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Chapter One What is a RTD Sensor?
An RTD, resistance temperature detector, is a passive temperature sensing device that operates on the principle that the resistance of a metal changes as the temperature changes. The electrical current that passes through the element or resistor of the sensor creates a resistance value that is measured by an attached instrument that correlates it to the temperature based on the resistance characteristics of the RTD sensor.
As the temperature of a metal rises, the metal‘s resistance to the flow of electricity increases. RTD sensors measure the temperature of materials that have a predictable change in resistance as the temperature of the material changes. The use of RTD sensors is due to their accuracy, repeatability, and stability.
Chapter Two: Types of RTD Sensor Elements
There are a wide variety of element types used to manufacture RTD sensors. Each of the various types conform to different standards, measure different temperature ranges, come in an assortment of sizes, and have differing standards for accuracy. Their basic function of measuring temperature by resistance is the same with each type having a pre-specified resistance value for their specific range of temperatures.
Resistance elements are the central part of an RTD and are to°ragile and sensitive to be used in their raw form, which means they must be protected and shielded. The readings of an RTD are measured in Ohms (Ω), an electrical unit for measuring resistance.
The various metals used as resistance elements have a resistance measurement for different temperature ranges.
Types of RTD Sensor Elements
The element of an RTD sensor is the sensing component that changes in resistance when there is a change in temperature. The most common element is platinum. Other element metals are copper, nickel, tungsten, Balco, and iridium.
Platinum sensor elements are made of pure platinum wire and have a positive temperature coefficient. The linear and long term stability of platinum RTD elements make them an extremely accurate sensor for industrial applications. Platinum RTD elements can use copper wire extension leads and are suited for industrial applications that have a wide temperature range that requires stability and linearity.
Nickel elements have a limited temperature range due to the amount of resistance per degree of temperature change and become non-linear over 300° or 572°. They have good corrosion resistance and are less expensive than platinum RTDs but age rapidly and lose their accuracy. Their temperature range is 80° to 260°C or – 112° to 500°.
Copper has good linear resistance in relation to temperature change but needs a longer element than platinum because of its low resistivity forces. The fact that copper oxidizes limits its use to temperatures under 150° or 302°. The uses of copper RTDs are limited to winding measurements for motors, generators, and turbines. Though copper elements have excellent linearity, they are less expensive than other types of RTDs. Applications that are free of oxidizing atmospheres use copper elements because of their linearity and low cost.
The Chart below offers a comparison of the effectiveness of copper, nickel, and platinum elements.
Sensors made of Balco are an annealed resistance alloy of 70% nickel and 30% iron. A Balco 500 ohm sensor provides a relative linear resistance at temperatures between -40° to 116° or -40° to 240°. Balco has the same thermal conductivity as nickel but with twice the resistivity. Much like copper, Balco RTD sensors are low cost with a high resistance coefficient and exceptional linearity as well as strong mechanical properties and some corrosion resistance.
The main characteristic of tungsten is its high resistivity, which makes it ideal for high temperature applications. The difficulty with tungsten is that it’s brittle and difficult to work and shape.
All metals produce a positive change in resistance for a positive change in temperature. Though this is true, certain metals have a better resistivity than others, which makes them better for RTD sensor use. Two metals that are seldom used for RTD sensors are gold and silver since they have a low resistivity factor.
When examining a metal for use in an RTD, the most important factor is its purity, since the purity of a metal strongly influences its resistivity. To specify and determine elements for use in an RTD, their resistance in ohms (Ω) is measured at zero degrees Celsius; this should result in 100 Ω of resistance.
Chapter Three: How an RTD Sensor Works
The working principle of an RTD sensor is rather simple. All forms of metal have a resistivity factor when the temperature of the metal rises. The resistance can be measured and used to supply temperature readings.
RTD sensors have become an essential part of manufacturing and sensitive temperature processes due to their accuracy and resistance to temperatures, vibrations, and shock. A limited amount of DC current is used to activate the sensor and avoid overheating from too much current.
