Pt100 Sensor – What It Is, How It Works & Applications

What is a Pt100 Sensor?

A Pt100 sensor is a type of platinum resistance temperature detector (RTD) that has a resistance of exactly 100 ohms at 0°C. The term “Pt” stands for platinum, and “100” refers to the resistance value in ohms. Pt100 sensors are widely used in industrial temperature measurement thanks to their excellent accuracy, repeatability, and long-term stability.

Pt100s operate according to the principles defined by IEC 60751, with a standard resistance change of approximately 0.385 Ω/°C across the 0°C to 100°C range. These sensors are commonly referred to as RTDs (Resistance Temperature Detectors) or PRTs (Platinum Resistance Thermometers).

We offer a wide range of Pt100 sensors designed and manufactured in the UK to meet industry standards. For applications requiring higher resistance, we also supply Pt1000 sensors.

IEC 60751 Standards & Pt100 Sensor Tolerances

The IEC 60751 standard governs the design and performance of Pt100 sensors and other platinum resistance temperature detectors (RTDs) used in industrial applications across the UK and worldwide. Under this standard, an RTD includes the sensing element, internal wiring, external terminals, and any protective sheath if present. Connection heads and mounting hardware may also be part of the assembly.

IEC 60751 applies primarily to sheathed RTDs operating within a temperature range of −200°C to +850°C. It defines two tolerance classes: Class A and Class B. These classes specify the maximum allowable deviation from the ideal temperature–resistance values as follows:

• Class A: ±0.06 Ω or ±0.15°C at 0°C
• Class B: ±0.12 Ω or ±0.3°C at 0°C

Most Pt100 RTDs use platinum wire with a temperature coefficient of α = 3.85 × 10⁻³/°C. While standard sensors typically have a nominal resistance of 100 ohms at 0°C, 10-ohm versions are also specified for high-temperature use (above 600°C) and use thicker wire for stability.

Note that for two-wire Pt100 configurations, the Class A tolerance does not apply due to the influence of lead wire resistance. Additionally, for 100-ohm sensors, Class A accuracy is valid only up to +650°C.

The standard outlines various requirements for sensor performance but does not prescribe construction methods. It ensures compatibility with both DC and AC measurement systems (up to 500 Hz), which introduces constraints related to inductance and electromagnetic coupling.

IEC 60751 also defines performance in areas such as:

• Insulation resistance and dielectric strength
• Response time and self-heating effects
• Immersion depth requirements and associated measurement error
• Thermo-electric effects and hysteresis testing
• Mechanical vibration, pressure resistance, and temperature cycling

Manufacturers may optionally declare additional electrical characteristics including:

• Thermometer inductance and capacitance
• Capacitance to earth
• Resistance of internal wiring
• Minimum usable immersion depth
• Self-heating power coefficients

This level of standardisation makes Pt100 sensors conforming to IEC 60751 fully interchangeable—a major advantage when sourcing or replacing sensors in critical UK-based industrial applications.

Basic Sheathed Pt100 RTD Assemblies

Our sheathed Pt100 RTD sensor assemblies are designed as cost-effective, general-purpose solutions for a wide range of industrial temperature measurement applications. These sensors typically operate across a temperature range of −100°C to +350°C, although custom versions can be manufactured for extended ranges.

As standard, each sensor contains a 100 ohm platinum element at 0°C, in line with IEC 60751 Class B tolerances. Optional Class A elements with enhanced accuracy—typically down to ±0.01%—are also available. These elements use high-purity, strain-free, doped platinum wire, encased in ceramic or glass insulation envelopes. Both thin-film and wire-wound RTD types are supported depending on the required stability and temperature range.

Sheathed assemblies can be configured with single, duplex, or triplex RTD elements, and are supplied with 2-wire, 3-wire, or 4-wire connections as standard. Four-wire configurations are ideal for precision bridge or potentiometric circuits and can be connected in either compensated or blind-loop format. Lead wires are typically made from Kapton®-insulated copper for temperature and chemical resistance.

A typical minimum immersion depth is around 50 mm. Recommended measuring current is below 5 mA to reduce self-heating effects. Insulation resistance is excellent, typically exceeding 100 MΩ at 240 V under ambient UK laboratory or factory conditions.

Tip designs are highly customisable to suit the application. These include reduced-diameter tips, thin-wall sheaths, and pierced shroud versions for measuring air and gas temperatures. The internal sheath is packed with inert insulating material to optimise heat transfer and reduce vibration sensitivity. Each unit is hermetically sealed to prevent moisture ingress and ensure long-term durability in harsh or corrosive environments.

