When a metal wire is heated the resistance increases. So, a temperature can be measured using the resistance of a wire. The RTD incorporates pure metals or certain alloys that increase in resistance as temperature increases and, conversely, decrease in resistance as temp decreases. RTDs act somewhat like an electrical transducer, converting changes in temperature to voltage signals by the measurement of resistance. The metals that are best suited for use as RTD sensors are pure, of uniform quality, stable within a given range of temperature, and able to give reproducible resistance temp readings. Only a few metals have the properties necessary for use in RTD elements.
RTD elements are normally constructed of platinum, copper, nickel or nickel- iron alloys. These metals are best suited for RTD applications because of their linear resistance temp characteristics as shown in Figure, their high coefficient of resistance, and their ability to withstand repeated temp cycles. The linear relationship of resistance-temp is given by the equation:
R = Ro (1 + ∝T)
R is the resistance at a temp T°C,
Ro is the resistance at 0°C, and
α is the temperature co-efficient of resistance
The coefficient of resistance is the change in resistance per degree change in temperature, usually expressed as a percentage per degree of temperature. The material used must be capable of being drawn into fine wire so that the element can be easily constructed.
Resistance Temperature Detectors
RTD elements are usually long, spring-like wires surrounded by an insulator and enclosed in a sheath of metal for protection. Figure shows the internal construction of an RTD. In the figure platinum is used as RTD element that is surrounded by a porcelain insulator. The insulator prevents a short circuit between the wire and the metal sheath.
Inconel, a nickel-iron-chromium alloy, is normally used in manufacturing the RTD sheath because of its inherent corrosion resistance. When placed in a liquid or gas medium, the Inconel sheath quickly reaches the temperature of the medium. The change in temp will cause the platinum wire to heat or cool, resulting in a proportional change in resistance. This change in resistance is then measured by a precision resistance measuring device that is calibrated to give the proper temperature reading. This device is normally a bridge circuit.
Advantages of RTDs
- Suitable for measuring high temperatures
- High degree of accuracy.
- Good stability and repeatability.
- Do not need a reference temp junction.
Disadvantages of RTDs
- Size is more than the thermocouple.
- Power supply required.
- Need auxiliary apparatus to get required form of output.
- Resistance element is more expansive than a thermocouple.
- Possibility of error due to self-heating and thermo-electric effect of the resistive element.