How Does an RTD Sensor Work?
When placing the sensor into an application, it is recommended to have a thermowell, a closed end tube mounted on the process stream. The process transfers heat to the thermowell wall and on to the sensor. A thermowell is a thermal conductive protrusion placed in a process line that allows the placement of sensing devices without needing to open a hole in the line.
Most RTD sensors have a protective sheath made of stainless steel or Inconel that protects the sensing element from the environment and mechanical impact. This allows the measuring end of the sensor to be placed directly into the measurement area. Termination wires connect it to the measurement recording device.
When placing the sensor in the line, it is important that the cables on the connecting head are straight and not twisted while screwing in the sensor. Leaving the wires disconnected when inserting the sensor avoids problems with the cables.
To ensure a good calibration, all of the connecting wires should be the same size and length. The insertion depth should be ten times the diameter of the stem.
The diagram below is an example of an inserted RTD sensor using a thermowell.
Frequency with which an RTD is calibrated depends on the temperature cycle, vibrations, and shock. In most cases, the frequency of calibration is determined by the user. The calibration of the sensor is achieved by comparing its resistance to a working standard. The best practice for calibration is t°omplete it when the sensor is in its working position.
Though most sensors have a protective sheath or are installed in a thermowell, part of the maintenance process is t°heck for damage from corrosion, shock, vibrations, or other factors. Damaged sensors should be replaced.
Chapter Four: Types of RTD Sensors
The different types of RTD sensors are categorized by the construction of the temperature sensing element. Tw°ommon types are thin film and wire wound. The type of RTD sensor to be used is determined by the environment it will be used in and the application.
The use of resistance temperature sensors began in the middle of the first industrial revolution, and they were assembled using copper wire and a galvanometer. Copper wire was replaced by platinum when it was discovered that platinum could measure a wider range of temperatures.
Types of RTD Sensors
Thin Film RTD Sensors
Thin film RTD elements have a thin layer of metal placed on the substrate of a ceramic material. The film of metal is etched into an electrical circuit pattern that offers the necessary amount of resistance. The image below is an example of a common form of resistance pattern. Lead wires are attached, and a protective coating is applied to the substrate and element.
Thin film RTD sensors are rugged, reliable, and resistant to shock and vibration damage. Since they are flat, they can be engineered t°it several applications and come in an assortment of resistance types, tolerances, sizes, and shapes.
Wire Wound RTD Sensors
The wire wound version of an RTD has a wire wound around the outside of a ceramic or glass housing, referred to as a bobbin in the diagram below. Glass core RTD sensors can be immersed in liquids. RTD sensors with ceramic cores can accurately measure extreme temperatures. Wire wound RTD sensors require skilled technical engineering and highly advanced manufacturing processes; this means they are more expensive than thin film sensors.
Coiled RTD Sensors
Coiled RTD sensors have thin wound wire enclosed in a ceramic or glass housing filled with a non-conductive powder. The resistance wire can expand and contract with the changes in temperature, minimizing errors that may be caused by mechanical strain. The tightly packed powder around the wire increases heat transfer, improving the response time of the sensor. The ceramic or glass housing is normally inserted into a protective metal sheath.
PT100 RTD Sensors
The “pt” in the pt100‘s designation indicates that the sensor is made with a platinum element. The 100 is its resistance factor. The pt100 RTD sensor is one of the most accurate temperature measuring instruments with a resistance factor of 100 Ω at 0° or 32°. There are several versions of the pt100 that have different temperature coefficients, represented by the Greek letter alpha or α. The most common is the “385”.
PT1000 RTD Sensors
The pt1000 is the second most used resistance sensor. It has a resistance factor of 1000 Ω and is mainly used in two wire applications. A pt1000 has greater resistance. The distortion in the lead wires is less significant and is only a small percentage of the total resistance.
Pt1000s have a higher resistance value and require less current. They are appropriate for configurations that use less power. Since the power consumption is low, they produce less heat and have fewer errors caused by self heating.
