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RTD – What Does it Stand For & How Are They Used in PLCs?

What Does RTD Stand For? 

The term ‘RTD’ stands for Resistance Temperature Detector. It’s a type of temperature sensor whose resistance changes with temperature changes. RTDs are popular temperature sensors because of their stability, and they also exhibit the most linear resistance signal in relation to temperature variations of a given electronic device. This means that, as the temperature of the electronic device increases, the resistance of the RTD sensor will also be increasing.

Note: RTDs are passive devices that do not produce their own outputs. Instead, they’re used with external electronic devices that measure the resistance of the RTD by allowing a small amount of electrical current to pass through the sensor, thereby generating a voltage. The electrical current that passes through the resistive element of the RTD sensor, creates a resistance value which is measured by the attached electronic device. The measuring instrument correlates the resistance value of the RTD sensor to its temperature, based on the resistance characteristics of the type of RTD resistive element used.

Since RTDs require external current for excitation, they are characterized by low sensitivity and slow response time, and they are prone to self-heating. Thus, to avoid the risk of self-heating, the amount of measuring/excitation current applied to a typical RTD sensor varies between < 1 mA to 5 mA (milliamperes).

Types of RTD Sensor Elements  

An RTD device consists of insulated copper wires and a resistive element. The most common number of copper wires used in RTDs is 2, but some RTDs have 4 or 3 wires. The resistive element is the temperature sensing component of the RTD, hence the name RTD sensor element. Its resistance changes when there is a change in temperature, so the actual temperature measurements of an RTD occur at the element.

RTD sensor elements are made of specific types of metal; as the temperature of the RTD element is determined depending on the resistance value measured on the metal. They are available with different resistance/temperature values depending on application requirements. The most common metals used for RTD sensor elements are Platinum (Pt), Copper (Cu), and Nickel (Nu). Each of the three metal elements has its advantages and limitations as discussed below:

A) Platinum (Pt) Elements: Platinum RTD sensor elements are made of pure platinum wire. They are characterized by a positive temperature coefficient, with their resistance changing by approximately 0.4 Ω (ohms) per ℃ (degree Celsius) of temperature change. Platinum has excellent resistance to corrosion and is highly stable over time due to its chemical inertness. Also, Platinum RTD elements offer an almost linear relationship between resistance and temperature, with a wide range of operating temperatures from -200 °C to 1000 °C. They make extremely accurate RTD sensors that are well suited for industrial applications requiring long-term stability, linearity, and a wide temperature range. However, they are the most expensive types of RTD sensor elements, since Platinum is classified as a noble metal. Note: RTD sensor elements made of Platinum can use wire extension leads made of Copper.  

B) Nickel (Ni) Elements: Compared to Platinum elements, Nickel RTD sensors are less expensive. They also depict a higher resistance value at 0℃ and they provide higher temperature sensitivity due to their high resistance ratio. However, their operating temperature range is limited to -80°C to 260°C or -112℉ to 500℉. This is because, with Nickel elements, the amount of resistance per ℃ of temperature change becomes non-linear at over 300℃ or 572℉. In addition, they have good corrosion resistance but they age rapidly and lose their accuracy over time.

C) Copper (Cu) Elements: Copper RTD sensors have excellent linear resistance in relation to temperature change compared to Nickel and Platinum elements. They are also a low-cost material with excellent linearity but due to their low resistivity forces, they require longer elements than Platinum. Copper RTDs are also less resistant to corrosion. In addition, the fact that Copper oxidizes at 200 ℃ limits the use of Copper RTD elements to a temperature range of -200°C to 150°C. They are thus limited to applications free of the oxidizing atmosphere such as taking winding measurements of generators, motors, and turbines. Other element metals used in RTD sensors are Tungsten (W), Iridium (Ir) ad Balco (annealed alloy of 30% Iron and 70% Nickel).

How Are RTDs Constructed?  

RTD construction is done in three main manufacturing configurations, namely: 

A) Wire-Wound RTDs 

In the wire-wound type of RTD, the sensing element consists of a small ultra-thin resistance wire (commonly platinum) wound around a non-conducting core made of ceramic or glass material referred to as a bobbin. This type of construction is often referred to as an outer-coil type. But in most cases, the small-diameter platinum wire is wound in a coil and placed inside a ceramic insulator or glass tube forming an inner-coil type.

External copper wires are then welded to the Platinum coil extending outside the insulator housing. Wire-wound RTD elements can be mounted inside metal sheaths or tubes to form sensing probes. This also increases their durability by protecting them from extreme environmental surroundings.

Wire-wound RTDs, especially the inner-coil type, are the most accurate types of RTDs. This is because the manufacturer has to carefully trim the length of the Platinum wire to achieve the specified resistance at 0℃. This resistance at 0℃ is called nominal resistance R0. Overall, all wire-wound RTDs are characterized with good accuracy over a wide temperature range. Those made of ceramic cores are readily used to accurately measure extreme temperatures (too low or too high), while those with glass tubes can be immersed in a variety of different liquids to measure temperature.

