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:

Duro-Sense Corporation
310-533-6877
https://duro-sense.com

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.

Duro-Sense Corporation
310-533-6877
https://duro-sense.com

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.

Duro-Sense Corporation
310-533-6877
https://duro-sense.com

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.

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.

DOWNLOAD THE DURO-SENSE RTD CATALOG HERE

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

Platinum Resistance Temperature Detectors

Platinum Resistance Thermometers Detectors (RTDs) rely on the fact that platinum, like many other metals, exhibits increased electrical resistance as temperature rises. For example, a conventional RTD designed to provide 100 Ohms at 0 °C has a resistance in the neighborhood of 80 Ohms at -50 °C and 120 Ohms at 50 °C, or a sensitivity of about 0.4 Ohms per degree. RTDs constructed to particularly exacting specifications, termed Standard Platinum Resistance Thermometers, are named as the defining measurement tools for interpolating temperatures under ITS-90. In general, RTDs can have high accuracy (0.01 °C), stability, and repeatability across a wide range of temperatures from -200 °C to 500 °C.

Typically the platinum element is formed into thick or thin films, or the platinum wire is arranged in two, three or four helical coils (see diagram, right) – the more coils, the higher the sensitivity. The film or wire is placed inside a glass or ceramic enclosure, and can be supported by loose or compacted MgO. Platinum-based leads connect the probe unit to the thermometer electronics, which convert the electrical signal to temperature.

RTDs are broadly divided into two groups: Industrial RTDs and Standard Platinum Resistance Thermometers, depending on sensitivity and robustness. ASTM and IEC define several classes of RTDs, each with a different set of specifications. An ASTM "Class A" unit, for example, has an out-of-the-box tolerance — maximum permissible error — that ranges from 0.47 °C at -200 °C to 0.13 °C at 0 °C to 0.98 °C at 500 °C.

Advantages

• Wide temperature range
• Resistance-temperature relationship is well characterized.
• Rugged construction in industrial RTDs
• Available in different shapes and sizes – application specific
• Can be used with a digital temperature read-out device.

Disadvantages

• Mechanical shock and vibration will cause drift.
• Deterioration at elevated temperatures (e.g., >500 °C)
• 2-and 3-wire devices need lead-wire compensation.
• Non-hermetically sealed RTDs will deteriorate in environments with excessive moisture.

Post abstracted from "Mercury Thermometer Alternatives: Platinum Resistance Thermometers (PRTs)" by NIST.

Precision RTD's (Resistance Temperature Detectors)

Duro-Sense RTDs, thermowells, and accessories provide high quality solutions to the aerospace, aviation, process control, medical, R&D, power generation, alternative energy, plastics, primary metals, high-tech and OEM industries.

https://duro-sense.com
310-533-6877

Temperature Sensor Basics: RTDs (Resistance Temperature Detectors)

RTD temperature sensor with
threaded connector (Duro-Sense)

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 offer considerably higher accuracy and repeatability than thermocouples and can be used up to 600 Deg. Celsius. 

Simple RTD diagram (courtesy of Wikipedia)

The RTD wire is usually a pure metal such as platinum, nickel or copper because these metals have a predictable change in resistance as the temperature changes. They are normally designed as a fine wire coiled around a bobbin (made of glass or ceramic), and inserted into a protective sheath. Because they are made of pure metals, they tend to more costly than thermocouples. RTD’s do need to be supplied an excitation voltage from the control circuitry as well. 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 many industries including the plastic processing industry, environmental test chambers, motor windings, pumps and bearings, ovens, kilns, waste treatment and the pulp and paper industry.  Because of their accuracy and repeatability, they are also commonly used in biomedical applications, aerospace, and semiconductor processing.

https://duro-sense.com
310-533-6877