Basics of Resistance Temperature Detectors (RTDs) ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Basics of Resistance Temperature Detectors (RTDs)

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Resistance temperature detectors (RTDs) are based on the principle that the electrical resistance of a metal increases as temperature increases. Thus, a temperature measurement can be inferred by measuring the resistance of the RTD element.. A basic construction of an RTD is shown below:

Materials used for RTD Construction
RTD’s are constructed of a resistive material with leads attached and usually placed into a protective sheath. The resistive material may be platinum, copper or nickel with the most common by far being platinum because of its high accuracy, excellent repeatability, and exceptional linearity over a wide range and it exhibits a large resistance change per degree of temperature change. The table below gives a summary of the basic characteristics of Nickel, copper and platinum materials as applied to RTD applications:
Element Material
Temperature Range
Base Resistance
TCR (Ω/Ω/°C)
 Platinum  -260 to 850°C  Best stability, Good  linearity  100 at 0°C  0.00385 (IEC  60751)
 0.003916 (JIS 1604 - 1981)
 Copper  -100 to 260°C  Best linearity  10 at 0°C  0.00427
 Nickel  -100 to 260°C  Low cost, high  sensitivity  120 at  0°C  0.00672

Copper and nickel are generally used in less critical industrial applications due to limited accuracy and linearity, and relatively narrow temperature ranges. Nickel elements have a limited temperature range because the amount of change in resistance per degree of change in temperature becomes very non-linear at temperatures over 300 °C. Platinum based sensors are the preferred choice these days.
Copper has a very linear resistance to temperature relationship but, since copper oxidizes at moderate temperatures, it should not be used over 150 °C. Copper RTDs are commonly used in winding temperature measurements of motors, generators and turbines.

RTD construction
The two most common styles of RTD sensors are wire-wound and thin film. Wire-wound RTDs are manufactured either by winding the resistive wire around a ceramic mandrel or by winding it in a helical shape supported in a ceramic sheath – hence the name wire-wound. Thin film RTD sensors are produced when a thin resistive coating is deposited on a flat (usually rectangular) ceramic substrate.

Resistance Temperature Formula for an RTD
The following law relates the resistance and temperature of an RTD 

R = R0(1 + αΔT)

α is the Temperature Coefficient of Resistance (TCR). 
R0 is the resistance at 0 degree C. 
ΔT is the temperature change. 

RTD sensors are usually manufactured to have a resistance of 100Ω at 0oC and the value of α is 0.00385 to 0.00390

Temperature Coefficient of Resistance (TCR)
RTD elements are characterized by their Tempera¬ture Coefficient of Resistance (TCR) also referred to as its alpha value. The IEC 60751-2008 standard defines these values for platinum element types. Shown below is a typical Temperature Coefficient of Resistance graph of a Pt 100 with TCR = 0.00385:
Resistance Versus Temperature Graph for a Pt100 RTD
The alpha value is the temperature coefficient for that specific material and composition. Copper elements have a different alpha value than platinum elements, and platinum elements themselves can vary depending on the purity of the platinum and any alloy content. Alpha values define sensor interchangeability. Various sensors with the same TCR value guarantees that the resistance versus temperature relationship will be the same for each sensor within a specified precision specification. When replacing an RTD sensor, the end user should ensure that the same material with the same resistance and alpha value is ordered and used.

Platinum Resistors
Platinum resistors used in RTD sensor construction are identified by the resistance at zero degree celsius (32 degree Fahrenheit). Pt 100 the most common types have a resistance of 100Ω at zero degree celsius (32 degree Fahrenheit). Other types such as Pt 500 and Pt 1000 have resistance values of 500 Ω and 1000 Ω respectively.

The ‘PT’ defines that the metal is Platinum and the ‘100’ is the resistance in ohms at ice point (or 0oC). These are generally wire wound and are quite common.

Again, the ‘PT’ defines a Platinum metal as the sensing element, but a resistance of 1000 ohms can be measured at 0 oC. These are generally thin film devices and are more expensive. 200 and 500 ohm Platinum RTDs are available, but are more expensive and less common.

Platinum is most popular for RTDs, it has good calibrated accuracy, is quite stable and has good repeatability, but is quite expensive. They are, however, not as sensitive as the Nickel and Balco devices. Nickel is not quite as repeatable, but is less expensive

RTD Lead Wires Configuration
Three basic types of RTDs exist in terms of wire configuration. They are:
(a) Two wire
(b) Three Wire
(c) Four Wire

Two Wire RTDs
These RTDs have two lead wires. They are used where lead length is short. There is no compensation for resistance of lead wires hence they are less accurate.