Sheath diameters are available from 0.5 mm to 13 mm in virtually any length or bend configuration required. Standard construction materials include 316 stainless steel, with optional upgrades to Inconel 600, Incoloy 800, nickel, and other specialist alloys. Fluoroplastic cladding is also available for demanding chemical or high-humidity environments.

A wide variety of end seals and terminal options are offered to match your requirements. These range from basic lab connectors and quick-release plugs, to heavy-duty industrial enclosures with protection tubes, thermowells, transmitters, and head-mounting systems. Whatever your UK application—from food production to petrochemical processing—we can supply or manufacture a compatible Pt100 assembly.

Mineral Insulated Metal Sheathed Pt100 RTD Assemblies (MI / MIMS)

One of the most popular formats for industrial Pt100 RTD sensors in the UK is the Mineral Insulated Metal Sheathed (MI or MIMS) type. This design is similar in construction to a mineral insulated thermocouple, consisting of a seamless metal sheath filled with compacted magnesium oxide (MgO) powder. The powder insulates and supports the RTD element and lead wires, forming a hermetically sealed, high-integrity probe.

MI Pt100 probes are compact, rugged, and flexible. Their semi-flexible construction allows them to be bent, twisted, and routed without damage, making installation easy even in confined or awkward spaces. Key benefits include:

• Excellent mechanical strength and self-armouring
• High insulation resistance (typically >100 MΩ)
• Fast response times with stable, long-term accuracy
• Reliable performance in high-vibration, vacuum, and corrosive environments

These sensors typically operate across a temperature range of −100°C to +600°C and use 100-ohm Class B RTD elements in either single or duplex configuration. Options are also available with Class A, 1/3 DIN, 1/5 DIN, or 1/10 DIN tolerance elements for higher accuracy requirements.

Like other sheathed RTDs, MI Pt100 sensors support 2-wire, 3-wire, or 4-wire connections as standard. The lead-out wires are typically made from insulated copper, and the recommended energising current is below 5 mA. Minimum immersion depth is approximately 50 mm to ensure measurement accuracy and thermal stability.

The typical sheath wall thickness is around 15% of the outer diameter, allowing good resistance to kinking or splitting. The construction supports bending with a radius of about 12 × OD (or down to 4 × OD when using a mandrel). Common diameters include 3 mm, 6 mm, and 8 mm, with custom lengths available to suit virtually any industrial application.

Standard sheath materials include 316 stainless steel and Inconel 600, offering excellent corrosion resistance. Inconel 600 sheaths are ideal for aggressive and carburising atmospheres (excluding high-temperature sulphur exposure). Optional fluoroplastic coatings can be applied for additional protection in chemically corrosive environments.

A wide selection of end seals, connectors, and mounting accessories is available, ranging from basic lab connectors to full industrial protection tubes, head-mount transmitters, and thermowell assemblies. These probes are cost-effective, readily available across the UK, and ideal for OEM or replacement use.

One important consideration is moisture ingress. If improperly manufactured or specified, MI RTDs may suffer from reduced insulation resistance due to water vapour, leading to instability or premature failure (commonly referred to as RTD poisoning). This risk is entirely avoidable with proper quality control and specification at point of order.

Extension Leads and Connectors for Pt100 RTD Sensors

Extension cables for RTD temperature sensors — including Pt100 assemblies — are available in various configurations and insulation types. Most follow the IEC 60751 colour coding standard for RTD wiring. The most common cable features three cores of stranded, silver-plated copper, interwoven and protected with a braid screen and durable outer sheath.

Depending on your installation and temperature range, different RTD cable types are available:

• PVC insulated RTD cable – Up to 105°C. Basic construction with tinned copper screen; cost-effective and flexible.
• PFA insulated cable – Up to 260°C continuous (300°C short term). Features nickel-plated copper braid for increased durability and chemical resistance.
• Fibreglass insulated RTD cable – Rated to 480°C. Includes stainless steel overbraid; ideal for high-temperature and harsh industrial environments.

Cable configurations include single-core, two-core, three-core (standard for Pt100), four-core, and six-core assemblies — enabling flexible options for both single and duplex RTD installations.

Although RTD sensors are less sensitive to extension cable characteristics than thermocouples, careful cable selection is still important. The main goal is to transmit low-level resistance signals with minimal interference or signal loss. Electrical noise, contactor switching, and thermoelectric effects can all impact accuracy if inappropriate materials or poor-quality terminations are used.

It's also important to avoid mismatched conductor materials at junctions, especially where potentiometric (voltage-based) measurements are involved, as this can introduce small but significant measurement errors.

As with thermocouple systems, RTD probes should be terminated using high-integrity industrial connectors designed for low-resistance, stable signal transmission. These are often the same connectors used in thermocouple installations and should be chosen based on environment, temperature, and sealing requirements.