Temperature sensors play a central role in the precise determination and monitoring of temperatures in metrology. The precise use of these sensors requires in-depth specialist knowledge of their functionality, areas of application and special features. This guide covers the key aspects for professional use.
The basics of temperature measurement in metrology
Temperature measurement is based on physical principles that enable temperature to be converted into measurable electrical signals. Various measuring principles utilise different material properties such as changes in resistance, thermoelectric effects or semiconductor properties. These basic principles largely determine the properties and areas of application of the respective sensors.
The accuracy of the measurement depends on numerous factors. These include sensor quality, correct installation and consideration of environmental influences. Metrological aspects such as measurement uncertainty, reproducibility and traceability play a decisive role in the quality assurance of measurements.
Basic knowledge of heat transfer and thermodynamic processes is essential for understanding the measurement processes. These physical relationships influence the measurement accuracy and must be taken into account when selecting sensors.
International standards and calibration
Temperature measurement is subject to strict international norms and standards. These ensure the comparability of measurements worldwide. The International Temperature Scale of 1990 (ITS-90) defines fixed points and calibration procedures for precise temperature measurements.
Calibrations are carried out by comparison with reference standards of higher accuracy. Systematic measurement deviations are determined and documented. Traceability to national or international standards ensures the quality of the measurements.
Regular inspection and recalibration of the sensors is essential for reliable measurement results. Calibration certificates document the measurement uncertainty and validity period of the calibration.
Types of temperature sensors
The variety of temperature sensors available enables optimum customisation to specific measurement tasks. Each sensor type has characteristic advantages and disadvantages in terms of measuring range, accuracy, response time and cost.
Non-contact infrared sensors are suitable for measurements in hard-to-reach places or with moving objects. Contact sensors, on the other hand, often offer higher accuracies and are less susceptible to interference from the environment.
Choosing the right type of sensor requires careful analysis of the measurement task and the ambient conditions. Factors such as temperature range, required accuracy, media resistance and costs must be weighed up against each other.
Resistance thermometer (RTD)
Resistance thermometers utilise the temperature-dependent change in the electrical resistance of metallic conductors. Platinum measuring resistors such as Pt100 or Pt1000 are characterised by high accuracy and long-term stability. The nominal resistance at 0°C defines the type designation, whereby Pt100 sensors have a basic resistance of 100 ohms.
The characteristic curve of platinum measuring resistors is almost linear and highly reproducible. Various tolerance classes in accordance with DIN EN 60751 define the maximum permissible deviations. The choice of tolerance class depends on the accuracy requirements of the measuring task.
Different designs enable adaptation to different installation situations. Thin-film sensors are suitable for compact designs, while wire-wound versions offer maximum accuracy. The mechanical robustness and chemical resistance of the sensor must correspond to the operating conditions.
Thermocouples
Thermocouples are based on the Seebeck effect, which occurs when different metals are combined. The resulting thermoelectric voltage is proportional to the temperature difference between the measuring point and the reference junction. Different material pairings are standardised for different temperature ranges and applications.
The thermocouple type is selected based on the temperature range and the required accuracy. Type K with nickel-chromium/nickel-aluminium is widely used and covers a wide temperature range. Precious metal thermocouples such as type S offer higher accuracy at a correspondingly higher cost.
Cold junction compensation is essential for accurate measurements. Modern transmitters perform this task automatically. The routing and shielding of the thermocouple wires influences the measurement accuracy and must be carefully planned.
Semiconductor temperature sensors
Semiconductor temperature sensors utilise the temperature-dependent properties of semiconductor materials. Thermistors (NTC/PTC) are characterised by their high sensitivity and compact design. However, the non-linear characteristic curve requires appropriate signal processing.
Integrated temperature sensors combine measuring element and signal processing in one component. Digital output signals simplify further processing. The accuracy is sufficient for many applications, but does not reach the precision of high-quality platinum measuring resistors.
The simple integration into electronic systems and the favourable costs make semiconductor sensors attractive for many applications. Long-term stability and the limited temperature range must be taken into account when making a selection.
Selection criteria for temperature sensors
Selecting the optimum temperature sensor requires a systematic analysis of the measurement task. Technical requirements such as measuring range, accuracy and response time must be weighed up against practical aspects such as costs and availability.
A structured selection process takes into account the specific operating conditions. Mechanical loads, chemical influences and electromagnetic interference can affect the measurement. The accessibility of the measuring location and the available power supply limit the selection options.
The total costs include installation, maintenance and calibration in addition to the purchase. Redundant measuring points increase availability for critical applications. The availability of spare parts and service should be guaranteed in the long term.
Measuring range and accuracy
The required measuring range defines the basic suitability of a sensor type. Safety margins at the upper and lower end of the measuring range ensure reliable measurements. The linearity of the characteristic curve influences the achievable accuracy over the entire measuring range.
The measurement uncertainty is made up of systematic and random components. Systematic deviations can be compensated for by calibration. The reproducibility of the measurements is limited by random influences. A realistic estimate of the achievable accuracy takes all influencing variables into account.
The resolution of the measuring system must correspond to the required accuracy. The quantisation during analogue-digital conversion and the noise of the measuring electronics limit the effectively usable resolution.
