What is an RTD Sensor? How Does it Work?

What is an RTD Sensor

How RTD Works? Resistance Temperature Detectors (RTD) Working Principle

A resistance temperature detector (RTD) is a sensor that measures temperature using a material’s resistance to temperature change.

RTDs are commonly used for industrial temperature sensing and measuring applications, as they are accurate and can measure a wide range of temperatures. They can also be more reliable and durable than other types of temperature sensors. 

In 1860, the original resistance thermometer was constructed using insulated copper wire, a battery, and a galvanometer. However, C.W. Siemens, the inventor of this device, found that a platinum element provided more precise measurements across a wider temperature range. 

As a result, platinum has become the preferred material for RTD sensing elements in temperature measurement, and it continues to be widely used today.

In this comprehensive guide, we’ll cover ‘what is an RTD sensor’, how an RTD works, its applications and the benefits of a resistance thermometer.

Did you know that we design and manufacture custom-built resistance thermometers? If you’re not sure what you need, please get in touch and we can help identify your requirements.

What is an RTD Sensor?

An RTD temperature sensor is a resistor made from a material like copper, nickel, or platinum. When the temperature changes, the material’s resistance will change so it can be measured, allowing it to be used as a temperature measurement device. There are two types of RTD construction; flat film and wire-wound. 

A flat film, or thin-film, RTD sensor is most commonly used due to its lower cost and resistance to shock, however flat film is not suitable for higher accuracies. 

A wire wound RTD sensing element typically comprises of a delicate, coiled wire wound around a core made of ceramic or glass. The most common number of wires is 3; however, some RTDs have 2 or 4 wires. Due to its fragility, the element is commonly shielded by a protective probe. 

The RTD element is constructed using a pure substance that has been thoroughly characterised for its resistance at different temperatures. As the temperature varies, the substance undergoes a predictable alteration in resistance, which enables the measurement of temperature.

The most popular RTD sensor probe is an RTD Sensor Pt100. They have been used for many years to measure temperature in laboratory and industrial processes, and have developed a reputation for accuracy, repeatability, and stability.

Related: What is a Temperature Sensor?

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How Does a Resistance Thermometer Work?

An RTD functions on the principle that as the temperature of a metal increases, so does the resistance to the flow of electricity. To measure this change in resistance, an electrical current is passed through the sensor, and the resistance element measures the current in Ohms. The resistance value is then converted into a temperature reading.

Resistance Temperature Detector (RTD) Applications

RTD sensors find applications in diverse industries, including automotive, power electronics, consumer electronics, food handling and processing, industrial electronics, medical electronics, military, and aerospace. 

They are utilised to measure temperatures in a variety of environments, ranging from hazardous industrial settings to medical operating rooms.

At Process Parameters, our RTD sensors are ideal for general-purpose industrial and OEM applications. We’re able to manufacture our resistance thermometers to your requirements for almost any environment.

Take a look at the industries we work with.  

RTD Sensors Types: Thin-Film and Wire-Wound 

RTDs are divided into two types depending on the construction of their temperature-sensing element: thin-film and wire-wound RTDs. The invention of the resistance thermometer was possible due to the discovery that the conductivity of metals decreases predictably as temperatures increase. 

Thin-film RTD sensors have a resistive material, typically platinum, that is sputtered onto a substrate, while wire-wound RTDs are constructed by winding a resistive material like copper or nickel around a ceramic core. 

Each type of RTD is suitable for specific applications, with thin-film RTDs limited to 500°C and typically only available with lower accuracy. They are however robust and resistant to shock and vibration.

Wire wound RTDs are more suitable for higher temperatures than flat film but problems start to occur at temperatures above 650°C. Wire-wound RTDs are available in all accuracy and tolerance bands but are less resistant to shock and vibration. Wire wound is also more expensive than flat film sensors.

What is the Difference Between RTD and Thermocouples?

