Resistance temperature detector

[1] A resistance thermometer/resistance temperature detector (RTD) is a positive temperature coefficient device sensor using substance's electrical resistance (like platinum) to measure temperature. Many RTD elements consist of a length of fine wire wrapped around a heat-resistant ceramic or glass core, etc. A RTD wire is often platinum (Pt), nickel (Ni), or copper (Cu). The material has accurate temperature–resistance relationship, used to indicate temperature.

[1] Ruthenium oxide is often used to make ones working at cryogenic temperatures. As RTD elements are fragile, they are often housed in protective probes called thermowells. They have higher accuracy and repeatability than thermocouples, as to why they are replacing them in industry usages under 600 °C.

"fine" wire: extremely thin wire, often 0.05 mm-0.127 mm diameter

Operation

[2] RTD sensors measure material resistances (often platinum) as temperature rises, causing the material resistance rise, allowing for precise temperature measurement based on such resistance change.

They often have either platinum, nickel, or copper wires, as such materials have a positive temperature coefficient, meaning temperature rise causes increased resistance, which is used to detect and measure temperature changes.

RTDs measures temperature by correlating it to its metal element's electrical resistance. As sensor's temperature rises, its resistance to current increases in predictable predictably.

[4]

[4] 2-wire RTDs uses 1 lead wire with 2 ends connected to a monitoring device. But since along the circuit/wires, the total resistance also includes wire resistances and RTD's element resistance, the readings will also have some errors. This is minimized by a larger gauge wire

3-wire RTDs use 3 wires to cancel out electrical resistance of the long cables connecting sensor to your controller, preventing false temperature readings.

[4] I.e., The 3rd wire measures how much resistance the wire itself creates ten substractes the difference so that only the element resistance is read by the system without only the wire's.

RTD and thermistors differences

What differs RTDs from thermistors involves in their material, temperature range, accuracy, linearity:

Materials

RTDs made of pure metals, often platinum.
Thermistors are made of treated ceramic materials or metal oxides.

Temperature range

RTDs measure wide temperature ranges, -200°C to 850°C.
Thermistors measure much smaller ranges, often -50°C and 150°C.

Accuracy and linearity

RTDs change resistance in a steady straight line as temperature rises, thus being predictable and accurate across various temperatures.
Thermistors have big and sudden resistance change, being very sensitive and precise, but only within small temperature spans.

Applications

RTDs are used in industrial settings, labs, and factories where tough and reliable measurements are needed across big temperature changes.
Thermistors used in daily electronics like toasters, coffee makers, and car engines to monitor specific temperature limits safely and cheaply.

Types

[2]

Platinum RTD

[2] A platinum RTD (Pt RTD) is identified by the element symbol "Pt" with a value denoting the nominal resistance at 0 °C, like Pt100 or PT100. A 100 Ω Pt RTD is of the most used types, often in industrial uses.

If unsure for the Pt RTD's functionality, learn how to check its performance by via resistance measurements at various temperatures.

They're popular due to great corrosion resistance, long-term stability, and broad temperature measurement range of -200…+850 °C. A Pt RTD is better for high-accuracy uses, but it's often more costly.

Nickel RTD

[2] A nickel RTD (Ni RTD) is identified by the prefix "Ni" with a value denoting the nominal resistance at 0 °C, like Ni120. They're cheaper than platinum and have good corrosion resistance. But, nickel ages lifespan and loses accuracy at higher temperatures. Nickel is limited to a measurement range of -80…+260°C.

Copper RTD

[2] A copper RTDs (Cu RTD) is identified by the prefix "Cu" with a value denoting the nominal resistance at 0 °C, like Cu10.

They have the best resistance to temperature linearity of the 3 RTDs. Copper is cheap but oxidizes at higher temperatures. Copper resistance vs temperature relationship helps in accurate measurements within limited temperature range, making it suitable for less demanding applications. Copper is limited to a measurement range of -200…+260°C.

Components

[D1] RTDs can have 2, 3, or 4 wires to compensate for lead wire resistance and have a range of -200-750 °C, but depends on protective sheathing material.

Most are covered stainless steel.

compensate (engineering): to actively adjust/offset errors

[2] RTDs are built in 3 ways: Wire wound RTDs, coiled element RTDs, and thin film RTDs. Understanding how they work in each of these methods is crucial to identify the ideal construction for your application.

Knowing to test an RTD sensor is crucial to verify accuracy and reliable performance.

Note: Higher AWG wire, longer length wire, causes more resistance in the lead wire. 5 ft of 26 AWG has 0.204 Ω resistance.

[2]

Wire wound RTD

[2] In wire wound RTDs, a resistance wire wraps around a non-conducting core, often made of ceramic (since ceramic base (substrate) can withstand high temperatures without damage), til it's 100 Ω resistance. Manufacturers trims the resistance wire's length to reach the resistance at 0°C, called the “R0” resistance. E.g., the Pt100's R0 resistance is 100 Ω.

substrate: base material/surface supporting electric component

Such construction offers high precision level and is ideal in usages for accurate and stable temperature measurements. Wire wound RTDs are often for systems where precise temperature monitoring is crucial.

[2] Lead wires are attached to the resistance wire, and then a glass or ceramic coating is applied over the wire for protection. As temperature increases, the length of resistance wire increases slightly. Care must be taken in the design to ensure that the resistance wire does not twist or otherwise deform as temperature increases. This is because mechanical strain causes a change in wire resistance.

Lab-grade RTDs used by calibration and standards laboratories eliminate this source of error by loosely winding resistance wire around a non-conducting support structure. This type of RTD can be extremely accurate, but is fragile and not suited for most industrial applications.

[3]

Numbers of wires

[D1] 2 wire leads aren't as accurate or common as 3 wire RTDs.

3 wire RTDs reduce the error caused by lead wire resistance.

4 wire RTDs are more common in scientific uses where accuracy is crucial.

[3]

[3] 2-lead RTDs cause leadwire resistance to the element resistance, thus the temperature reading is high. This graph shows the temperature error, from 2 leads of variopuos sizes and lengths, for a 100 Ω Pt RTD at 100 °C.

Coefficients

RTDs have resistance coefficients (α) to tell you how much the resistance changes per degree temperature change. Usual Pt RTDs (like Pt100) have coefficient α of 0.00385 Ω/Ω/°C (IEC standard), i.e., its resistance increases by ~0.385% per °C.

E.g., A Pt100 is 100 Ω at 0°C. At 1°C → ~100.385 Ω

References

My documents

[D1] ELE8931 Week 2 theory (Canva)

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Resistance temperature detector

Operation

RTD and thermistors differences

Types

Platinum RTD

Nickel RTD

Copper RTD

Components

Wire wound RTD

Numbers of wires

Coefficients

References

My documents

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