Cars, trucks, appliances, climate control systems, industrial processes, and life science instruments have a common need for temperature measurement. Platinum resistance temperature detector (RTD) elements are specifically designed for the precise measurements required by these and dozens of other applications. This article explores the inherent properties of platinum temperature sensors, the types of RTD elements available, and typical applications.
Platinum as a temperature sensor
A platinum RTD element is a thermal resistor -- a resistor with a value that changes by a predictable amount in response to temperature changes. Platinum RTD elements have a positive temperature coefficient (PTC), meaning the resistance increases with increasing temperature. A virtually linear change in resistance, coupled with intrinsic chemical and temperature resistance, makes platinum an ideal base material for temperature sensing. Platinum has been used in temperature sensing since the early 1900s and has a long history with precision temperature measurement applications.
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The elemental properties of platinum contribute to several characteristics beneficial to temperature sensing.
High precision: Strict controls on platinum RTD element manufacturing processes and raw materials minimize deviations from the ideal resistance versus temperature response.
High signal resolution: A relatively high (approximately 3,900ppm) change in resistance vs. temperature provides good signal resolution. Choosing an element resistance higher than the standard Pt100 (100Ω), such as Pt1000 (1,000Ω), Pt2000 (2,000Ω), or Pt10k (10,000Ω), further enhances resolution by increasing the ohms/°C response to temperature variation.
Wide operating temperature range: Platinum RTD elements are available for operation from -196°C to 1,000°C.
Long-term stability: The typical long-term drift of thin-film platinum RTD elements is 0.04 percent maximum after 1,000 hours at 500°C.
Interchangeability and standardization: The IEC 60751 specification defines the requirements, tolerance classes, and resistance versus temperature function for RTD thermometers made with a platinum element having a temperature coefficient of 3.851 x 10-3°C-1, commonly expressed as TC3851, TC3850, or 3850ppm. The defined tolerance classes allow for interchangeability between two elements of the same resistance value. Other available temperature coefficients include 3750ppm, 3770ppm, and 3916ppm.
Linearity: The resistance versus temperature relationship for a platinum RTD element can be described by the polynomial Rt = R0(1+at+bt2), where R0 is the resistance at 0°C, t is the temperature, a and b are experimentally derived constants, and Rt is the resistance at t. The value for b is so small that a linear relationship between temperature and resistance may be assumed for most applications. The linearity simplifies signal processing, and minimizes measurement error.
Platinum RTD sensor manufacturing process
Two major types of platinum RTD temperature sensors are available: thin-film and wirewound. As their names imply, thin-film elements, including surface-mount devices (SMDs), are manufactured via thin-film photolithography, while wirewound sensors are constructed from a precision-wound platinum wire.
I agree. This is a very nice article. I would like to add that in selecting thin film or wire wounds the engineer needs to factor in installation harness length . Thin films are driven by microamps, whereas wire wounds are driven by milliamps. Therefore, the signal to noise ratios need to be taken into account when checking your sensor accuracies. A short run harness can use a thin film element with no issues, but if you have a long run, wire wounds are typically less sensitive to harness noise. There are ways to compensate for this, but that adds cost, which affects the component selection process. All in all, this covers the best points of both sensing technologies.
Nice Article, one other big benefit of RTD sensors over thermocouples is that you can use standard copper wire between the sensor and the controller. Thermocouples require compensated conductors all the way to the controller.
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