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RTDs are available in a variety of metals with platinum, nickel, copper, and tungsten being the most common. Fig. 1 shows a table that indicates the temperature range and the resistance coefficient (a) for these RTDs. The temperature coefficient is the amount of resistance change one may expect for each °C of temperature change. For example, the resistance of a platinum RTD is 100 Ω at 0°C. Since its resistance coefficient is 0.0039, it must be multiplied by 100 Ω to determine the amount of resistance change per °C.
0.0039 x 100 Ω = 0.39 Ω/°C
Since the platinum RTD will change 0.39 Ω for each °C, if the temperature changes 100°, the new resistance would change 39 Ω to the original 100 Ω resistance at 0°C and the new resistance for the RTD would be 139 Ω at 100°C. The following calculations show this.
0.39 °C/ Ω x 100° = 39 Ω
39 Ω + 100 Ω = 139 Ω at 100°C
This table shows the temperature range for platinum as -184°C - 815°C and for nickel as -73°C - 749°C, which is the linear range for these RTDs. The extended range for platinum is -270°C - 982°C, and for nickel the extended range is -101°C - 315°C. These RTDs don't maintain a linear curve when they are measuring temperatures in their extended ranges.
Above: Fig. 1 A table for RTDs that shows the type
of material, temperature range, and amount of resistance per °C.