Response time and dynamic behaviour
The response time describes the reaction of the sensor to temperature changes. Small thermal masses and good heat transfer enable fast response times. The design of the sensor and the installation situation have a significant influence on the dynamic behaviour.
Rapid temperature changes require correspondingly fast sensors. The sampling rate of the measuring electronics must be matched to the dynamic behaviour of the sensor. Filters in the signal processing reduce interference, but extend the effective response time.
The thermal coupling between the measurement object and the sensor determines the achievable dynamics. Thermally conductive paste improves the thermal contact with solid bodies. In the case of liquids and gases, the flow velocity influences the heat transfer.
Ambient conditions and interferences
Environmental influences can significantly affect the measuring accuracy. Temperature fluctuations in the area of the connection technology lead to thermal stresses. Electromagnetic fields induce interference voltages in the measuring lines. Protective measures such as shielding and filtering reduce these influences.
Mechanical stress caused by vibration or shocks requires a robust sensor design. Chemical resistance to aggressive media ensures long-term function. Moisture and dirt can impair the insulation and cause leakage currents.
The installation location must allow representative measurements. Temperature gradients and heat radiation distort the measurement results. Accessibility for maintenance and calibration should be taken into account during planning.
Practical implementation
Correct installation is crucial for reliable temperature measurements. The selection of suitable installation fittings protects the sensor and enables it to be replaced. Cable routing and connection technology must fulfil the electrical and mechanical requirements.
Signal processing includes amplification, filtering and digitisation. Modern transmitters offer additional functions such as linearisation and diagnostics. Integration into higher-level systems requires suitable communication interfaces.
Careful documentation of the installation facilitates subsequent maintenance work. Test reports and calibration certificates are part of quality assurance. Regular checks ensure the long-term function of the measuring system.
Mounting and installation
The mounting position has a significant influence on the measuring accuracy. Sufficient immersion depths ensure thermal coupling. Heat dissipation errors due to heat conduction via the thermowell must be minimised.
Standardised process connections simplify installation. Sealing materials and screw-in depths correspond to the process conditions. The mechanical fastening prevents vibrations and ensures the position of the sensor.
The electrical installation is carried out in accordance with the applicable regulations. Shielded cables and separate cable routes reduce interference. Earthing and potential equalisation are part of the EMC concept.
Signal processing and data acquisition
Signal processing converts the sensor signals into usable measured values. Analogue input stages must be matched to the sensor type. Precision amplifiers and filters improve the signal-to-noise ratio. The compensation of systematic errors increases the measuring accuracy.
The analogue-to-digital conversion digitises the measured values for further processing. The resolution and sampling rate of the ADC determine the achievable accuracy. Modern transmitters offer integrated functions such as linearisation and averaging.
Data acquisition stores and visualises the measured values. Trend displays enable process monitoring. Limit value monitoring and alarms warn of deviations. Archiving the measurement data is used for documentation and analysis.
Quality assurance and maintenance
Regular maintenance ensures the reliability of the temperature measurement. Inspections detect mechanical damage and wear. Cleaning the sensors prevents measurement errors caused by contamination. Electrical connections must be checked for tightness.
A systematic maintenance concept defines intervals and inspection points. The documentation of maintenance work is part of quality assurance. Spare parts for critical components should be kept in stock.
Preventive measures extend the service life of the measuring system. The analysis of faults helps with optimisation. Training of maintenance personnel ensures that the work is carried out correctly.
Regular calibration and inspection
Regular calibration ensures measurement accuracy. Calibration intervals are based on requirements and experience. Traceability to national standards must be ensured.
Comparative measurements with reference thermometers enable intermediate checks. The documentation of calibrations is part of quality management. Deviations outside the specification require corrective measures.
Modern sensors offer self-diagnostic functions for monitoring. Analysing the diagnostic data enables predictive maintenance. Trend analyses show gradual changes in sensor properties.
Error sources and troubleshooting
Systematic fault analysis facilitates troubleshooting. Typical sources of faults include mechanical damage, electrical faults and process influences. Possible causes of faults are excluded in a structured manner from the sensor to the evaluation unit.
Electrical measurements check the supply voltage and signal level. Insulation measurements detect the effects of moisture and ageing. Checking the parameterisation prevents software errors. Defective sensors show characteristic error patterns.
The documentation of faults supports future troubleshooting. Empirical values help to solve problems quickly. Replacement sensors enable the measurement function to be restored immediately.
New technologies and materials
Innovative sensor concepts extend the range of applications. Fibre optic sensors enable distributed measurements. Microelectromechanical systems (MEMS) combine sensor and electronics.
New materials improve stability and response behaviour. Nanostructures optimise heat transfer. Biocompatible coatings open up medical applications.
Development focusses on sustainability and resource efficiency. Energy self-sufficient sensors utilise energy harvesting. Recyclable materials protect the environment.
Conclusion and recommendations for action
The successful implementation of temperature measurements requires a systematic approach. Careful planning and professional installation are crucial. Regular maintenance and calibration ensure measurement quality.
Digitalisation offers new opportunities for optimisation. Modern sensor technology supports efficient processes. Integration into networked systems requires holistic concepts.
Future-proof solutions take technological trends into account. Investments in quality pay off in the long term. Continuous training ensures the expertise of employees.