Type Approval
Type Approval, also commonly known as Pattern Evaluation, is the fundamental gatekeeping process in legal metrology. It is the rigorous, independent evaluation of a new design (or "type") of a measuring instrument to verify that it complies with all applicable statutory and regulatory requirements before it can be legally sold or used for commercial, health, or safety purposes.
The primary objective of type approval is to determine whether an instrument is robustly designed to maintain its metrological integrity under the environmental and operational conditions it will likely encounter in the field. It is not merely a test of initial accuracy, but a test of sustained reliability and resistance to interference.
The Evaluation Process
The type approval process is exhaustive and typically conducted by a designated National Metrology Institute (NMI) or an accredited third-party laboratory. The evaluation includes:
- Document Review: Thorough analysis of the manufacturer's engineering drawings, schematics, software architecture, and operating manuals to ensure the design principles are sound and compliant.
- Metrological Testing: Evaluating the instrument's basic accuracy, repeatability, and linearity across its entire specified measuring range under reference laboratory conditions.
- Influence Quantity Testing: Subjecting the instrument to variations in environmental conditions, such as temperature extremes, humidity, changes in atmospheric pressure, and fluctuations in power supply voltage, to ensure these factors do not cause measurement errors exceeding legal limits.
- Disturbance Testing: Evaluating the instrument's immunity to external disturbances, such as electromagnetic interference (EMI), radio frequency interference (RFI), electrostatic discharge (ESD), mechanical vibration, and shock.
- Software and Security Verification: Analyzing the instrument's software to ensure it cannot be easily manipulated or hacked to provide fraudulent readings, and verifying the presence of secure audit trails and digital seals.
Maximum Permissible Errors (MPE)
A critical concept in type approval is the Maximum Permissible Error (MPE). This is the extreme value of measurement error, with respect to a known reference value, permitted by legal regulations for a given instrument.
Type approval testing often utilizes a "stepped" MPE approach. For instance, the MPE during initial type evaluation under reference conditions is typically smaller than the MPE allowed during subsequent in-service inspections, acknowledging that instruments may experience slight degradation in performance over time in harsh field environments.
The Type Approval Certificate
If an instrument successfully passes all required evaluations, the regulatory authority issues a Type Approval Certificate. This document formally recognizes that the design is legally acceptable.
The certificate details the specific model, its authorized measuring range, environmental operating limits, required software versions, and the precise locations where physical or digital security seals must be applied to prevent unauthorized tampering. Once certified, the manufacturer may proceed to produce the instrument, subject to individual unit verification before deployment.