Using RTD Sensors

Resistance Temperature Detectors (RTDs) are temperature sensors that contain a resistor that alters resistance value as its temperature changes. For many years, RTDs have been used to measure temperature in industrial and laboratory processes, and they have built a reputation for stability, repeatability, and accuracy.

Why Use an RTD Instead of a Thermocouple or Thermistor Sensor?

A specific set of conditions are possessed by each type of temperature sensor for which it is best suited. RTDs provide the following benefits:

• Long-term stability
• Good accuracy (better than thermocouples)
• Good interchangeability
• A wide range of temperature (approximately -200 to 850 °C)

RTDs, with a temperature range up to 850 °C, can be used in all but the highest-temperature industrial processes. When RTDs are made with metals such as platinum, they become extremely stable and not affected by oxidation or corrosion.

While nickel, nickel-iron alloy, copper, and other such materials have also been used to make RTDs, they are not generally used because they have lower temperature capabilities, and are not as repeatable or stable as platinum.

RTD Standards

Two standards are available for platinum RTDs: the American standard and the European standard (also called the DIN or IEC standard).

The European standard, also called the DIN or IEC standard, is regarded as the universal standard for platinum RTDs. This DIN/IEC 60751 (or simply IEC751) standard requires the RTD to have a temperature coefficient of resistance (TCR) of 0.00385 Ω/Ω/°C between 0 and 100 °C and an electrical resistance of 100.00 Ω at 0 °C.

Two resistance tolerances are indicated in DIN/IEC751:

• Class A = ±(0.15 + 0.002*t) °C or 100.00 ±0.06 Ω at 0 °C
• Class B = ±(0.3 + 0.005*t) °C or 100.00 ±0.12 Ω at 0 °C

The following resistance tolerances are used in industry:

• ¹/₃ DIN = ±¹/₃* (0.3 + 0.005*t) °C or 100.00 ±0.10 Ω at 0 °C
• ¹/₁₀ DIN = ±¹/₁₀* (0.3 + 0.005*t) °C or 100.00 ±0.03 Ω at 0 °C

The combination of temperature coefficient and resistance tolerance defines the resistance versus temperature characteristics for the RTD sensor. If the element tolerance is large, the sensor will deviate more from a generalized curve, and there will be more variation from one sensor to another (interchangeability).

OMEGA employs a resistance versus temperature curve from 200 to 850 °C together with resistance values specified for each degree Celsius. The below interchangeability table shows how the temperature coefficient and tolerance influence the sensor’s specified temperature in degrees Celsius:

Other Resistance Value Options

RTD elements can also be purchased with resistances of 200, 500, 1000, and 2000 Ω at 0 °C. These RTDs have the same temperature coefficients as described previously, but due to their higher resistances at 0 °C, they offer more resistance change per degree Celsius, enabling greater resolution.

RTD Element Construction

There are two types of platinum RTD elements available: wire wound and thin film

Thin Film

Thin-film RTD elements are made by depositing a thin platinum layer on a substrate. A pattern is then produced that offers an electrical circuit, which is trimmed to provide a specific resistance. Then, lead wires are attached and the element is coated to protect both the platinum film and wire connections.

OMEGA’s F2020, 100 Ω, Class “A” thin-film element

Thin film elements are supplied in European standard (0.00385 Ω/Ω/°C) that exhibit a temperature coefficient of 0.00375 Ω/Ω/°C. A specialized version is used mainly in the appliance industry.

Wire Wound

RTD elements are also available in wire-wound constructions. Two types of wire-wound elements are available: those wound around a ceramic or glass core and covered with additional ceramic or glass material (used in more specialized applications), and those with coils of wire packed within a glass or ceramic tube (the most frequently used wire-wound construction).

Typical wire-wound RTD element

Wiring Arrangement

The RTD element should be connected to some sort of control or monitoring equipment in order to measure temperature. Since the measurement of temperature is dependent on the element resistance, measurement error will occur if any other resistance (connections, lead wire resistance, etc.) is added to the circuit.

The four basic wiring methods are shown below.

Except from the 2-wire configuration, the above wiring arrangements enables the control or monitoring equipment to factor out the unnecessary lead wire resistance and other resistances that take place in the circuit.

Sensors that use the 3-wire construction are most commonly used in monitoring applications and industrial process. As long as all of the lead wires have the same resistance, the lead wire resistance is factored out or else, errors can occur.

Sensors that use the 4-wire construction are commonly found in laboratories and other applications where there is a need for more precise measurements.

Wire Materials

When specifying the lead wire materials, users should carefully choose the correct lead wires for the environment and temperature where the sensor will be exposed to in service. The following table lists the capabilities of the three most favored constructions:

Configuration

The physical construction of the sensor should be considered after selecting the RTD element, wire arrangement, and wire construction. The final configuration of the sensor will depend on the application.

Different sensor configurations are required to measure the temperature of a liquid, a gas stream, or a surface.

Liquid Measurements

Often, liquids are measured using probe-type sensor styles. These sensors can be as simple as the general-purpose PR-10 and PR-11 constructions, or as involved as the PR-12, 14, 18, or 19—with transmitters and connection heads. The quick-disconnect sensor is a popular option, that can be used with compression fittings for flexible installation, or with OMEGA’s PRS plastic handle for a handheld probe.

While measuring the temperature of adverse environments such as highly pressurized systems or plating baths, sensors can be coated with a PFA material, or they can be placed in a thermowell to protect the sensor from extreme environments. Customers can speak to OMEGA’s application engineers about their unique measurement challenges.

Air and Gas Stream Measurements

Air and gas stream measurements are very difficult because the rate of transfer of temperature from the fluid to the sensor is slow compared to liquids. Therefore, sensors specifically designed for use in gas or air place the sensing element as close as possible to the media.

The RTD-805 and 806 sensors from OMEGA enable the sensing element to be almost in direct contact with the air stream. This construction, with a housing design consisting of slots that enable the air to flow past the element, is highly preferred for measuring air temperature in clean rooms, laboratories, and other locations.

Surface Temperature Measurements

It is very difficult to determine surface measurements accurately. A wide range of styles are available that can be selected depending on the way users want to connect the sensor, the sensor’s level of sensitivity to temperature changes, and whether the installation will be permanent.

The OMEGA SA1-RTD sensor is the most accurate and fastest-responding surface RTD. When the sensor is applied to a surface, it almost becomes a part of the surface it is measuring. Surface sensors can also be screwed, bolted, cemented, or glued into one place.

The RTD-830 has a pre-machined hole in the housing to enable easy installation with a screw. The RTD-850 includes a housing with threaded tip that enables it to be easily installed into a standard M4 threaded hole.

This RTD is useful for measuring the temperature of structures or heat sinks where screw holes may already exist.

Element and Wire Assemblies

If users require a simple RTD sensor with element and leads, or want to build their own sensor, there are a wide range of element and cable configurations available to choose from. The element and wire assemblies from OMEGA can also be cemented directly to a structure.

These sensors can be produced with any of OMEGA’s RTD elements and can contain bare lead wires, fiberglass, or PFA to suit users’ specific application. Users can call the company’s application engineers to select from these vast resources. If the required product is not available, OMEGA can turn it around quickly to meet its customers’ requirements.

This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.

For more information on this source, please visit OMEGA Engineering Ltd.