Resistance thermometers – precise temperature monitoring

Pt100/0 measuring resistors are electrical resistors routinely used as sensor elements in resistance thermometers for technical temperature measurement. This resistor is made of the purest platinum and reaches 100 Ohm at 0°C.

As the temperature rises, the intrinsic resistance of the Pt100 also increases, reaching 138.5 ohms at 100°C.

The Pt100/0 measuring resistor is available in two models as standard: as a wirewound resistor and a flat-film measuring resistor.

The wirewound resistor is wound around a ceramic or glass mandrel and protected from external influences by ceramic or glass housing.

A ceramic substrate is coated in platinum for the flat film measuring resistor. This platinum layer features meandering conductor paths and is sealed with ceramic or glass adhesive.

Both resistor types are equipped with pins.

A resistance thermometer is a type of electrical contact thermometer. It utilizes the dependence of the electrical resistance of metallic or semiconductor resistors on temperature.

The Pt100/0 resistance thermometer is the type most commonly used for industrial purposes, with basic resistance of 100 ohms at 0°C.

Pt1000, Pt500 or Ni100 measuring resistors and NTC or PTC measuring resistors are also used for industrial purposes.

When the temperature of the metals changes, so does their electrical resistance. The electrical resistance increases when the temperature increases and decreases when the metal cools down. The way in which electrical resistance changes depending on the temperature varies from material to material.

This effect is utilized in resistance thermometers. Certain requirements need to be met to achieve accurate measurements on this basis. To start off, a material with a known temperature behavior that can be calculated is required. Then the basic resistance of this metal needs to be precisely set at a defined temperature through calibration and adjustment. Finally, the temperature of metallic resistance must be very close to the true temperature of the medium to be measured.

For Pt100/0 platinum measuring resistors, the first two requirements have been standardized in DIN EN 60751 and supported with a calculation function, temperature resistance tables and accuracy classes. The final thermal coupling requirements depend on a combination of the application situation and a fitting sensor design.

Depending on the manufacturing quality of the measuring resistors, resistance thermometers can be divided into different accuracy classes. According to the DIN EN 60751 standard, resistance thermometers have the following permissible deviations:

When the temperature of the metals changes, so does their electrical resistance. The electrical resistance increases when the temperature increases and decreases when the metal cools down. The way in which electrical resistance changes depending on the temperature varies from material to material.

This effect is utilized in resistance thermometers. Certain requirements need to be met to achieve accurate measurements on this basis. To start off, a material with a known temperature behavior that can be calculated is required. Then the basic resistance of this metal needs to be precisely set at a defined temperature through calibration and adjustment. Finally, the temperature of metallic resistance must be very close to the true temperature of the medium to be measured.

For Pt100/0 platinum measuring resistors, the first two requirements have been standardized in DIN EN 60751 and supported with a calculation function, temperature resistance tables and accuracy classes. The final thermal coupling requirements depend on a combination of the application situation and a fitting sensor design.

Depending on the manufacturing quality of the measuring resistors, resistance thermometers can be divided into different accuracy classes. According to the DIN EN 60751 standard, resistance thermometers have the following permissible deviations:

A traditional resistance thermometer consists of one or more measuring resistors, electrical conductors, a thermowell, insulation material and suitable connections.

In essence, resistance thermometers can be split into line sensors and measuring inserts. While measuring inserts are installed in fittings with a thermowell, connection socket and connection head and can be replaced at any time, line sensors consist of a flexible line and a sensor element.

Along with measuring resistance, the accuracy of the resistance thermometer is ultimately determined by the selected circuit type.

Two-wire circuits are the most cost-effective yet most inaccurate measuring method. The two connection pins on the measuring resistor are extended by one conductor and connected to the measuring device. This requires minimal material as the intrinsic resistances of the conductor are fully incorporated in the measurement signal. As a result, the conductors need to be as short as possible or have a large cross-section, or any errors need to be arithmetically compensated for.

The four-wire circuit requires the most materials and evaluation, yet yields the most accurate measurements. A smart combination of a constant current across the resistance provided by a red-white conductor pair and the measurement of the voltage drop across the measuring resistance by the second conductor pair ensures the intrinsic resistances of the conductors are fully compensated. The measurement signal is measured without any distortion.

The compromise between these two circuits is the three-wire circuit.

A bridge circuit such as the Wheatstone bridge, is used to compensate for the intrinsic resistances in a circuit. The challenge lies in the fact that the three conductors must be completely identical in terms of structure, material properties and external conditions to ensure measurements can be taken without any distortion. However, the differences arising from the actual design of the conductors and the actual operating conditions are significantly smaller than with the two-wire circuit.

Digital sensors are a combination of an analog sensor such as a resistance thermometer and a signal converter such as a measuring unit or a transducer. The transducer modulates the analog signal of the resistance thermometer into a discrete signal via a HART signal, for example. Digital displays in connection heads are also possible. Digital sensors require an external or an internal voltage source as well as a compatible communication interface.

NTC and PTC thermistors are sensors whose electrical resistance changes depending on the temperature. Two behaviors are possible in the process:

The resistance increases as the temperature increases. Conductivity is therefore best at lower temperatures. This behavior is exhibited by thermistors with a Positive Temperature Coefficient (PTC for short). Examples of PTC thermistors include metals such as the platinum in a Pt100/0.

NTC thermistors behave in exactly the opposite way. When the temperature increases, the resistance of the thermistors decreases – they exhibit a Negative Temperature Coefficient (NTC for short). As a result, conductivity is best at high temperatures. Examples of NTC thermistors include metal oxides/ceramics or semiconductors such as silicon.

In practical applications, both thermistor types can be used to measure temperatures. However, the partially strongly non-linear signal characteristics of NTC sensors must be taken into account.