B) Thin-Film RTDs

Thin-Film RTD elements are made by depositing a very thin layer of resistive platinum metal onto a ceramic substrate material. The Platinum metal film is etched or laser cut into an electrical circuit pattern to provide the specified amount of resistance. External lead wires made of copper are then attached, and a thin film coating of epoxy or glass is applied to the entire element. The coating protects the deposited platinum metal film, and also functions as a strain relief for the attached lead wires. Manufacturers adjust the nominal resistance (R0) of a thin-film RTD element by opening parallel shunts in the electrical pathway with a laser beam. The more the number of shunts that are opened, the higher the nominal resistance (resistance at 0 °C) of the thin-film element.

Thin-Film RTDs are popular due to their cost effectiveness, reliability, and ruggedness. They perform better in vibration prone applications than other types of RTDs because their thin-film elements are more resistant to damage from vibration and mechanical shock. Also, their flat physical profile allows for design flexibility and versatility in terms of resistance types, shape, size, and tolerance options. This makes them ideal for use in a variety of different industrial instrumentation and control applications.

C) Coiled-Element RTDs

In this type of RTD, the resistance wire (typically Platinum) is rolled into small coils that loosely fit into a ceramic or glass housing, which is then filled with a non-conductive powder. The loosely fitting resistance wire is free to expand and contract with changes in temperature, this minimizes errors that may result from mechanical strain. The tightly packed non-conductive powder increases heat transfer around the resistive Platinum coils, thereby improving the overall response time of this type of RTD. The glass or ceramic housing the coiled-element RTDs is usually inserted into a protective metal sheath, forming RTD probes.

Comparison Between Wire-Wound and Thin-Film RTDs 

Construction of wire-wound RTD elements requires highly advanced manufacturing processes and skilled technical engineering; they are thus more expensive than thin-film RTDs. On the other hand, thin-film RTD sensors are easily mass-produced, cheaper, and more widely available since they are capable of achieving higher nominal resistance with fewer Platinum films. In addition, wire-wound sensors are longer in length and tend to be more delicate and vibration-sensitive. While thin-film sensors are smaller and more resistant to vibration and mechanical shock damage.

Thin-film RTDs are not as accurate as wire-wound RTDs, because the parallel shunting method cannot adjust nominal resistance
R0 as precisely as the trimming of the platinum wire in the wire-wound RTD types. In addition, thin-film RTDs are more prone to strain errors at higher temperatures than wire-wound RTDs due to the slightly different expansion rates between the ceramic base and platinum film.

Moreover, because of their smaller sizes, the applied excitation current in thin-film RTD introduces a slightly higher accuracy error due to self-heating. However, compared to wire-wound RTDs, thin-film RTDs have a faster response rate, which is desirable in many industrial applications. While the three RTD types (wire-wound, thin-film, and coiled-element RTDs) discussed above are the most widely used in industrial applications, there are other more exotic configurations of RTD elements including:

  • Carbon Resistor RTD Elements: They are popularly used at ultra-low temperatures ranging between -273 °C to -173 °C. They are cheaper and they give very reproducible readings at extremely low temperatures. 
  • Strain-Free RTD Elements: They consist of a Platinum wire coil that is minimally supported within a sealed housing full of inert gas. The wire is loosely coiled over the support structure, thus this RTD element is free to expand and contract with temperature changes. These sensors function reliably at higher temperatures of up to 961.78 °C. But they are highly susceptible to vibration and mechanical shock, as the platinum wire loops can sway back and forth, resulting in deformation.  

How Does an RTD Sensor Operate? 

The working of an RTD sensor is based on the correlation principle between temperature and electrical resistance of pure metals, characterized by a linear positive change in metal resistance with increasing temperature. Such that, as the temperature of the metal rises, so does its resistance to electricity flow.

So, when an electric current is passed through the RTD sensor, the resistive element can be used to measure the RTD’s resistance to the current flowing through it. And as the temperature of the RTD resistive element increases, its electrical resistance increases as well. This electrical resistance is measured in Ohms (Ω), and the value can be converted into temperature based on the material characteristics of the resistive element used.

Each metal used as an RTD resistive element has a certain electrical resistance measurement at different temperatures. For this reason, RTD sensor elements are mainly specified according to their nominal resistance R0, defined as the resistance in Ohms (Ω) at 0℃. Typical nominal resistance values for Platinum-based RTDs (mainly thin-film RTDs) are 100 Ω and 1000 Ω; designated as Pt100 and Pt1000 respectively.

In RTD sensor elements, the relationship between temperature and resistance is nearly linear. It’s known to follow this equation: 

For < 0℃        RT = R0 [ 1 + aT1 + bT2 + cT3 (T − 100) ]………….(i)
For > 0℃        RT = R0 [ 1 + aT1 + bT2 ]………….(ii)

where,  

RT = Resistance value at Temperature T  

R0 = Nominal Resistance value  

a, b, and c = Constant used for scaling the RTD sensor. 

