Tempering a commercial-sized vat of chocolate requires accurate temperature sensing. Did you know that it would be almost impossible to achieve that smooth, shiny chocolate you see in high-end chocolate shops if it weren’t for the Resistance Temperature Detector or RTD? Because chocolate tempering requires narrow temperature tolerances of ±0.5°F (0.2778°C), an RTD sensor is the best solution.

Besides food production, there are thousands of manufacturing processes that require heat. RTDs are critical to these processes, from immersion heaters for large tanks to packaging equipment, sterilizers, and dozens of other applications.

This blog will focus on RTD sensors, how they work, and their applications. 

Definition of RTD Temperature Sensors

RTD sensors are temperature sensing devices that use the resistance of a resistor wire to measure temperature. It relies on the principle of electrical resistance. As the temperature of metal increases, so does its resistance or the ability of electrons to pass through it. Based on the resistance information, the sensor’s electronics determine the temperature of the wire. Once calibrated, a change in resistance directly corresponds to the temperature value of whatever system it measures. Another name for this device is a Resistance Thermometer.

RTD sensors can monitor process temperatures or measure the air temperature in buildings and are also an inexpensive way to measure relative humidity and pressure. They can measure the surface temperature of an object or the internal temperature of a substance in a tank. RTDs have a wide temperature range, good accuracy, a long lifespan, and are easily integrated into industrial systems.

How Do RTD Sensors Work?

RTD sensors measure the electrical resistance of a wire as it changes with temperature. As the temperature increases, the wire’s resistance increases. When a low amount of current passes through the element, the voltage drop is proportional to the wire’s resistance. 

The temperature coefficient of resistance (TCR) is the relationship between the changes in resistance and temperature. The temperature is directly proportional to the resistance, then measured and converted to temperature calibration units. An RTD sensor with a resolution of 0.1°C can detect changes in temperature as small as 0.1°C.

This resistance of the metal uses Ohms as a measurement. The accepted standard in most countries for Platinum wire is 100 ohms at 0°C with a temperature coefficient (TC) of 0.00385 ohms per ohm degree centigrade. Depending upon the type of RTD, its response time may be as fast as 0.5 seconds. From this information, the sensor’s electronics determine the temperature of the wire. 

RTDs can use different resistance wires, with platinum-iridium alloy providing the most accurate temperature readings or resolution. The RTD connects to other components using two, three, or four insulated copper lead wires.

We’ll go into more detail about the lead wires in a moment. First, we need to understand the properties of the resistance wires.

RTD Sensor Element Materials

The correlation between temperature and resistance of the sensing element should be linear and stable. Materials whose temperature vs. resistance characteristics remain linear to higher temperatures are the best choice.

There are four primary metals used for temperature-sensing elements.

• Platinum Sensor Elements

Platinum resistance temperature detector elements are pure platinum wire with copper wire extension leads. Platinum RTD sensor elements are highly accurate for industrial applications. They have a wide temperature range and are suitable for applications that require stability and linearity.

• Nickel Sensor Elements

Nickel elements don’t have the temperature range of platinum. They require error corrections above 300 °C or 572 °F. Nickel has good corrosion resistance and is less expensive than platinum RTDs, but it doesn’t last as long. Their working temperature range is -80 °C (-112 °F) to 260 °C (500 °F).

• Copper Sensor Elements

Copper has good linear resistance relative to temperature change. However, it needs a longer element than platinum because of its low resistivity forces. It’s limited to processes under 150°C (302°F) because it oxidizes. It’s a viable low-cost option for temperature measurement in motors, generators, and turbines.

• Balco Sensor Elements

Balco is not as common as the other three metals. It is a nickel-iron alloy consisting of 70% nickel and 30% iron. It’s a low-cost metal with similar resistance as nickel but twice the resistivity. It provides a relative linear resistance at temperatures between -40°C (-40°F) or 116°C to (240°F).

The two most essential factors in RTD elements are resistance and metal purity since it affects the resistance. For example, gold and silver have low resistance, so they are not a good choice.

The factors determining which element to use are the cost, temperature limits, and accuracy required.

Knowing the properties of these metals, manufacturers build three primary types of RTDs.

Types of RTD Sensors

C.W. Siemens, the founder of the Siemens corporation, assembled the first resistance thermometer in 1860. He created one from insulated copper wire and a galvanometer connected to a battery. Through experimentation, he found platinum to be the most accurate and have the widest temperature range. Today, there are three methods for creating RTDs.