The main difference between an RTD and a thermocouple is the way they measure temperature. An RTD measures temperature by measuring the resistance of a material, while a thermocouple measures the voltage difference between two different metals. 

Thermocouple sensors are typically less expensive than RTDs, and they can measure higher temperatures than RTDs. RTDs are also more sensitive to temperature changes, and they are better suited for applications that require stability and repeatability. Therefore, the choice of which to use should be based on the application and the requirements of the user.

Related: What is a Thermocouple?

What is the Difference Between RTDs and Thermistors?

Resistance temperature detectors and thermistors are both types of resistors that have a resistance value which changes in relation to temperature. RTDs are typically composed of a metal element, often platinum, and are protected within a probe, sheath, or ceramic substrate. 

On the other hand, thermistors are composed of metal oxides such as manganese, nickel, or copper, paired with binding agents and stabilisers. In recent years, thermistors have become more popular due to improved meters and controllers that allow users to accommodate a wide range of thermistors. 

RTDs are more accurate than thermistors, but thermistors are cheaper and more easily manufactured. Additionally, thermistors are better at measuring rapidly changing temperatures, while RTDs have greater stability and are better suited for applications that require repeatability. 

RTDs can measure a wider range of temperatures than thermistors, and they are made of high-quality metals. Thermistors are more sensitive to changes in temperature and they can measure lower temperatures than RTDs. Therefore, the choice of which to use should be based on the application and the requirements of the user. 

Advantages of Resistance Temperature Detectors

Temperature monitoring is critical for many industrial processes for optimal operation. RTD sensors are a reliable way to achieve precise temperature control. While they are more expensive than other methods, they offer cost savings due to their accuracy, longevity, repeatability, and stability. 

These sensors are highly sensitive and offer quick and accurate readings due to their linearity, which ensures that resistance changes reflect temperature changes. Additionally, RTD sensors offer consistent readings over a long period of time and are made of high-quality metals, making them suitable for a wide range of temperatures.

Summary of What is an RTD Sensor

In conclusion, RTD sensors are passive temperature-sensing devices that are used due to their accuracy, repeatability and stability. The principle of operation is simple: as temperatures increase, the resistance of a metal changes. 

Different types of RTD sensors are categorised by the construction of the temperature sensing element and they come in various sizes and standards for accuracy. With the right selection, an RTD sensor can be used in a wide variety of applications, providing accurate and reliable temperature readings.

We offer a wide range of custom-built RTD temperature sensors for industrial applications. With our high-quality RTD sensors and reliable customer service, we are a leading UK manufacturer of temperature measurement solutions. 

If you would like some advice or a free quote, please do not hesitate to contact the team at Process Parameters. Call us at 01628 778688 or email us here.

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Resistance Temperature Detector FAQs

Why Does an RTD Have 3 Wires?

An RTD with three wires has two wires to measure the resistance, and a third wire to provide resistance compensation for the wire used between the sensing element and measuring instrument. An RTD can also have four wires, which does the same as three but better, and a two-wire RTD has no resistance compensation. 

Does an RTD Need a Power Supply?

In most cases, an RTD does not require a power supply. An RTD can measure resistance without a power supply, using the voltage difference between its two wires. Ohm’s law is used to measure the resistance of the sensor by applying a small current. The measuring instrument does this for the user and there is no need for a power supply. In fact, applying too much current causes the detector to heat up, it acts like a small heater.

Why is Platinum Used in RTD?

Platinum is used in RTDs because of its consistent resistive properties. Platinum has a low-temperature coefficient of resistance, which means that its resistance changes very little over different temperatures. This makes it ideal for measuring temperature accurately and reliably.

What is an RTD Sensor Used for?

RTD (resistance temperature detectors) sensors are popular in industrial applications because of their accuracy, longevity and stability in detecting temperatures. Known to be more reliable than other temperature detection devices, RTDs are especially useful when precision measurements are required, such as in production and machinery.

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