Therefore, the most significant characteristic of the pure metals used as RTD resistive elements is the temperature coefficient. Defined as the linear approximation of resistance vs temperature relationship over a temperature span of  0℃ to 100℃. It is denoted by α and has units of Ω/Ω.℃(ohm/ohm/°C), expressed mathematically as: 

α = (R100− R0) / (100℃ × R0)

where,  

R0 = Resistance of the RTD resistive element at 0 °C

R100 = Resistance of the RTD resistive element at 100 °C 

Pure Platinum metal has a temperature coefficient of α = 0.003925 Ω/Ω .℃ in the 0℃ to 100℃ temperature range. Knowing the temperature coefficient value is important because it defines how much the resistance of the RTD sensor will change in temperature. The greater the temperature coefficient value, the more the expected change in resistance for a given temperature range.

RTD sensors are therefore used to measure the temperatures of materials with predictable resistance changes as the temperature of the material in question changes. They provide accurate, repeatable, and stable temperature measurements. However, the typical response time of an RTD sensor varies between 0.5 to 5 seconds, they’re thus more suitable for applications that don’t require an immediate response.

Benefits of Using RTD Sensors 

The benefits of using Resistance Temperature Detectors over other temperature sensors such as thermistors or thermocouples, are:

  • High Levels of Accuracy: RTDs can produce temperature readings with an accuracy of 0.1℃, which makes them far more accurate for industrial applications.  
  • Excellent Repeatability: With RTDs, temperature measurements can be made routinely over time and obtain the same readings or output under repeated identical operating conditions. 
  • Long-term Stability: In general, Platinum-based RTD sensors are more stable over time due to their chemical inertness and excellent resistance to corrosion. They can thus provide precise and accurate temperature readings for a long time. 
  • Wide Operating Temperature Range: The temperature range offered by an RTD sensor depends on the type of resistive element material used. Platinum-based RTD resistive elements provide a wider operating temperature range.  
  • Well-suited for Industrial Environments: RTD sensors are relatively immune to electrical noise. This makes them highly suitable for temperature measurements around high-voltage industrial equipment like generators and motors. Thin-film RTDs are also ideal for industrial applications as they are more resistant to vibration and shock damage. 

How Are RTDs Used in PLCs? 

A Simple Circuit of an RTD Input Module in PLCs 

RTD sensors are used as inputs for Programmable Logic Controllers (PLCs) through RTD input modules. They deliver a special type of input signal to the PLC, much like an analog signal or digital signal. Using the input signals provided by the RTDs, the PLC can monitor and control the temperature of various field devices that are fitted with RTD sensors.

The RTD input modules are used as PLC expansion modules to measure temperature, and they work when directly connected to RTD sensors. So, the module provides an interface between the PLC processor and the RTD sensors. Therefore, to use an RTD sensor as a PLC input it must always be connected to an RTD input module so that the CPU can integrate its input. A typical RTD module for PLC applications consists of a temperature-sensing element connected by 3-wire, 2-wire, or 4-wire RTD sensors that provide input signals to the module.

The module provides two channels for measuring the resistance of up to four RTD sensor units; from any combination of the various types of RTD resistive elements including Platinum(Pt), Nickel(Ni), Copper(Cu), or Nickel-iron alloy. In most cases, the type of RTD sensors used as PLC inputs are Platinum-based with a nominal resistance of 100 Ω denoted as Pt100, and with a temperature coefficient of α = 0.003925 Ω/Ω .℃. The RTD module also provides an onboard RTD temperature scaling in ℃ and ℉, or resistance scaling in Ω (ohms). For this reason, RTD sensors can provide inputs to the PLC in form of temperature or resistance values.

The RTD sensors used with PLCs are designed to measure temperature using two dissimilar electrical conductors that form an electrical junction. While the RTD module supplies small amounts of electrical current to each RTD unit connected to its inputs (it has up to 4 RTD input channels). The current supplied by the module acts as an RTD excitation current because as previously stated, an RTD sensor is a passive device that cannot provide an output on its own. The diagram to the right illustrates a simplified circuit of a PLC RTD input module:

The module monitors the temperature signals from the RTD sensor units within a fixed temperature range. It then receives and stores the digitally converted analog temperature or resistance signals from the connected RTD units into its image data table for retrieval by the PLC processor. From there, the processor will act on the digital signals and actuate the appropriate outputs for temperature control. Note: RTD input modules have Analog-to-Digital Converter (ADC) for converting the analog input signals from the RTD units into digital signals for use by the PLC. 

This entry was posted on March 14th, 2022 and is filed under Allen-Bradley, Education, Technology, Uncategorized. Both comments and pings are currently closed.

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