What Is an RTD?
(Submitted by Randall Gauthier, RdF Corporation, http://www.rdfcorp.com/)
An RTD (Resistance Temperature Detector) is basically a temperature sensitive resistor. It is a positive temperature coefficient device, which means that the resistance increases with temperature. The resistance of the metal increases with temperature. The resistive property of the metal is called itsresistivity. The resistive property defines length and cross sectional area required to fabricate an RTD of a given value. The resistance is proportional to length and inversely proportional to the cross sectional area : rXL R= -----------A Where R = Resistance (ohms) r = Resistivity (ohms) L = Length A = Cross sectional area RTD Materials The criterion for selecting a material to make an RTD is: •the material must be malleable so that it can be formed into small wires. • it must have a repeatable and stable slope or curve. • the material should also be resistant to corrosion. • the material should be low cost • it is preferred that the material have a linear resistance verses temperature slope Some of the common RTD materials are: Platinum with a temperature coefficient of 0.00385 -0.003923 Ω/Ω/°C and practical temperature range of -452 to +1100°F (-269 to +593°C). The platinum RTD has the best accuracy and stability among the
common RTD materials. The resistance versus temperature curve is fairly linear and the temperature range is the widest of the common RTD materials. Platinum has a very high resistivity, which means that only a small quantity of platinum is required tofabricate a sensor and making platinum cost competitive with other RTD materials. Platinum is the only RTD commonly available with a thin film element style. Primary uses: Platinum is the primary choice for most industrial, commercial, laboratory and other critical RTD temperature measurements. Copper, nickel and nickel iron are also commonly used RTD materials. They are mostly used in lower costnoncritical applications and will not be detailed in this article. Platinum RTD Performance Specifications : Temperature Coefficient ( ): Platinum RTDs are manufactured with two distinct types or temperature coefficients (∝). The temperature coefficient (∝) is the slope of the platinum RTD between 0°C to 100°C. It is calculated as follows: R100 - R0 ∝ = 100 X R0 ∝= Temperature Coefficient (Ω/Ω/°C)R100 = RTD resistance at 100°C R0 = RTD resistance at 0°C DIN Grade Platinum: The DIN grade, sometimes referred to as the European standard, has a temperature coefficient of 0.00385Ω/Ω/°C (+/- 0.000012). A consortium of European standards committees developed the curve that all manufacturers of platinum RTDs could conform to. The platinum that is used to 1
achieve the DIN standard is pureplatinum that is alloyed with a controlled small amount of platinum group metals to reproduce the curve. The DIN curve has captured a majority of the market for industrial RTDs worldwide. Thin film sensors are only manufactured with DIN platinum. Reference Grade Platinum: Reference grade platinum is made from 99.999% pure platinum. It will produce a maximum temperature coefficient of0.003926Ω/Ω/°C. The maximum temperature coefficient can only be achieved in Standard Platinum Resistance Thermometers (SPRT) for laboratory use. The practical range of temperature coefficients for industrial use is 0.003902 to 0.003923Ω/Ω/°C. Reference grade platinum is still the choice for critical applications including aerospace and nuclear. Accuracy: Platinum RTDs typically are provided in two classes, class Aand Class B. Class A is considered high accuracy and has an ice point tolerance of +/- 0.06 ohms. Class B is standard accuracy and has an ice point tolerance of +/-0.12 ohms. Class B is widely used by most industries. The accuracy will decrease with temperature. Class A will have an accuracy of +/-0.43 ohms (+/-1.45°C) at 600°C and class B will be +/- 1.06 ohms (+/- 3.3°C) at 600°C. The...
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