RTD theory

RTD stands for Resistance Temperature Detector. It is a type of temperature sensor that measures temperature by changes in the electrical resistance of a metal wire or film as temperature changes. RTDs are commonly used in applications where high accuracy and stability are required, such as industrial process control, HVAC systems, and laboratory equipment.

The theory behind RTDs is based on the fundamental principle that the electrical resistance of a conductor changes with temperature. In RTDs, this change in resistance is typically linear and predictable over a certain temperature range.

Here's a brief overview of the theory behind RTDs:

1. Temperature-Resistance Relationship: RTDs are typically made of materials with a predictable and linear relationship between resistance and temperature. The most common material used for RTDs is platinum (Pt), although other metals such as nickel (Ni) and copper (Cu) can also be used. Platinum RTDs offer excellent stability, linearity, and accuracy over a wide temperature range.

2. Positive Temperature Coefficient (PTC): Most RTDs exhibit a positive temperature coefficient, meaning that as temperature increases, the resistance of the RTD also increases. This relationship is described by the Callendar-Van Dusen equation, which is used to calculate the temperature from the measured resistance.

3. Resistance-Temperature Curve: The resistance of an RTD is typically measured using a Wheatstone bridge circuit, where the RTD is one of the arms of the bridge. By applying a known excitation voltage and measuring the voltage across the RTD, the resistance can be calculated using Ohm's law. The resistance value is then converted to temperature using calibration curves or polynomial equations specific to the RTD type and material.

4. Temperature Coefficient of Resistance (TCR): The temperature coefficient of resistance (TCR) is a measure of how much the resistance of a material changes with temperature. For RTDs, the TCR is specified by the manufacturer and determines the sensitivity and accuracy of the sensor. Platinum RTDs typically have a TCR of around 0.00385 Ω/Ω/°C, meaning that the resistance changes by 0.385% per degree Celsius change in temperature.

5. Linear Range: RTDs have a linear temperature-resistance relationship over a specific temperature range, typically from -200°C to +850°C for platinum RTDs. Beyond this range, the relationship may deviate from linearity, and special calibration may be required.

In summary, RTDs operate on the principle of measuring the change in resistance of a metal wire or film as temperature changes. This change in resistance is predictable and linear over a certain temperature range, allowing RTDs to provide accurate and stable temperature measurements in various applications.