

• Class A (+/- .15 + .002*t)
• @ temp of 100DegC = +/- .35DegC

When ordering an RTD, a tolerance class will be part of the order, dependent on the application. IEC 751 stipulates that the RTD be marked with their nominal R0 value, their tolerance class, the wiring configuration and the temperature range.

3-wire configuration

• Pt100 / A / 3 / -100/+200  = Platinum 100 Ohm / Class A / 3-Wire / -100 to +200 Deg.C

The most common RTD configuration is the 3-wire type. This configuration is more than adequate for 99.9% of applications. If absolute accuracy is needed, a fourth wire can be introduced, but rarely is it worth the added cost.

2-wire configuration

Another configuration is a two wire RTD with a stand-alone loop. (Probably rarely used today).

Since the RTD is a resistance device, the resistance of the wires used to connect the RTD to the measurement meter introduces errors and must be known. This is the reason a third (or fourth), wire is used.

3rd wire used to cancel wire error

First the meter reads the resistance of the two common wires to determine the value of R_wire. For a three wire RTD, it is assumed that this resistance is the same as that of one common and one non-common wire.

Then the meter reads the resistance of one of the common wires, the RTD, and the non-common wire to determine R_total

Meter reading 2 common wires

Meter electronics and software then subtract R_wire from R_total to get R_t which is then converted to a temperature.








Rt = R_total – R_wire