Calibration of thermodynamic measurands

Definition: The calibration of thermodynamic measurands is the documented determination of the relationship between the indications of a measuring instrument and the quantity values realised by traceable standards (e.g. temperature, relative humidity, pressure or flow) under defined thermodynamic and ambient conditions. It provides measurement errors and measurement uncertainties as the basis for reliable measurement results. Requirements for competence, traceability, measurement uncertainty assessment and documentation are laid down in DIN EN ISO/IEC 17025 for calibration laboratories.

Practical relevance: Temperature, humidity and pressure measurements are safety- and quality-critical in process plants, laboratories and testing facilities.
Calibration provides measurement errors, stability information and measurement uncertainties that are used to assess the suitability of a measuring device for specified limit values. A statement of conformity is only made if it has been commissioned and is based on a defined decision rule. Traceability is achieved via recognised reference scales and standards (e.g. ITS-90 for temperature or calibrated pressure and climate standards) and is decisive for process safety, product quality and regulatory compliance.

Decision-making perspectives:

• Technical decision-makers: Ensuring correct temperature and pressure control in plants and test rigs.
• Purchasing/project management: Definition of suitable calibration intervals as well as selection of competent, preferably accredited calibration laboratories.
• Science: Assessment of thermodynamic measurement uncertainties, reference scales (e.g. ITS-90) and long-term stability to validate experimental results.
• Insurance/law: Verification of standard-compliant monitoring of measuring equipment including traceability and decision rules in damage or compliance cases.

Typical testing or verification methods: Comparison calibration in temperature baths, furnaces or climate chambers under defined stability and equilibrium conditions, humidity and pressure calibration using traceable reference sensors or standards, determination and documentation of the measurement uncertainty including thermodynamic influencing variables (e.g. gradients, drift, stability), documentation of the traceability chain to recognised reference scales and standards, issuing of a calibration certificate where applicable with a statement of conformity based on a defined decision rule

FAQ:

• Why is temperature calibration safety-relevant?
• Incorrect temperature measurements can lead to overheating, material damage or process instability.