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Temperature is a thermodynamic parameter daily perceived and evaluated based on our senses. Our subjective assessment might not be accurate, since we could feel heat or cold at the same temperature due to different relative humidity, the presence of wind, or changing personal well-being. Uncertainties of subjective assessment can be avoided by measuring temperature using different types of thermometers or other temperature meters. However, can we trust the displayed values? At SIQ we know the answer to this question since we also calibrate the temperature and other associated quantities.

The temperature is not only related to our perception and well-being, but it is also encountered daily (i.) the temperature in refrigerators should not be too high to maintain shelf life of food, (ii.) many industrial processes occur only at a certain temperature, e.g., melting of iron in a foundry, (iii.) the optimal operation of the various machines and their service life are often associated with specific operation temperature range, (iv.) new device needs to be exposed to adequate temperature and humidity cycles in climate chambers to ensure operational lifetime and device safety, (v.) test laboratories and especially calibration laboratories must control the temperature and humidity very precisely (strictly) to ensure traceability and repeatability of results; additionally the environmental conditions must be continuously monitored and recorded (e.g., a constant temperature of 23±2 ° C and relative humidity 50%±20% must be ensured according to ISO 17025 standard).

Temperature/humidity measurement is generally based on electrical elements (sensors) that change their electrical properties by changing temperature/humidity, usually resistance or DC voltage. The temperature/humidity is then indirectly determined by measuring electrical quantities. Different types of resistors exist that change the resistance with temperature (e.g., the resistance of RTD sensors and Pt100/Pt1000 resistors increases linearly with increasing temperature, on contrary the resistance of thermistors decreases exponentially with increasing temperature, etc.). The second group of temperature meters are thermocouples, which generate temperature-dependent DC voltage between two different metals at the sensor junction, i.e., thermoelectric effect. Different combination of metals covers the temperature range from ‑210 °C up to 1800 °C. The thermocouples are marked with standard designations B, E, J, K, N, R, S, T, C, A, U, etc., accordingly.

The number of new product testing in temperature chambers increases due to restriction of novel standards. Products need to be cycled in different temperature and humidity profiles within a required time interval. The temperature in the chamber is inhomogeneous, therefore the tested products placement in a chamber must be chosen very carefully. Temperature fluctuations can also occur due to non-ideal chamber regulation or due to mechanical or electrical changes during the testing (i.e., the influence of tested device). The same also applies  for climate chambers, which regulate relative humidity in addition to temperature.

Several measuring instruments related to temperature measurement can be calibrated at SIQ:

  • all listed types of thermocouples (temperature calibration is usually performed at a reference temperature of 0 °C, external or internal reference is possible, the measurement can also be modified according to customer’s requirements)
  • RTD displays and simulators
  • temperature indicators and simulators
  • temperature and climate chambers
  • measuring instruments that can also measure or generate temperature based on DC voltage or resistance (calibrators, temperature meters) in addition to other measuring parameters
  • temperature probes at 23 °C (reference internal junction) in an oil bath

Calibration of test equipment at SIQ

Temperature and climate chambers

Calibrations of temperature and climate chambers are performed in the customer’s premises. We calibrate the following parameters:

  • Temperature: The temperature is measured using measuring system with nine probes arranged throughout the chamber. The calibration of each temperature requires at least 4 hours due to temperature stabilization. Therefore, the total calibration time can last several days if multiple temperatures need to be calibrated. Accredited calibrations are performed in the temperature range from –70 °C to +180 °C. Several sets of measurements in different measuring system arrangements are required for chambers larger than 2 m3.
  • Relative humidity: Six humidity meters are placed in the chamber. Each measurement of relative humidity lasts at least 4 hours. However, both relative humidity and temperature can be calibrated at the same time.
  • Temperature gradient measurement: The key parameter for certain chambers is the temperature gradient, i.e., the information how the temperature changes in a certain time. This parameter can be also calibrated non‑accredited using the measured temperature and time.

RTD displays/RTD simulators

RTD displays measure the resistance of the sensor at the input terminals and indicate the temperature of the sensor according to calculation. On contrary, the RTD simulators generate a certain resistance at the output terminals according to temperature settings. Both types of instruments are calibrated using reference resistance decades or by measuring the resistance (three-wire or four-wire connection). We also check the internal reference of the calibrated instrument, if available. Similarly, we also perform calibrations of displays and simulators based on Pt100/Pt1000 sensors, by measuring of generation of resistance and recalculation according to SIST EN 60751 and SIST EN 60751 standards.

Temperature meters/temperature calibrators for thermocouples

Temperature meters for thermocouples measure DC voltage at the input terminals and after the calculation give the temperature for a certain thermocouple type. On the other hand, temperature calibrators generate DC voltage at the output terminals according to set temperature and thermocouple type. Accredited calibration for both types of measuring instruments for thermocouples type B, E, J, K, N, R, S, T, C, A, U is performed indirectly by measuring or by generating DC voltage and recalculation according to IEC 584-1 standard. We can also calibrate data loggers which can have several channels/modules available for different types of thermocouples. The scope of calibration is usually defined according to customer’s needs. We can also provide accredited calibrations of temperature probes and humidity meters by cooperation with our external partners.

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