Three Wire RTDs
These RTDs have three lead wires. They are the most common type of RTD assembly you can find. Typically, they are connected to standard bridge circuit, which allows lead wire resistance to be compensated. They are more accurate than two wire RTDs and are used in most industrial applications.

Four Wire RTDs
They have four lead wires. They are used where higher accuracy is demanded. Lead wire resistance errors are eliminated in this configuration by measuring the voltage across the RTD element supplied with a constant current.

Two-wire RTDs are generally used in HVAC applications, whereas three-wire RTDs are commonly found in industrial situations. Four-wire RTDs are used in high-precision services requiring extremely good accuracy.

Advantages of RTDs:
  • Good accuracy (better than thermocouples)
  • Long term stability
  • Good interchangeability
  • A wide temperature range (-200 to 850°C)
  • Disadvantages of RTDs
  • Bulky in size and fragile
  • Slow thermal response time due to bulk size
  • Self heating problems
  • More susceptible to electrical noise
  • More expensive to test and diagnose
With a temperature range up to 850°C, RTDs can be used in all but the highest-temperature industrial processes. When made using metals such as platinum, they are very stable and are not affected by corrosion or oxidation.

When to Use RTDs
RTD sensors are used in various temperature measurement applications. However, there are certain application requirement that warrants the use of an RTD sensor instead of a thermocouple. Below are the specific application situations/conditions where the use of an RTD is inevitable:
1. Where accuracy and stability are a requirement of the given application
2. When accuracy must extend over a wide temperature range
3. When area, rather than point sensing improves control
4. When a high degree of standardization is required.

Difference Between RTDs and Thermocouples
 They are used for low temperature  applications of up to 850°C for  industrial RTDs
 They are used for high temperature  applications of up to 1700°C
 They are more accurate and stable  than thermocouples
 They are less accurate and stable than  RTDs
 They are expensive than  thermocouples
 They are very cheap compared to RTDs
 They are not suitable for high  vibration and mechanical shock  environment
 They are well suited for environment  where high vibration and mechanical  shock is present

RTD International Standards
Several international standards define the relation¬ship between resistance and temperature for RTD sensors. However, these days, only two standards are now very common: ASTM 1137 (American) and IEC 60751 (International). The International Electrotech¬nical Commission IEC 60751 standard describes the ideal relationship between the resistance of platinum RTDs and the temperature being measured.

Released in 2008, IEC60751- 2008 standard includes new tolerance classes, specifies wire colors for RTDs a, and expands the range of alpha (α) values that can be used with RTDs. IEC 60751 is equivalent to and supersedes the DIN 43760 and the BS-1904 standards.  IEC 60751 also is equivalent to the Japanese standard JS-C1604.

The ASTM (American Society for Testing and Materials) E1137 standard applies to platinum RTDs with an average temperature coefficient of resistance of 0.00385 between 0 and 100 °C and nominal resistance at 0°C of 100 Ω or other specified value. This specification covers platinum RTDs suitable for all or part of the temperature range between -200 to 650 °C.

Another standard in use is the JJG 229. It is a Chinese standard known as “Regulations of Industry Platinum and Copper Resistance Thermometers”. It is similar to the IEC 60751 standard. Below are the common international standards for RTDs and their equivalents:
International Standards
Standard Remarks
 IEC 60751  Defines class A and B performance for 100Ω, 0.00385 alpha Pt RTDs
 DIN 43760  Equivalent to IEC 60751
 BS-1904  Equivalent to IEC 60751
 JIS C1604  Equivalent to IEC 60751. Adds 0.003916 alpha
 ITS-90  Defines temperature scale and transfer standard

RTD Tolerances
There are two resistance tolerances specified in DIN/IEC751:
Class A = ±(0.15 + 0.002*t)°C or 100.00 ±0.06 Ω at 0ºC
Class B = ±(0.3 + 0.005*t)°C or 100.00 ±0.12 Ω at 0ºC

Two resistance tolerances used in industry are:
1⁄3 DIN = ±1⁄3* (0.3 + 0.005*t)°C or 100.00 ±0.10 Ω at 0ºC
1⁄10 DIN = ±1⁄10* (0.3 + 0.005*t)°C or 100.00 ±0.03 Ω at 0ºC

The combination of resistance tolerance and temperature coefficient define the resistance vs. temperature characteristics for the RTD sensor. The larger the element tolerance, the more the sensor will deviate from a generalized curve, and the more variation there will be from sensor to sensor (interchangeability). This is important to users who need to change or replace sensors and want to minimize interchangeability errors. 

The American standard, used mostly in North America, has a resistance of 100.00 ±0.10 Ω at 0°C and a temperature coefficient of resistance (TCR) of 0.00392 Ω/Ω/°C nominal (between 0 and 100°C).

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