Temperature Sensors: RTDs and Thermocouples

What Is a Temperature Sensor?

An industrial temperature sensor consists of a sensing element housed in a protective metal sheath, the lead wires that connect this element to the external circuit, and a mounting adapter that fixes the sensor to the process or to a connection head. Two technologies dominate in process plants: resistance temperature detectors (RTDs) and thermocouples. In an RTD, the electrical resistance of a platinum element rises with temperature, and the temperature is calculated from the measured resistance. In a thermocouple, two dissimilar metals are joined at a measuring junction, and the temperature difference between this junction and a reference junction generates a small voltage.

The sensor never works on its own in the measuring chain. In most installations a thermowell shields it from flow forces, pressure and corrosive media, while a temperature transmitter or a control-system input card linearizes its raw signal and converts it into engineering units. Sensor selection should therefore be considered together with the bore and length of the thermowell, the input type of the transmitter and the design of the connection head.

The deciding parameters are the measuring range, the required tolerance class, the level of vibration and mechanical shock, the response time needed and the hazardous-area classification of the installation point. As a general rule, RTDs are preferred up to 600 °C where high accuracy and long-term stability matter; thermocouples are chosen for higher temperatures or where a very fast response is required.

How RTD and Thermocouple Sensors Work

RTDs and thermocouples detect temperature through different physical effects, and this difference defines both the achievable accuracy and the wiring method. The measuring chain can be summarized in the following steps:

1.      Resistance change in the platinum element: The resistance of platinum increases as temperature rises, and this relationship is stable and repeatable over a wide range. The resistance-temperature curve of a Pt100 element is defined by a temperature coefficient of α = 0.00385 — the curve most widely used internationally.

2.      Lead wire resistance compensation: In an RTD, the lead wires form part of the measuring circuit. With a 3-wire connection, the third wire compensates for lead resistance on the assumption that all wires have equal resistance. With a 4-wire connection, a constant current of roughly 150 µA flows through two wires while the voltage is picked up on the other two by a high-impedance circuit, so lead resistance has no influence on the reading.

3.      Thermoelectric voltage from the Seebeck effect: In a thermocouple, two dissimilar metals are joined at the measuring point (hot junction). A non-linear voltage develops that is proportional to the temperature difference between the hot junction and the reference point (cold junction). Its magnitude depends on the metal pair and sets the measurement resolution.

4.      Cold junction reference and signal processing: The cold junction is the transmitter, input card or signal conditioner where the voltage is measured; its temperature is known and compensated for. The transmitter converts the RTD resistance or thermocouple voltage into temperature using the relevant standard curve or sensor-specific constants.

Thin-Film and Wire-Wound RTD Elements

Thin-film elements consist of a fine platinum pattern deposited on a ceramic substrate and are more resistant to vibration and mechanical shock. The standard thin-film element operates from –50 to 450 °C and the high-temperature thin-film element from –60 to 600 °C. In wire-wound elements, high-purity platinum wire is coiled in a helix inside a ceramic body; with a range of –196 to 600 °C, they suit low-temperature duties below –60 °C and points that call for a tighter tolerance.

Thermocouple Types and Operating Atmosphere

Type E (nickel-chromium/constantan) has the highest output of the five types, at about 68 µV/°C, and is recommended for oxidizing or inert atmospheres. Type J (iron/constantan) is used in vacuum, reducing and inert atmospheres and becomes brittle below 0 °C. Type K (nickel-chromium/nickel-aluminum) is a general-purpose type, mostly used above 538 °C in oxidizing or neutral conditions. Type N (nickel-chromium-silicon/nickel-silicon) offers higher thermoelectric stability than Type K but must not be used in vacuum or reducing atmospheres. Type T (copper/constantan) combines good linearity with resistance to moisture at cryogenic and moderate temperatures.

RTDs provide higher accuracy and long-term stability; thermocouples provide a wider temperature range and a shorter response time. Which technology is selected depends on the process temperature and on whether the point serves control or monitoring.

Key Features

The sensor family is configured so that a single mechanical platform can be adapted to very different process points.

·         One standard outside diameter: All sensor types and lengths share a 6 mm (¼-inch) nominal outside diameter, which simplifies spare-parts stocking and makes it easier to match the thermowell bore.

·         Three Pt100 element designs: Options include a vibration-resistant thin-film element, a high-temperature thin-film element and a wire-wound element that holds a tight tolerance at low temperatures.

·         Five thermocouple types: Type E, J, K, N and T thermocouples are supplied with a 321 stainless steel or Alloy 600 sheath, depending on the measuring range and operating atmosphere.

·         Choice of tolerance class: RTDs are available to IEC 60751 Class A or Class B; thermocouples to IEC 60584 Class 1/Class 2 or to ASTM E230 Special/Standard tolerance.

·         Single and dual elements: Single-element 3- or 4-wire and dual-element 3-wire RTDs, plus single or dual, grounded or ungrounded (isolated) thermocouples, make redundant measurement possible.