Thin-film RTDs

Thin-film-type RTD elements have a thin layer of metal, typically platinum, laminated to a ceramic base material. A laser etches or cuts out the electrical circuit pattern. After attaching lead wires, a thin glass coating protects the unit.

Thin-film Resistance Temperature Detectors are reliable, low-cost, and more resistant to shock or vibration damage than other types. Their flat profile makes for design flexibility in different industrial control and instrumentation applications, such as industrial immersion heaters.

Wire-wound RTDs

Manufacturers make wire-wound RTDs by winding thin platinum wire around the outside of a ceramic or glass housing, known as a bobbin. Glass core RTD sensors measure temperatures of liquids in tanks and vessels. RTDs with ceramic cores can accurately measure extreme temperatures. Wire wound RTD sensors require skilled technical engineering and highly advanced manufacturing processes. They are usually more expensive than thin film sensors.

Coiled RTD Sensors

Coiled sensors look like springs. They are thin platinum sensing wires coiled inside a ceramic or glass core packed with a non-conductive powder. The ceramic or glass housing is protected inside a metal sheath. The powder allows the resistance wire to expand and contract with the temperature changes, minimizing errors caused by mechanical stress. The tightly packed powder also increases the sensor’s heat transfer and response time. 

Wire-wound RTDs are the most accurate but are more expensive to manufacture than thin-film types. Mounting inside a core protects them when submerged, such as inside a tank with flanged immersion heaters. 

Attaching either type of RTD requires copper lead wires.

Lead Wires for Connectivity and Accuracy

The RTD connects or terminates with other components to deliver temperature data. Lead connecting wires, typically copper, connect the RTD to temperature recording or process control instrumentation. The lead wire resistance can slightly affect the temperature coefficient. There are two methods to correct this issue.

1. A three-wire configuration reduces the error by connecting two lead wires to one side and one wire to the other. A circuit will measure the extra wire’s resistance and subtract the value from the element’s resistance to make the correction.
2. A four-wire configuration is the most accurate because two wires connect to each side. The application circuitry measures the total resistance of the lead wires and subtracts it from the RTD’s known resistance value.

Regardless of the number of lead wires, these sensors have unique properties.

Properties of Resistance Temperature Detector Sensors

Here are the properties that make RTDs essential for processes requiring accurate temperature measurement. 

• Accuracy – The accuracy of RTD sensors depends on the length and diameter of the sensor wire. The larger the wire, the less accurate the measurement is because the temperature at the sensor’s surface is higher than the temperature of the substance or surface it is measuring. The sensor’s wire should be as short as possible for greater accuracy. 
• Linear Response – RTD sensors have a very linear response producing a straight line when the sensor’s temperature and the substance’s temperature are plotted on a graph. 
• Temperature Range – Temperature sensor ranges vary widely, typically between -200°C (-328°F)  and +600°C (1112°F).
• Response Time – RTD sensors have a swift response time, generally less than one second.
• Stable Output – RTD sensors have a very stable output, which means that the voltage produced by the sensor is almost constant between readings.

Sometimes Resistance Temperature Detectors get confused with other thermal controls.

Difference Between RTD, Thermocouples, and Thermistors

We’ve already discussed RTDs, but there are other similar sensors. The simplest way to differentiate these devices is: RTDs are pure metals, thermistors are polymer or ceramic, and thermocouples are two wires of different metals.

Thermistor

An electrical device made of ceramic or polymer, thermistors measure electricity and temperatures accurately. They detect temperature changes quickly, allowing controllers to make fast temperature-related decisions. Commonly used in home appliances such as freezers, water heaters, and air conditioning units, thermistors are not appropriate for industrial use. Their maximum working temperature is only 130°C (266°F).

Thermocouples

Thermocouples are mechanical, electro-mechanical, or fluid-mechanical components. 

They use two different metal wires, usually iron and copper, to detect temperature changes through a small voltage signal generated by the junction of the two dissimilar metals. Although not highly accurate, the response time is very fast. 

