New to RTDs? Consider this.
According to Knowledge.Ni.Com:
RTD Overview “A platinum resistance temperature detector (RTD) is a device with a typical resistance of 100 Ω at 0 °C. It consists of a thin film of platinum on a plastic film. Its resistance varies with temperature and it can typically measure temperatures up to 850 °C. Passing current through an RTD generates a voltage across the RTD. By measuring this voltage, you can determine its resistance and, thus, its temperature. The relationship between resistance and temperature is relatively linear.
Figure 1. Physical Architecture of an RTD
RTD Fundamentals “RTDs operate on the principle of changes in electrical resistance of pure metals and are characterized by a linear positive change in resistance with temperature. Typical elements used for RTDs include nickel (Ni) and copper (Cu), but platinum (Pt) is by far the most common because of its wide temperature range, accuracy, and stability. “RTDs are constructed using one of two different manufacturing configurations. Wire-wound RTDs are created by winding a thin wire into a coil. A more common configuration is the thin-film element, which consists of a very thin layer of metal laid out on a plastic or ceramic substrate. Thin-film elements are cheaper and more widely available because they can achieve higher nominal resistances with less platinum. To protect the RTD, a metal sheath encloses the RTD element and the lead wires connected to it. “Popular because of their stability, RTDs exhibit the most linear signal with respect to temperature of any electronic temperature sensor. However, they are generally more expensive than alternatives because of the careful construction and use of platinum. RTDs are also characterized by a slow response time and low sensitivity, and, because they require current excitation, they can be prone to self-heating. “RTDs are commonly categorized by their nominal resistance at 0 °C. Typical nominal resistance values for platinum thin-film RTDs include 100 and 1000 Ω. The relationship between resistance and temperature is nearly linear and follows this equation: For <0 °C RT = R0 [ 1 + aT + bT2 + cT3 (T – 100) ] (Equation 1) For >0 °C RT = R0 [ 1 + aT + bT2 ] Where RT = resistance at temperature T R0 = nominal resistance a, b, and c = constants used to scale the RTD “The resistance/temperature curve for a 100 Ω platinum RTD, commonly referred to as Pt100, is shown in Figure 2.
Figure 2. Resistance-Temperature Curve for a 100 Ω Platinum RTD, a = 0.00385
This relationship appears relatively linear, but curve fitting is often the most accurate way to make an accurate RTD measurement. “The most common RTD is the platinum thin-film with an a of 0.385%/°C and is specified per DIN EN 60751. The a value depends on the grade of platinum used, and also commonly include 0.3911%/°C and 0.3926%/°C. The a value defines the sensitivity of the metallic element, but is normally used to distinguish between resistance/temperature curves of various RTDs. Table 1. Callendar-Van Dusen Coefficients Corresponding to Common RTDs
* For temperatures below 0 °C only; C = 0.0 for temperatures above 0 °C.
|Standard||Temperature Coefficient (a)||A||B||C|