·         Constant contact with the thermowell tip: Spring-loaded adapters are based on 13 mm of spring compression, and each sensor is manufactured with an extra 6 mm over the length ordered; the tip is pressed against the bottom of the thermowell, improving response time and vibration performance.

·         Secondary seal with the welded adapter: In the welded adapter the sensor capsule is welded to the adapter. This joint allows direct immersion and, in a thermowell installation, forms a secondary process seal rated for 3500 psi.

·         Lead wire extension options: Twisted, PTFE-wrapped shielded or PVC/PTFE-jacketed armored cable extensions allow the transmitter to be moved away from heat sources or to an accessible location.

·         Calibration and CVD constants: RTDs can be supplied with a single-point resistance calibration, or with a calibration and certificate giving Callendar-Van Dusen constants over standard or custom ranges.

·         Automatic recognition via sensor ID tag: An optional ID tag built into the sensor hands over the calibration data, element type and wire count to a compatible transmitter, eliminating manual entry errors.

Technical Specifications

The values below are typical data for the RTD and thermocouple sensor family covered on this page. Final values should be confirmed at project stage for the selected element type, tolerance class, mounting adapter and approval option.

The temperature range stated for each sensor type represents the element's complete working range; a tolerance class applies only within the range for which that class is defined. Compact adapters are not explosion-proof and are not used in Division 2/Zone 2 approved configurations.

Key Advantages

·         The right technology for each point: Offering both RTD and thermocouple options on the same mechanical platform allows an informed trade-off between accuracy and temperature range at every measuring point.

·         Measurable system accuracy: Entering sensor-specific CVD constants into the transmitter replaces the ideal curve with the sensor's actual curve, typically improving the accuracy of the measuring point by a factor of three to four.

·         Redundant measurement: Dual-element RTD and thermocouple options keep critical points measuring if one element fails and allow the deviation between the two elements to be monitored.

·         Easy sensor replacement: In a thermowell installation, the sensor can be removed for calibration or replacement without shutting down the process; the spring-loaded adapter ensures that the new sensor seats against the bottom of the well.

·         Suited to demanding site conditions: Vibration-resistant thin-film elements, armored cable extensions and stainless steel connection heads support dependable operation at vibrating, corrosive or hard-to-reach points.

·         Standardized wire colors: RTD wire colors follow IEC 60751 and thermocouple wire colors follow IEC 60584 or ASTM E230, reducing the risk of wiring mistakes in the field.

·         Complete measuring point supply: The sensor can be supplied assembled with the thermowell and transmitter, process-ready — torqued and wired.

Application Areas

RTD and thermocouple sensors are used wherever temperature must be measured continuously and reliably for process monitoring and control. The application groups below summarize the situations in which the choice of element type matters most.

Process Monitoring and Control Loops

For temperature control of reactors, heat exchangers, columns and pipelines, Pt100 RTDs are usually preferred for their tight tolerance and long-term stability. On critical loops, dual-element sensors provide redundant measurement.

Liquid and Gas Custody Transfer

The sensors are offered with calibration options that meet MID requirements for liquid and gas custody transfer; these options depend on a 4-wire connection and a minimum sensor length. They are used for temperature compensation in flow measurement.

Cryogenic and Low-Temperature Applications

Down to –196 °C, wire-wound RTDs or Type T thermocouples are used. Low-temperature connection head options are available for ambient temperatures down to –51 °C and –60 °C.

High-Temperature Process Points

Above 600 °C, Type K or N thermocouples with an Alloy 600 sheath measure up to 1,200 °C. The transmitter electronics must be protected from process heat by an extension or remote mounting.

Lines with Vibration

At vibrating points such as pump and compressor discharges, thin-film RTD elements and spring-loaded adapters that press the tip against the thermowell bottom increase mechanical robustness and measurement continuity.

Hazardous (Classified) Areas

Flameproof connection heads and intrinsically safe sensor configurations are intended for gas and dust atmospheres covered by ATEX and IECEx. The scope of approval is determined by assessing the sensor, head and cable gland together.

How to Select a Temperature Sensor

Sensor selection determines both the accuracy and the maintenance needs of the measuring point. For a technical assessment we recommend clarifying the following:

Process Conditions

·         Minimum, normal and maximum process temperature (limits of the element type and sheath material)

·         Process atmosphere: oxidizing, reducing, inert or vacuum (for thermocouple type selection)

·         Level of vibration and mechanical shock

·         Whether a thermowell will be used or direct immersion is required

Accuracy and Element Configuration

·         Required tolerance class (IEC 60751 Class A/B, IEC 60584 Class 1/2 or ASTM E230)

·         3-wire or 4-wire RTD connection; 4-wire preferred for long cable runs

·         Single or dual element (need for redundant measurement)

·         Thermocouple junction: grounded (fast response) or ungrounded (isolated, less susceptible to noise)

·         Need for a calibration certificate and CVD-based sensor-transmitter matching

Mechanical Design and Mounting

·         Thermowell lagging length (H) and immersion length (U); sensor length L = H + U