Two Wire RTD Sensors
The two wire type of RTD is the simplest circuit design. A single lead wire connects to each end of the element. The resistance in the circuit is calculated by measuring the resistance in the lead wires and connectors. This results in some degree error or readout that is higher than the actual measured temperature. This can be eliminated with calibration.
Three Wire RTD Sensors
The three wire configuration is the most used in industrial applications. Two wires are connected to one end of the sensor, A and B, and to the monitoring device. The third wire, C, is connected to the element. The three wires are of equal length, so their resistance is equal. The three wire configuration also has errors that have to be adjusted by calibration.
Four Wire RTD Sensors
The four wire configuration is the most complex, time consuming, and expensive to install but produces the most accurate and precise readings. DC current is provided through two leads, A and C. The voltage drop is measured by the other two leads, B and D. The voltage drop and current are known, making the resistance easy to read as well as the temperature across the system.
A variation of the four wire design has two red wires connected to the element with a white configuration that is looped. This design is a combination of the three and four wire methods.
Chapter Five: The Difference Between RTDs and Thermocouples
RTDs and thermocouples are sensors used to measure heat in Fahrenheit or Kelvin units. Both instruments convert temperature readings into electrical signals. RTDs work on the principle of resistance, which happens uniformly with changes in temperature. Thermocouples operate on the principle that when two metals are joined together, there is a potential difference, at the point of contact, that varies with changes in temperature.
Since both instruments are designed to measure a range of temperatures under varied conditions, it is difficult to decide which of the instruments is better than the other. It is more useful to compare them by examining some of their specific qualities.
The Difference Between RTDs and Thermocouples
The majority of temperature readings are taken in inhospitable environments where there is corrosive, oxidizing, and reducing atmospheric conditions. In addition to the uncomfortable conditions, there are vibrations, noises, and electricity.
RTDs are wire wound in protective casings and are rugged and immune to harsh and hazardous conditions. For added protection, RTDs can be coated with perfluoroalkovy (PFA) polytertrafluoroethlyene for use in plating baths and pressurized systems.
Thermocouples, with metal cases, are very capable of dealing with corrosive and oxidizing conditions. When exposed thermocouple junctions are used, care must be taken.
A thermocouple costs far less than RTDs, which can cost two to three times more, capable of reading the same temperature range. The difference in cost between thermocouples and RTDs is due to the lower production costs for producing thermocouples.
RTDs are capable of measuring temperatures up to 1000°C though it can be difficult to get accurate readings above 400°C. Thermocouples can measure temperatures up to 1700°C. When making the choice of which instrument to use, the general rule is to use a RTD for temperatures below 850°C. For temperatures above 850°C, it is best to use a thermocouple.
The majority of industrial applications operate between 200°C and 400°C, which makes RTDs the best choice.
RTDs and thermocouples respond quickly to variations in temperature with thermocouples being slightly faster. There are various adjustments that can be made to RTDs to enhance their response time.
There is little difference in the dimensions of the two instruments. They are small with a diameter of 0.5 mm. Though it is doubtful, it may be necessary to check the mounting location to see which device will fit.
The construction and design of RTDs makes them susceptible to failure in environments where there are vibrations. Thermocouples are unaffected by vibrations and are capable of supplying readings in those conditions.
RTDs require a power supply and voltage to operate. The necessary power is 1mA up to 10 mA and is minimal but can cause the RTD’s platinum element to heat up, which will affect its accuracy. Thermocouples do not require a power supply and are unaffected by heat.
Long Term Use
RTDs are far more stable and are capable of providing accurate and precise readings for a long time. Thermocouples produce electromagnetic fields (EMFs) that change over time because of oxidation, corrosion, and the changes in metallurgical properties of the sensing elements. Once a thermocouple begins to drift, the effect is irreversible.
For industrial uses, RTDs are far more accurate and can produce readings with an accuracy of 0.1C. A thermocouple’s accuracy is far less at 1C.
