How the Standards Apply to Accelerometer Calibration
Familiar with standards like ISO17025 and ISO16063-21? Already operating under an ISO9001 Quality management system? The world of standards, controlled processes, documentation and audits can sometimes be daunting... To help demystify this world when it comes to accelerometer calibration, we’ll begin by discussing the relationships between the various parts of each these quality system pieces.
With the broad based acceptance and application of the ISO9001:2000 Standard for Quality Management Systems , most companies that own/operate test and measurement equipment have the obligation to maintain and control the acceptable function performance of this equipment. For this reason, many test engineers are already family with terms and procedures like, “Control of Measurement and Test Instrumentation”.
While working within the over-arching framework of the ISO9001, users often ask if there are any applicable standards to calibration of accelerometers. At broadest level for calibration, users should be familiar with the ISO17025:2005 Standard entitled, “General Requirements for the Competence of Calibration and Testing Laboratories”. The 17025 standard (formerly ISO Guide 25) provides the global quality framework for the calibration/metrology laboratory much the same way ISO9001 does for the entire organization. Like ISO9001, the 17025 standard covers control of instrumentation, vendor management, communication and documentation, however it also covers much deeper calibration specifics like uncertainty calculations and proficiency testing. When written properly the manuals, procedures, forms and instructions of 17025 flow seamlessly within an ISO9001 framework. In fact, if you operate under an existing quality system, much of the general process is probably already under control.
If you are not running under a current quality system, the ISO10012:2003 standard may be a good choice for your organization as it includes pertinent organizational aspects of the more sweeping ISO9001 standard and the full technical scope of the ISO17025 laboratory standard. In fact, once a lab is certified to both ISO9001 and ISO17025, it has reached compliance with ISO10012.
Regarding US standards, you may have also heard of the “Z540” standard. Formally called, ANSI/NCSL Z540-1-1994, Calibration Laboratories and Measuring and Test Equipment – General Requirements, the standard is essentially the same structure and content as the ISO17025 standard. When being certified to 17025, a laboratory can gain dual certification (ISO17025 and ANSI Z540) with only 3 additional questions which deal with subtle variations on a few clauses (use of non-standard methods, subcontracting and complaints).
Of particular interest and scrutiny during the accreditation process are the uncertainty calculations and proficiencies for each calibration methodology. These uncertainties are composed of both systemic and random components and are created by the environment and the component equipment selection used to create the calibration system.
In the case of accelerometers, the ISO standard 16063-21:2003 specifies a methodology for vibration calibration by comparison to a reference transducer (back-to-back method). An electrodynamic calibration shaker provides oscillatory (sinusoidal) excitation to both the reference sensor and the co-mounted sensor-under-test (SUT). Sensitivity is measured at a reference frequency (normally 100Hz or 160Hz). Sweeping through the desired range of frequencies then generates a frequency response curve of the transducer, while changing the level of excitation at a certain frequency allows the sensor linearity to be measured.
This simple overview begins to show the basic relationships between the applicable standards. In other editions of the Sensors & Calibration Tips newsletter we will discuss various calibration system considerations when operating under the 16063 standard, as well as provide tips and tricks for accelerometer calibration practitioners.