A Pt100 is the most commonly used sensor element in RTD temperature probes. One of its applications is for the placement of circuit boards. It changes its ohmic resistance in relation to the temperature. When connected to an evaluation unit, the sensor temperature can be determined from the measured resistance. Often the Pt100 is also called Pt100 sensor or resistance sensor.
The Pt100 is usually used as a platinum chip temperature sensor and consists of a ceramic carrier to which a thin platinum layer is applied by a sputtering process. The term thin-film sensor is also commonly used due to the very thin layer thickness. The platinum coating is then structured in a meandering pattern using a photolithographic process. The connection wires are welded to pads after which these contact surfaces are then strain-relieved with a glass layer. A further – melted – layer of glass acts as a covering layer to protect the platinum meanders from external influences and also functions as insulation.
A wired Pt100 is mainly used in industrial electrical temperature measurement in RTD temperature probes.
Thin-film sensors are also available in surface-mounted device design type – SMD design type – which are specifically designed for automated placement on circuit boards. These Pt100 sensors do not have wire connections, but are soldered directly to the circuit board via solderable connection areas. Their small size allows a high placement density.
Pt100 sensors in DMD design type are suitable for surface or ambient temperature measurement on circuit boards and are the preferred choice for temperature monitoring or compensation circuits. In addition, a popular use of SMD sensors is the construction of measuring inserts for temperature probes. A circuit board is automatically equipped with the Pt100 sensors in SMD design type to form a preassembled measuring insert that can be slid into a thermowell. Such probes are used, for example, in large numbers for heat quantity measurement.
Platinum chip temperature sensors have been around since the 1980s. They largely replaced the Pt100 sensors with a platinum wire winding that had been used until then. The winding of these sensors is either fused into glass (glass sensor) or placed in holes of a ceramic tube (ceramic sensor).
Measurements of up to 800 °C are possible with the sensors. In special applications, the wirewound sensors are still used today (e.g. in laboratories). As a rule, however, the platinum chip temperature sensors described above are used.
"Pt" is the designation for platinum and the number 100 designates the basic resistance of 100 ohms at 0 °C.
At 0 °C, a Pt100 has a basic resistance of 100 Ohm. With increasing temperature, the sensor changes its resistance by about 0.38 ohm/kelvin (i.e. per °C temperature change). The characteristic curve is standardized (DIN EN 60751) and thus evaluation units can determine the sensor temperature from the measured resistance.
For more information on the application of Pt100 sensors in electrical thermometers, watch the video.
The behavior of a Pt100 described above can easily be read from a Pt100 resistance table.
Pt100 reference values of the resistor in steps of 1 °C:
For metallic temperature sensors a polynomial can be used to calculate which sensor resistance results for different sensor temperatures.
The basic formula for the second order polynomial is:
R(T) = R0 x (1 + A x ϑ + B x ϑ2)
R0: Basic resistance at 0 °C
ϑ: Temperature in °C
A, B: Individual coefficients of the sensor
For a Pt100, the base resistance is 100 Ω and the coefficients are 3.9083 ×10-3 °C-1 (A) and -5.775 ×10-7 °C-2(B). Substituted into the polynomial, the Pt 100 formula is:
R(T) = 100 Ωx (1 + 3.9083 ×10-3°C-1 x ϑ - 5.775 ×10-7°C-2x ϑ2)
The formula is valid for positive temperatures.
Platinum sensors also exist with basic resistances such as 1000 ohms (at 0 °C). Other nominal resistances are also producible, but rare.
The thin film technology allows measurements in a range from -70 to +600 °C.
The tolerances are also specified in the standard. For the thin-film resistors, there are tolerance classes or Pt100 accuracy classes from F 0.1 (high accuracy) to F 0.6 (low accuracy). The tolerance or limit deviation is determined according to the standard by inserting the measuring temperature into the respective formula.
Here are two examples:
Determination of the tolerance for a sensor of class F 0,1 and a measuring temperature of 100 °C:
±(0,1+0,0017 x I100I= ± 0,27 K (corresponds to ±0,27 °C)
Determination of the tolerance for a sensor of class F 0,3 and a measuring temperature of 400 °C:
±(0,3+0,005 x I400I= ± 2,3 K (corresponds to ±2,3 °C)
Pt100 sensors are very stable in the long term, which is mainly explained by the resistance material platinum. The characteristic curve of the sensors is standardized. If the characteristic curve is available in an evaluation unit, the evaluation of the temperature signal is possible. Due to the standardization, the procurement of spare parts for the sensors or assembled thermometers is very unproblematic.