The Resistance Thermometer (shorten called RTD – Resistance Temperature Detectors) is a temperature sensor (see also the Thermocouple) that determines the temperature to which it is subjected by exploiting the principle that the resistance of a given metal varies with the temperature.
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The resistance thermometer works on a basic relationship between metals and temperature. As the temperature changes, they have a corresponding variation of resistance derived from the sensing element made by certain metals (usually Platinum, Nickel)
By supplying the resistance thermometer with a constant current and measuring the voltage drop of the resistor it is possible to calculate the electrical resistance of a metal and determine the temperature.
Similarly, as the temperature of the resistance element (sensing element) increases, the electrical resistance, measured in ohms (Ω), increases. RTD elements are generally identified based on their resistance in ohms at zero degrees Celsius (0 C).
The types of resistance thermometers are classified with the abbreviation of the material used for their construction followed by the nominal resistance value, expressed in Ohm, with reference to the temperature of 0 ° C (ice melting point), according to the IEC reference standard. 751 (EN 60751).
Normally resistance thermometers are identified with the code of the material used for their construction (Platinum = Pt, Nickel = Ni) followed by their nominal resistance at a temperature of 0 ° C (example Pt100).
The range of use of industrial resistance thermometers is between -200 and + 850 ° C as reported in the EN 60751 standard. In the case of platinum, the relationship between resistance and temperature is described by the Callendar-Van Dusen equation (EN 60751).
The Pt100 resistance thermometer is the most common: at 0 ° C the sensing element assumes a nominal resistance of 100 Ω.
The Pt500 resistance thermometer at 0 ° C the sensing element assumes a nominal resistance of 500 Ω.
The Pt1000 resistance thermometer at 0 ° C the sensing element assumes a nominal resistance of 1000 Ω.
The most common materials used for the construction of the sensitive element are:
It is the most used in industrial applications as platinum has excellent corrosion resistance, excellent long-term stability and a wide range of temperatures (-200 … +850 ° C).
The EN 60751 standard stipulates that resistance thermometers must have a nominal value at 0 ° C. As mentioned, the most used values are 100 ohm (Pt100), 500 ohm (Pt500), 1000 ohm (Pt1000).
They are harmonized by DIN 43760 regulation, are less expensive in terms of construction than the platinum element and less dependent on corrosion. However, nickel is more subject to wear and tear over time and loses accuracy at elevated temperatures. Furthermore, the temperature range is more limited (-80… +260 ° C).
To date, their scarce use in the industrial sector makes them more expensive than common Platinum elements.
The resistance element is usually built in three ways: wire wound, spiral or film.
A resistance thermometer can be configured with 2, 3, 4 connection wires.
The choice of configuration must consider several factors for an appropriate temperature measurement process.
The 2-wire connection is the least accurate as it is not possible to calculate the lead wire resistance from the sensor measurement. They are mainly used in processes with short connecting wires and where accurate accuracy is not required.
The 3-wire construction is widely used in industrial applications.The third wire helps compensate for the average lead wire resistance from the sensor measurement. The 3-wire resistance thermometer is a valid alternative to a 4-wire configuration when there are long distances between the sensor and the measuring / control instrument.
The 4-wire resistance thermometer is used in processes where high accuracy is required in temperature measurement, such as in laboratories. The 4-wire circuit works with the volt-amperometric method, where one pair of wires is dedicated to measuring the voltage and the second pair is dedicated to generating the known current. With this method the resistance is obtained, and the circuit allows to compensate any differences in the cable resistances.
All thermometers with a tolerance class higher than class B must have a 3 or 4-wire configuration, to avoid the error introduced by the resistance of the conductor to which the sensitive element is connected. Resistance thermometers can be made with one or two sensitive elements and with internal configurations of the connection wire according to the following table (EN 60751):
The tolerance values of the thermometric resistance thermometers are classified in the following table according to the EN 60751 standard. These tolerances apply to thermometers with any R0 value.
Special tolerance classes (e.g. 1/3 DIN, 1/5 DIN …) are made as multiples or fractions of the values of tolerance class B. A special tolerance class must be accompanied by the operating temperature range in which it can works. Example for the definition of the tolerance class 1/3 DIN and 1/10 DIN 1⁄3 DIN = ± 1⁄3 * (0.3 + 0.005 * t) ° C or 100.00 Ω ± 0.10 Ω at 0 ° C 1⁄10 DIN = ± 1⁄10 * (0.3 + 0.005 * t) ° C or 100.00 Ω ± 0.03 Ω at 0 ° C
Good stability High accuracy Good linearity Limited drift Good interchangeability Long-term stability Connectable with common copper wires
Expensive Power needed Absolute low resistance Auto heating Sensitive to vibrations
