Resistance thermometer

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SPRT Glass Capsule - RTD

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Thin Film PRT

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Wire Wound PRT

Coil Element PRT

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Resistance thermometer, also known as a resistance temperature detector (RTD), is a type of temperature sensor used to measure temperature by correlating the resistance of the RTD element with temperature. Most RTD elements consist of pure metal, with platinum being the most widely used due to its stable and repeatable resistance-temperature relationship. RTDs are known for their accuracy and stability over a wide temperature range.

Principles of Operation

The operation of a resistance thermometer is based on the principle that the electrical resistance of a metal changes with temperature. In the case of a platinum RTD, the resistance increases as the temperature rises and decreases as the temperature falls. This relationship can be described by the Callendar-Van Dusen equation, which provides a standard method to calculate the temperature from the measured resistance.

Construction

An RTD consists of a fine wire (usually platinum, but also nickel or copper) wrapped around a ceramic or glass core. The wire is protected by an outer sheath of metal or ceramic. RTDs can be made in different shapes and sizes, including thin film, wire wound, and coiled elements.

Types

There are several types of RTDs, categorized by their construction and the material used. The most common types are:

• Platinum RTDs: Known for their high accuracy and stability, platinum RTDs are widely used in industrial applications.
• Nickel RTDs: Less expensive than platinum, but with a more limited temperature range and less stability.
• Copper RTDs: Used for their high conductivity but have a very limited temperature range.

Applications

Resistance thermometers are used in a wide range of applications, from industrial process control to scientific research. They are particularly valued in applications where precision and long-term stability are crucial, such as in the pharmaceutical and food industries, aerospace, and environmental monitoring.

Advantages and Disadvantages

The main advantages of RTDs include their high accuracy, stability, and repeatability over a wide temperature range. However, they are generally more expensive than thermocouples, another common type of temperature sensor. RTDs also require a current source to operate, which can introduce measurement errors due to self-heating.

Comparison with Other Temperature Sensors

RTDs are often compared with thermocouples and thermistors. Thermocouples can measure higher temperatures and respond more quickly to temperature changes but are less accurate and stable than RTDs. Thermistors, on the other hand, offer a higher sensitivity over a limited temperature range but are not as stable or accurate as RTDs over time.

Installation and Maintenance

Proper installation and maintenance are crucial for accurate temperature measurement with RTDs. Factors such as the installation environment, vibration, and electrical noise can affect the accuracy of RTD measurements. Regular calibration is also necessary to ensure ongoing accuracy.

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