Here is a breakdown of the three devices:

	RTD	Thermocouple	Thermistor
Temp. Range	-260°C (-436°F) 850°C (1562°F)	-270°C (-454°F) 1800°C (3272°F)	-80°C (-112°F) 150°C (302°F)
Sensor Cost	Moderate	Low	Low
Stability	Best	Low	Moderate
Temp. Sensitivity	Moderate	Low	Best
Linearity	Best	Moderate	Poor
Applications	General purposes, highest accuracy, and temperature averaging	Highest temperatures	Best sensitivity, narrow ranges, and point sensing

[Source- Chart]

RTDs provide accurate and precise readings for a longer time than thermocouples. RTDs are much more accurate, producing temperature readings with an accuracy of +/- 0.1°C, while a thermocouple’s accuracy is only about +/- 1°C.

Applications of RTD sensors

We find sensors in thousands of applications in almost every industry.

Immersion Heaters

Industrial immersion heaters are electrical heating devices. One of the practical applications of RTDs is monitoring temperatures alongside immersion heaters in tanks. The heater is a metal coil heated by an electrical current and submerged in a liquid or material. It heats as the current passes through it.

There are various types, including over-the-side immersion heaters and flanged immersion heaters. Chemical processing plants and other industrial applications where high temperatures are required use immersion heaters.

Visit our blog to learn more applications for immersion heaters.

Building Automation

RTDs are essential for building automation, industrial control, and agriculture. They detect temperature changes, which help measure temperature, room occupancy, power consumption, or other conditions. 

Packaging Systems

RTD sensors are integral to hot melt packaging systems because of their precision in sensing and proportioning temperature. They consistently maintain a temperature control stability of +/- 1°F, critical for plastics and hot glue application systems.

Automotive Industry

You’ll find these sensors in every vehicle engine monitoring temperature of the oil, air, fluid levels, and other critical measurements.

Pharmaceutical Production

The production of drugs and medicines requires strict temperature control. Temperature fluctuations can alter the chemicals and damage a batch. The pharmaceutical industry requires precise instrumentation that meets research, formulation, testing, and production requirements.

Chemical Processing

The chemical industry has strict temperature control requirements for chemical purity and safety. Production systems integrate RTD sensors as monitors and controls to ensure thermal accuracy and safety.

Other Applications

Other typical applications for RTDs include agricultural monitoring, industrial control systems, and data logging. 

The versatility of RTD sensors makes them essential components in power electronics, computers, consumer electronics, food handling and processing, industrial electronics, medical electronics, military, and aerospace.

Advantages of RTD Sensors

There are many reasons why applications require RTD sensors. Below are some of the key benefits: 

• Wide Temperature Range – RTD sensors have a wide temperature range for various applications.
• Excellent Accuracy – RTD sensors have superior accuracy, usually within +/- 0.1°C of the measured temperature. 
• Fast Response – RTD sensors can quickly and accurately detect temperature changes in as little as 0.5 seconds. 
• Stable Output – A very stable output means that the voltage produced by the sensor is almost constant between readings.
• Affordable Technology – They are very reasonable compared to other types of sensors and are easy to maintain.
• Linear and Predictable – More linearity compared to a thermocouple.

Most of these sensors are low maintenance, reliable, have a long service life, and work in many applications. 

Disadvantages of RTD Sensors

Despite the many advantages, there are a few disadvantages. These include: 

• Expensive – RTD sensors are costly and have high maintenance costs. 
• Installation – RTD sensors are bulky and heavy, which makes them difficult to install. They also require a dedicated space for installation due to their large size. 
• Calibration – RTD sensors require calibration to ensure that they read the actual temperature of the measured substance. Calibration is necessary when first installed and must be recalibrated if it is relocated.
• Sensitive to Vibration – These sensors are not suitable for applications where vibration or sudden shocks happen.
• Require Power – RTDs require power from 1 mA up to 10 mA. Although minimal, it can still heat the platinum wire and affect the readings.
• Slower than Thermocouples – The reaction time of an RTD is less than that of a thermocouple.

Thermocouples do not require power to work and are unaffected by heat. For high-temperature measurements, a thermocouple might be a better solution.

Essential Temperature Control in Manufacturing

Resistance temperature detectors (RTDs) monitor temperatures in many operations within a facility, such as hydraulics, air handlers, ovens, and immersion tank heaters, to name a few.

RTDs provide stability, accuracy, and repeatability for thermal systems. A limited amount of direct current activates the sensor and avoids overheating from too much current.

RTD sensors are fast and work well in a wide temperature range. One of their most significant advantages is linearity, meaning their temperature readings are almost constant throughout their effective temperature range. They have high repeatability, accurately measuring identical temperatures even after repeated exposure to multiple heating and cooling cycles. RTDs are an ideal solution for most industrial temperature monitoring applications.