The chart below offers a brief comparative tool for examining RTDs, thermocouples, and thermistors.
Types of Thermocouples
Thermocouples are classified into types with each type being suitable for specific temperature conditions. To accommodate the various environments, each class of thermocouple has a construction to match an specific application.
Thermocouple types are:
- E: suitable for vacuum, inert, mildly oxidizing, or reducing conditions
- J: used where there is limited oxygen
- K: requires metal or ceramic protection
- N: resists oxidation from sulfur
- T: used in oxidizing or reducing environments
- S, R, and B: must be protected with a form of tubing as conditions dictate
Chapter Six Uses for RTD Sensors
Sensors are a necessary part of manufacturing used to measure physical phenomena using the properties of metals and fluids. An essential measuring device is the resistance temperature detector, a precise, sturdy, and accurate piece of equipment that supplies data for application monitoring.
The linear nature of RTD sensors, as well as their stability, has increased their use. Since the resistance of materials presents a predictable change, the use of RTD sensors provides consistent and accurate temperature measurements.
Uses for RTD Sensors
Automotive RTD Sensors
RTD sensors are widely used in the automotive industry to measure engine temperature, air temperature, external temperature, and water levels. The benefit of RTD sensors for the auto industry is that they don‘t heat up and are flexible and adaptable.
Solar Power RTD Sensors
In solar power applications, even distribution of heat is critical to the efficient and effective production of electricity. RTD sensors do not overheat and are ideal for use with heating applications. They are placed in solar panels to monitor the temperature of the panels. This is also true of grid connected wind turbines as a means of measuring the fluctuation in temperature.
Pharmaceutical RTD Sensors
The production of drugs requires close temperature monitoring and control. Increases and decreases in temperature can damage a batch and its formulation. Achieving the proper thermal capability is an essential part of research, formulation, testing, and production. The unique nature of the pharmaceutical industry requires the construction of precise instruments designed to meet the requirements of diverse temperature readings.
Chemical RTD Sensors
Much like the pharmaceutical industry, the chemical industry has strict requirements regarding temperature control. The results of research and experimentation necessitate maintaining an accurate and precise environment. The various special chambers and integrated systems use RTD sensors as monitors and controls to ensure accuracy and safety.
Semiconductor RTD Sensors
Recent developments have led to an increasing demand for high temperature control and thermal heating solutions for the semiconductor industry. The requirements of the semiconductor industry necessitate temperature measuring devices specifically designed and engineered t°it their manufacturing environment. In the complex conditions of wafer processing, RTD sensors provide the necessary repeatability, accuracy, and stability.
The industries listed above are only a few of the many applications that require the accuracy and precision of RTD sensors.
Every aspect of food production requires constant monitoring of temperature. RTD sensors are used during manufacturing, storage, and shipping.
RTD sensors are used for monitoring temperature, fire detection, and climate control.
The use of RTD sensors for aerospace is somewhat like their application in the auto industry. In aerospace, they monitor the temperatures of engines, coolant, and compressors as well as fuel tanks and fire control equipment.
All machinery and electric motors have to be monitored for increases in temperature, which could significantly damage production. This is also true of windings, generators, ovens, and microwave power.
Temperature control is crucial to patient care, especially in cases of infant incubators, respiration devices, and dialysis equipment.
In sound production, amplifiers and transmitters use tremendous amounts of heat producing electricity that has to be controlled and monitored.
An endless number of consumer products use RTD sensors as a means of controlling temperature. From coffee makers and cellphones t°lothing washers and electric blankets, RTD sensors ensure constant and safe temperature control.
Chapter Seven: Construction of an RTD Sensor
Resistance temperature detectors have six major components, which are the resistant element, wire, tubing material, connection fitting, outer diameter, and termination. The configuration of these components is what separates the various types of RTD sensors and their accuracy and capabilities. Though the capabilities of the various sensors are different, their basic components are the same.
A typical RTD has a resistant element made of platinum, nickel, or copper. These metals produce the most accurate and positive temperature coefficient. Platinum is the most common type of element because of its excellent corrosion resistance and long term stability.
