Theory temperature sensors

Temperature sensors are indispensable in many technical applications in order to obtain exact information about the temperature of a system. They play a decisive role in areas like climate technology, process control, medical technology and automobile industry. The basic operating principle of temperature sensors is based on measurement of physical properties changing alongside temperature. There are different kinds of temperature sensors, each of them relying on different measuring principles. In this technical report, the basic principles of function of temperature sensors are explained and their scopes of application as well as their advantages and disadvantages are discussed.

PT100 und PT1000

These sensors are based on the principle of the platinum resistor. The PT100 has a resistance of 100 ohms at 0°C, the PT1000 a resistance of 1000 ohms at 0°C. The resistance changes in a linear fashion as temperature rises. The difference between the PT100 and the PT1000 is merely that the resistance of the latter has a tenfold value. Because of that the influence of wire resistance is reduced by factor 10.

The temperature range for both sensors normally lies between -200°C and +850°C. The reaction speed is usually high, which means they can detect temperature changes fast. The average reaction speed lies at t90%>45s and at t63%>12s. Besides that, temperature drift is generally low. Platinum resistors are defined in DIN EN 60751.

NTC (Negative Temperature Coefficient)

NTC-sensors use a semiconductor with a negative temperature coefficient. That means that their resistance declines with rising temperature, therefore they are also called high temperature conductors. The temperature range of NTC-sensors varies from model to model, but normally lies between -50°C and 150°C. The response speed is generally fast, similar to platinum sensors. The temperature drift however can be a bit higher than in platinum sensors.

The Beta-value (also called B-value) is a measure of temperature dependency of the resistor for an NTC. It indicates how strongly the resistance varies with the temperature. A higher Beta-value means a bigger change in resistance per degree of temperature change. The Beta-value is normally indicated in Kelvin.

NTCs are defined in DIN 43760. More on DIN 43760 can be found at the bottom of the page.

K-type

K-type-sensors are based on the principle of thermocouple principle and consist of two different metals which are connected at one end. The temperature range of K-type-sensors is normally very broad and can range from -200°C to +1350°C. When heated over 850°C, irreversible changes of the thermoelectric properties can occur due to oxidation. The response speed is generally fast, like the other sensors. The temperature drift, however, can be a bit higher than in platinum sensors.

This sensor was developed mainly for oxidative environments and must be particularly protected in other environments. Type K can also be used in low-temperature range and low temperature measuring technology up to -250°C. For K-type-sensors there are different tolerance categories. These limit deviations are defined by IEC 60584-1.

Ni-120

Ni-120-sensors are based on the principle of nickel resistors. The temperature range for Ni-120-sensors typically lies between -80°C and +260°C. At 0°C, Ni-120 have a resistance of 100 ohms. This resistance changes approximately linear as temperature changes and can be calculated with DIN 43760 via the Callendar-Van-Dusen-equation. Nickel is more sensitive than platinum and therefore this sensor is very well suited for applications in which a fast reaction time is needed.


Semiconductor sensors

SMT172

The SMT172 is a highly precise temperature sensor, developed especially for the application in demanding environments by Angst+Pfister. It delivers reliable measurement results in real time with an accuracy of ±0,1°C and a resolution of 0,01°C. The semiconductor temperature sensor also features pulse width modulation (PWM) which can be evaluated analogously or digitally. Furthermore, the sensor is applicable very flexibly because it supports a supply voltage ranging from 2,7 V to 5,5 V. Additionally, the SMT172 has a high long-term stability as well as a low temperature drift. Therefore, it is suitable for long-term surveillance applications and a reliable solution. Also, the SMT172 is to be emphasized for its energy efficiency. It is one of the most efficient temperature sensors currently on the market with an energy consumption of only 0,36 µJ per measurement. With a t63% of 0.6s it is very fast compared to other technologies. Because of the option of evaluating this sensor via PWM, wire resistance plays no role. There, sensor signals can theoretically be transported over infinite cable lengths.

SMT172

DIN 43760

DIN 43760 is a norm which sets the specifications for NTC temperature sensors. It defines the electrical and mechanical properties of NTCs and specifies the requirements of their accuracy, temperature range, resistor values and tolerance.

The norm DIN 43760 contains, amongst others, indications on the resistor values at certain temperatures, the tolerances of said resistor values, the operating and storage temperatures, the long-term stability, the insulation strength and the mechanical resilience of the NTCs.

Compliance of the norm DIN 43760 ensures a certain standardisation and comparability of NTC temperature sensors from different manufacturers.


Sensor evaluation

4-conductor-method resp. interface measuring method

The 4-conducter-method is a measuring method used to measure the resistance of an electronic component with a high accuracy. It is often used in electronics, electrical engineering and material testing.

With the 4-conductor-method, four ports are being used: two for the measurement of the current and two for the measurement of the voltage. Both current connections are connected to the component to be measured or the wire, while both voltage connections are utilized to measure the voltage over the component or the wire.

Current is fed into the component or the wire via both current ports. Because of these ports having a very low resistance, voltage drop over the ports can be ignored. The voltage over the component or the wire is then measured via the two voltage ports.

By using four components, the influence of resistance of the connection lines is eliminated. This enables an exact measurement of the resistance of the component or wire without having to consider the resistance of the connection lines.

The 4-conductor-method is especially useful for measuring very small resistors or resistors in circuits with high accuracy. It is also used for measuring the resistance of materials, e.g. to determine the specific resistance of metals.

In summary it can be stated that the 4-conductor-method allows for an exact measurement of resistance by eliminating the influence of connection lines and therefore delivers more accurate measurement results.

Evaluation unit

Both voltages at Rref and Pt100 are measured according to the 4-conductor-method, i.e. there is no current flowing in the wires A, B, C and D. Current flows from E to F. With this principle, the UTI can measure different values of unknown resistors (Pt100, Pt1000, Thermistors etc.).

From the UTI-signals, the relation of the resistance of Pt100 and Rref can be derived. Then, the resistance of Pt100 is derived from the relative value and the value of Rref (measured by another reference device), and finally the temperature value is extracted.

If there are other sensors to be evaluated, Rref must be adjusted, and the resistance must be interpreted that way that it approximately corresponds to the same height.

PT100 and PT1000 can also be evaluated directly via a PLC-interface. There are analogue interfaces which are construed for a platinum resistor. At 22°C, the PLC gives out a value of 220 without needing a transducer.

Figure 1: Evaluation unit/PT100 can be replaced arbitrarily

Finished assemblies

With temperature sensor assemblies you have the possibility to adapt sensors to your individual requirements. No matter if it involves standard temperature sensors with cables or calibrated measuring ranges, montage options, case dimensions or cable outlets, these assemblies can be configurated the way they fit your requirements perfectly. This ensures a simple integration of our assemblies in existing systems. With the possibility of tailoring sensors exactly to the requirements of your application, costs can be reduced. Angst+Pfister Sensors and Power offers assemblies with various technologies: No matter if PTC, NTC, PT100, PT1000, K-type or also Ni-120. For existing sensors from different manufacturers or for new projects we gladly prepare a comparative offer.

If you’d like to learn more about how our custom-specific temperature sensor assemblies can optimize your processes, we are gladly at your disposal. Contact our team for an individual consulting and experience how you can profit from this exciting technology.


Glossary