Resistive temperature device (RTD) elements are wire windings or other thin-film serpentines that exhibit changes in resistance with changes in temperature. They are usually made of metallic elements or alloys such as copper, nickel, or nickel-iron. The most linear, repeatable devices are made of platinum, a precious metal that is suitable for temperature measurements over a wide operating range.

Technology

RTD elements use two basic sensing technologies.

• Wire-wound devices consist of a coil of insulated wire wrapped around a ceramic or glass core.
• Thin-film devices also use ceramic substrates, but are smaller than traditional wire-wound devices.

Multi-element products combine two or more RTD elements in a single sensor/transducer package to provide redundancy in case the primary element fails. Resistive temperature device (RTD) elements are often used with RTD probes and interfaced to RTD temperature transmitters, devices that convert resistance measurements to current signals. 

Temperature Sensing

Industrial and commercial temperature sensors are typically separated from a controller, recorder, or other instrumentation by some amount of distance. The most common remote sensor technologies are RTD's, thermocouples, and thermistors. The application usually dictates the sensor type as each technology offers its own benefits and limitations. The following chart outlines each sensor's capabilities.

Resistance Measurement

Resistive temperature device (RTD) elements detect temperature indirectly by measuring the resistance in the sensing element. The relationship between resistance and temperature is a function of the nominal resistance (Ro), the resistance of the RTD in ohms (Ω) at 0ºC, and the temperature coefficient of resistance (TCR), also referred to as the alpha coefficient (α), of the metal used in the RTD.

Temperature coefficient of resistance (α)

The value of α is a ratio of the fractional change in resistance per degree change in temperature. Most materials used to manufacture RTD possess a positive α value meaning that resistance increases with an increase in temperature. The value is defined as the average fractional change in resistance per degree Celsius over a temperature interval of 0°C to 100°C with the unit (Ω/Ω/ºC).

Where:

R₁₀₀ = Resistance at 100°C (Ω)

R_o = Resistance at 0°C (Ω)

Nominal resistance (Ro)

The nominal resistance (R_o), the resistance (Ω) at 0ºC, is used to describe the resistance value of the sensing element. The most common value for R_o is 100 Ω although RTD's are available with other values with the following being most common:

• 100 Ω
• 120 Ω
• 200 Ω
• 500 Ω
• 1000 Ω

Wiring Configuration

Measurement wires leading to the sensor may contribute to a significant source of error. This is due to the fact that the nominal resistance and temperature coefficient of resistance are relatively small values that when combined with the lead wire impedance, which may be several to tens of ohms, creates a significant alteration to the sensor's signal. It is therefore important to select an RTD whose wiring configuration is suitable for the industrial application it is to be used for. RTD elements are available with 2-, 3-, or 4-wire elements.

Two-wire configuration

Two-wire configurations are the simplest and least accurate RTD wiring arrangements. Lead wire resistance is incorporated into the sensor signal and accuracy decreases as the length of the lead wires increases. Two-wire configurations are suitable for use with short lead wires or may require thicker gauge lead wires to decrease the lead wire resistance.

Three-wire configurations

In a three-wire configuration the effects of lead wire resistance are minimized by subtracting the average lead wire resistance. This is done by subtraction the resistance of a two wire closed loop from the sensor signal. Three-wire configurations assume all three wires carry the same resistance and are suitable for transmission over distances of up to 600m of cable.

Four-wire configurations

Four-wire configurations increase the accuracy of measurement by eliminating lead wire resistance. The circuit is a true 4-wire bridge, using wires 1 and 4 to power the circuit while wires 2 and 3 are used to measure the sensor signal and compensate for any differences in lead wire resistances.

Types of Sensors

Resistive temperature device (RTD) elements use two types of sensors: standard (Class B) and precision (Class A). Both types are defined by the International Electrotechnical Commission (IEC), a global organization that prepares and publishes standards for electrical and electronic technologies.

• Class B sensors are the industry standard for platinum RTD elements. They use a single ice-point calibration and provide an accuracy of .3° C for temperature measurements at 0° C.
• Class A accuracy is multi-point and more precise, providing tighter control of the temperature coefficient and an accuracy of .3° C at 0° C.

Although the American Society for Testing Materials (ASTM) also uses Class A and Class B designations, these categories specify different permissible deviations than IEC standards.

Specifications

Specifications for resistive temperature device (RTD) elements include operating temperature, nominal resistance, temperature coefficient of resistance (α), response time, accuracy, length, and width or diameter.

• Operating temperature is the range of temperatures in which devices are designed to operate. RTD elements are capable of measuring ranges up to -200 ºC to +850 ºC. Typically, the upper and lower 5% of the operating range is non-linear.
• Nominal resistance, R_o, is the electrical resistance of the wire coil or thin-film element when measured at 0 ºC.
• The temperature coefficient of resistance (α) is a ratio of the fractional change in resistance per degree change in temperature.
• Response time is the amount of time elapsed before an accurate measurement can be made. An RTD element's response time is dependent upon the media in which devices are immersed and the media flow rate. Response time is either an approximation or average value.
• Accuracy is the percent difference between the measured value and the true value. The accuracy of an RTD element typically varies with respect to temperature, due to the metallic components' varying temperature coefficient of resistance at given temperature ranges.
• Length is the greatest dimension across a plane or solid face of the RTD element.
• The diameter or width is the shortest dimension across a plane or solid face of the RTD element.

Standards

IEC 60751 - This standard specifies requirements for industrial platinum resistance thermometer sensors whose electrical resistance is a defined function of temperature. The standard covers thermometers suitable for all or part of the temperature range -200 ºC to +850 ºC with two tolerance classes. It is primarily concerned with sheathed elements suitable for immersion in the medium whose temperature is to be measured.

ASTM E-1137 - This specification covers the requirements for metal sheathed industrial platinum resistance thermometers (PRT's) suitable for direct immersion temperature measurement. It applies to PRT's with an average temperature coefficient of resistance between 0 and 100°C of 0.385 %⁄°C and nominal resistance at 0°C of 100 Ω or other specified value. This specification covers PRT's suitable for all or part of the temperature range -200 to 650°C. The resistance-temperature relationship and tolerances are specified as well as physical, performance, and testing requirements. 

Resources

RTD Theory

Taking Temperature Measurements with RTDs

What Are RTD Sensors? 

Image credits:

National Instruments | Thermometrics Corporation