There are several types of temperature sensors used in industrial environments, the most common being Resistance Temperature Detectors (RTDs). Their main operating principle is based on the resistance of the material, which varies depending on the ambient temperature. The most common resistance temperature sensors are made of fine platinum, nickel or copper wire wrapped around a heat-resistant core. Each of these materials has a different temperature dependence of resistance, so a temperature sensor with a platinum wire is considered to be the most suitable and with the most linear relationship between temperature and resistance.
Table of contents:
PT100, where PT stands for the material (platinum abbreviation) and 100 for the resistance in ohms at 0 degrees Celsius, is the most common type of temperature sensor. There is also a type PT1000 which, as the name suggests, has a resistance of 1000 ohms at 0 degrees Celsius.
PT100 temperature probes can be made from either a fine platinum wire wrapped around a ceramic core or a fine micro-plate. In contrast, the PT1000 is only available in a micro-plate version. The purity of the platinum element and its physical arc length are also key physical differences that account for the different resistances.
The figure shows a typical RTD sensor structure, showing the fine metal wire that makes up the sensor element. This wire is painstakingly arranged to ensure a stable and accurate temperature measurement.
Using metal wires such as platinum ensures the sensor's linear properties and accuracy over a wide temperature range. Platinum's high purity and precision make it an ideal material for this type of temperature sensor.
The total resistance of a temperature sensor is made up of two components - the resistance of the platinum element itself and the resistance of the sensor cable. The lower the resistance of the sensor and the longer the sensor cable, the lower the effect of the sensor on the overall resistance of the circuit and hence the temperature accuracy. Therefore, PT1000 sensor types are recommended where longer cables from the temperature point to the receiver are required and only a 2-wire sensor can be used. To reduce the measurement error in 2-wire arcs, temperature sensors with 3 and 4 wires have been developed. To achieve the lowest temperature measurement error, it is recommended to use a PT1000 with a 4-wire circuit.
| Temperature sensor | Connection type | Temperature offset class | Measurement error °C |
| PT100 | 2 defects | B | 5.25 |
| PT100 | 2 defects | A | 4.65 |
| PT100 | 4 defects | B | 1.05 |
| PT100 | 4 defects | A | 0.45 |
| PT1000 | 2 defects | B | 1.47 |
| PT1000 | 2 defects | A | 0.87 |
| PT1000 | 4 defects | B | 1.05 |
| PT1000 | 4 defects | A | 0.45 |
*Data used for the calculation - temperature 150 °C, cable length 10m, conductor cross-section 0.22mm².
RTD temperature sensors are used in many industries such as food and beverage, pharmaceutical, chemical and oil refining. They provide accurate and reliable temperature measurements, which are essential in these processes.
RTD sensors are often connected to a transducer to ensure accurate temperature measurement. The transducer converts the sensor's resistance signal into an analog or digital format that is easy to understand for control systems. Most commonly, the transducer converts the signal into a 4-20 mA current loop, which is the industry standard for long-distance signal transmission with minimal signal loss.
RTD sensors can be connected to different types of devices such as programmable logic controllers (PLCs), data acquisition systems and supervisory control and data acquisition (SCADA) systems. The connection can be made using a two, three or four-wire system, where the four-wire system provides the highest measurement accuracy as it compensates for the resistance of the wires.