Construction of an RTD Sensor
All metals produce resistance when there is a change in temperature. Elements for RTD sensors are specifically chosen for their linearity during temperature change. The first RTD sensor used copper as its element; it was eventually discovered that platinum performed better and gave more accurate readings.
In the production of modern RTD sensors, copper, nickel, and platinum are commonly used with Balco, a nickel alloy, because of its low cost.
Copper is the most common metal used for wire leads for two, three, or four wire RTDs and is insulated with a variety of materials that include fiberglass, Teflon, and various types of plastic. Wire leads are required to be a specific length to meet the resistance requirements of the RTD and connect it to the read out device.
Common tubing materials are stainless steel and Inconel. Stainless steel is recommended for RTD sensors used at temperatures up to 500° or 260°. When the application temperature is above 500° or 260°, Inconel is recommended. The tubing material has to match the durability of the RTD sensor and be adaptable to the many conditions in which the sensor will be used.
The connection fitting securely joins the RTD sensor with the application and includes fittings used for other temperature measuring equipment. Two common metals used for producing fittings are brass and stainless steel. Brass is chosen for its corrosion resistance, while stainless steel is corrosion and chemical resistant. Fittings are designed for easy installation of the sensor and securing of the wire leads such that they will not be twisted or crimped.
The outer diameter of RTD sensors come in a wide range of sizes but are normally between 0.063 inches (1.6 mm) and 0.5 inches (12.7 mm).
The termination connects the RTD sensor to the monitoring device. The connection can be completed in a variety of ways, including soldering and crimping. The termination on the cold end comes in several forms that include bare plain wires and different types of plugs and jacks.
Chapter Eight: The Benefits of RTD Sensors
Since the invention of RTD sensors during the first industrial revolution, their use has spread to every manufacturing process. Their stability and exceptional accuracy has made them an ideal temperature monitoring device that provides instantaneous data with little effort.
Temperature, in an industrial operation, must be closely monitored to ensure the proper function of an application. Close control and monitoring is required so that an optimal temperature is reached and maintained. The precision and accuracy of RTD sensors make for a reliable means of achieving that goal.
Benefits of RTD Sensors
RTD sensors are more expensive than other temperature measuring methods. Cost savings are realized in their accuracy, longevity, repeatability, and stability.
A major requirement for a sensor is that it provides data instantly. RTD sensors provide temperature readings quickly and accurately. Improvements to thin film pt100s have substantially improved sensor response time.
The main reason for the popularity of RTD sensors is their accuracy, which is within 0.1°. The exceptional accuracy of RTD sensors is due to their linearity.
Linearity is the ability of a sensor to respond to changes in temperature as they occur across the full range of possible temperatures. A device with high linearity provides a resistance change that matches that of the device. This particular factor is the main reason that RTD sensors are so accurate and reliable.
The stability of a device is its ability to provide accurate and precise readings over a long period of time. Readings from RTD sensors are constant, stable, and repeatable for longer than any other form of temperature sensor. The stability of a sensor is measured by its amount of drift or lack of linearity.
RTD sensor elements are made of pure high quality metals; this is essential for their performance and the quality of their readings.
A pt100 RTD sensor is capable of measuring temperatures from -330° F (-201° C) to 1560° F (848° C). Its variable and wide temperature range make it adaptable for most industrial applications.
- An RTD, resistance temperature detector, is a passive temperature sensing device that operates on the principle that the resistance of a metal changes as the temperature changes.
- There are a wide variety of element types used to manufacture RTD sensors. Each of the various types conform to different standards, measure different temperature ranges, come in an assortment of sizes, and have differing standards for accuracy.
- The popular use of RTD sensors is due to their accuracy, repeatability, and stability.
- The working principle of an RTD sensor is rather simple. All forms of metal have a resistivity factor when the temperature of the metal rises.
- The different types of RTD sensors are categorized by the construction of the temperature sensing element.