·         Mounting type: spring-loaded, welded, compression fitting or DIN plate

·         Extension length or lead wire extension to protect the electronics from process heat

·         For a replacement sensor: length measured with the spring uncompressed

Electrical Connection and Hazardous Area

·         Connection head material (aluminum/stainless steel), need for a display cover and cable entry thread

·         Head-mounted transmitter or terminal block for remote mounting

·         Area classification and required protection type (flameproof, intrinsically safe, Zone 2, dust)

·         Minimum ambient temperature and low-temperature head option

Sensor-transmitter or sensor-thermowell assemblies that are only hand-tightened do not meet hazardous-area approval requirements; process-ready, torqued assemblies should be specified for these areas. To keep their dust and ingress protection ratings, spring-loaded and DIN-style sensors need to be fitted inside a thermowell.

Tolerance Standards and Hazardous-Area Approvals

The performance, tolerance and safety characteristics of the sensors are defined by the standards and approvals below. The applicable scope depends on the ordered configuration.

The scope of approval is determined by assessing the sensor, connection head, cable gland and mounting type together, and should be confirmed for each project with the TLY Enerji engineering team.

Temperature Sensor Solutions from TLY Enerji

At TLY Enerji we do not treat the temperature sensor as an isolated component, but as part of a system that works together with the thermowell, the transmitter and the control system. Our engineers evaluate process temperature, atmosphere, vibration and area classification data to decide between RTD and thermocouple and to define the element type, tolerance class and wiring configuration.

In existing plants we help determine replacement sensor lengths, check compatibility with thermowell lagging and immersion lengths, select extension lengths and transfer sensor-specific calibration constants into the transmitter. Working alongside site teams through project documentation, PLC/DCS integration and commissioning, we aim for a measuring point that delivers accurate, traceable data from day one. Rationalizing the temperature sensor types used across different units and standardizing spare parts are also part of this approach.

Frequently Asked Questions

What is the difference between an RTD and a thermocouple?

An RTD works on the principle that the resistance of a platinum element changes with temperature, and it offers higher accuracy, repeatability and long-term stability; the RTDs on this page cover –196 to 600 °C. A thermocouple uses the thermoelectric voltage generated where two dissimilar metals meet; it can measure up to 1,200 °C and responds faster, especially in the grounded design. RTDs are favored for control and high-accuracy points, thermocouples for high temperatures.

What does Pt100 mean?

Pt100 refers to an RTD with a platinum (Pt) sensing element that has a nominal resistance of 100 Ω at 0 °C. Its resistance-temperature relationship is defined by a temperature coefficient of α = 0.00385, the curve most widely used internationally. Pt100 sensors are built with either a thin-film or a wire-wound element.

What is the difference between Class A and Class B RTDs?

Class A and Class B are interchangeability tolerances defined in IEC 60751. At 0 °C, for example, Class A allows ±0.15 °C and Class B ±0.3 °C; at 100 °C the figures are ±0.35 °C and ±0.8 °C respectively. Class A applies from 0 to 300 °C for thin-film elements and from –100 to 450 °C for wire-wound elements. The high-temperature thin-film element is offered in Class B only.

Should I choose a 3-wire or a 4-wire RTD?

A 3-wire connection compensates for lead resistance on the assumption that all wires are equal, which is sufficient for many applications. In a 4-wire connection, current and voltage travel on separate wires, so lead resistance does not affect the measurement; it should be preferred for long sensors or cable runs and for high-accuracy points. If required, a 4-wire sensor may also be wired as a 2- or 3-wire sensor.

What is the difference between grounded and ungrounded thermocouples?

In a grounded thermocouple the measuring junction touches the sheath, so it responds faster (T90 about 4.0 s in flowing water) but is more exposed to noise from ground loops. In an ungrounded (isolated) thermocouple the junction is insulated from the sheath; the reading is more stable, but the response time is longer (T90 about 7.3 s). The same distinction applies to dual-element versions.

Which thermocouple type suits which atmosphere?

Type K is a general-purpose thermocouple used up to 1,200 °C in oxidizing or neutral atmospheres. Type N covers the same range with higher thermoelectric stability but is not suitable for vacuum or reducing atmospheres. Type J is intended for vacuum and reducing conditions; Type E gives the highest output voltage; Type T offers good linearity at cryogenic and low temperatures.

How is the right sensor length for a thermowell determined?

For new installations, the sensor length is the sum of the thermowell lagging length (H) and immersion length (U): L = H + U. To replace an existing spring-loaded sensor, measure it with the spring uncompressed from the tip to the point 13 mm into the thread engagement, then subtract 6 mm. Spring-loaded sensors are made a little longer than the length ordered so that they bottom out in the thermowell.

How does sensor-transmitter matching improve accuracy?

The actual resistance-temperature curve of every RTD differs slightly from the ideal curve. By testing the sensor at several temperatures, its Callendar-Van Dusen constants (R0, α, δ, β) are determined and then entered into the transmitter at the factory or in the field. The transmitter then uses the sensor-specific curve, typically improving the accuracy of the measuring point by a factor of three to four.