This is a common question when it comes to understanding the quality system covering the calibration of sensors, like accelerometers. More and more often customers are requiring dynamic test service providers to be certified to the requirements of
ISO/IEC 17025:2005 - the standard which covers the “General requirements for the competence of testing and calibration laboratories.”
Fundamentally, ISO17025 is a quality system (like ISO9001) which is structured to ensure the quality and repeatability of a product or service business. To that end, the standard does not define exactly how the processes should be carried out, but
rather an overarching set of guidelines and controlled outcomes that specific procedures should ensure. For example, the 17025 standard requires that the calibration of reference sensors have adequate control and calibration traceability. How
each of these areas is accomplished is up to the practitioner (such as choosing an appropriate calibration interval); but in the end, the choices for process/procedure must be documented, repeatable, defensible, and provide a quality output. The
quality system is proven by its outcomes. “Don’t just talk the talk (procedure) but also walk the walk (proven by objective evidence)” as reviewed in both the internal and external audit process.
Prior to any work on a 17025 compliant quality system, you’ll need to decide on your scope of competency. This scope defines the list of test and calibration services that your organization will be operating under the 17025 quality system. For dynamic
sensor users, this may be a list including items such as accelerometer calibration, force sensor calibration, and/or pressure sensor calibration. The following links are included as examples of different kinds of scope for dynamic calibration.
PCB Piezotronics scope and The Modal Shop scope.
To fulfill the requirements of the 17025 standard there are 5 basic components that compose the structure to meet the detailed clauses of the standard. They are: scope, procedures, control (including conformance/feedback), uncertainties, and proficiencies.
Overall, this is again very similar to the structure of ISO9001 in the use of procedures, control and audits. In fact, as far as procedures go, much of the same verbiage in place for an existing quality system can be used in fulfillment of a
17025 compliant system. The common parts to the two quality systems can be in areas like purchasing, document control, quality systems auditing/findings, and correct/preventive action.
The definition of calibration uncertainties is unique to ISO17025 and one core to the specific concept of calibration. The calculation of uncertainties (both systematic measurement bias and random components) is a critical part of both the understanding,
operation, and report documentation of sensor calibration. In fact, the repeatability of calibration within a prescribed uncertainty is the first level of performance proficiency. The area of uncertainties is typically one of the most intensely
discussed during the 17025 compliant system formulation, as well as during the certification audits. It is in this area that practitioners can show that they clearly understand the inner workings / cause and effect of the calibration system measurement
The concept of proficiency is extended through round robin testing. In a round robin, the same test sensor is calibrated by multiple organizations. Each organization provides calibration data and uncertainties so that the results may be compared.
Through the discussion of both calibration values and uncertainties, any unacceptably large deviation can be viewed, isolated, and discussed for root cause analysis, thereby improving the measurement and calibration process.
Ensuring all these parts conform to 17025, a laboratory can then enlist the services of a certifying body. The certifying bodies in the United States, such as A2LA or ACLASS , will assign an independent auditor who reviews the procedures/documentation of the quality system and ultimately makes an onsite visit to test the system for conformance with proof of objective evidence. Upon
satisfactory audit examination, the facility receives certification which is subject to regular renewal audits.
In future months we’ll break down the pieces of this in more detail, starting with calibration uncertainties.