Metrology Monday! #105 Building a calibration station in accordance with ISO/IEC 17025 – Equipment

Last week we talked about the importance of a good procedure as the basis of your test station.  This week, we move on to equipment.  Your equipment needs to be capable of achieving the appropriate accuracy/uncertainty required to produce a valid result for the products that you calibrate (6.4.5).  When you are selecting equipment, it is a good practice to look for equipment that is at least 4 to 5 more times accurate than the products you intend to calibrate. Yes, this is the one place where using Test Accuracy Ratios is appropriate.  The reason for this is that when you complete your final uncertainty analysis, your evaluation of expanded uncertainty should produce acceptable Test Uncertainty Ratios.  My recommendation is to do the math and complete a formal Test Accuracy Ratio analysis before you settle on the final equipment.  When you have chosen your equipment, don’t forget to make available to your staff procedures for handling, transport, storage, and a develop maintenance plans as necessary for the equipment (6.4.3).

Most of your equipment for the calibration station will require calibration. 6.4.6 provides excellent guidance regarding what equipment requires calibration.  You must calibrate the equipment if 1) the equipment uncertainty affects the validity of reported results or, 2) it disseminates measurement traceability for devices that you calibrate.  These requirements apply to accessories such as shunts, loads, or cabling if they affect measurement results.  You need to ensure that you maintain a calibration program for all of your equipment (6.4.7) that includes the adjustment of calibration intervals for your equipment so that it maintains an appropriate level of confidence as measured by End of Period Reliability (See post 90 for more information).  All equipment also needs to be labelled so that its calibration status is clearly discernible. (6.4.8).

It is often very important to think about correction factors (6.4.11) if your calibration station uses them.  Sometimes you need to correct for things such as reference resistor or shunt calibration values.  You also may need to correct for attenuator measured values at particular frequencies.  If your test station is automated, you must have a process to get these correction values into the calibration station, and more importantly, develop standard work for updating them after the equipment in question was calibrated.  As an assessor, I have cited a laboratory for having corrections in a calibration station that was from several calibration periods before the present one.  As it is with any of your equipment, you must also make sure that you have taken practicable measures to safeguard your equipment from unintended adjustments (6.4.12).  The key word in this sentence is “practicable”.  That means that this is not an iron-clad requirement because the authors of the 2017 version of ISO/IEC 17025 recognize that it may not always be possible to protect your equipment from unintended adjustments.  For older equipment with physical adjustments, it is fairly easy to put a “calibration void” seal on equipment.  Today however, most equipment is adjusted through firmware.  If this is the case, there may or may not be a password to protect access to the adjustment menu.  This is where everyone needs to ensure that practical measures were taken to protect the equipment.

Lastly, your equipment’s calibration must be metrologically traceable (6.5) to the SI (International System of Units).  This can be done by several ways, such as sending the equipment to an organization that can perform an accredited calibration, sending it to an appropriately recognized National Metrology Institute, or doing a direct realization of the SI.  Even if the laboratory you send this to is accredited, you should review their scope of accreditation to ensure that 1) the accreditation is still in effect, and 2) the scope of accreditation contains the measurements that you need with an appropriate level of uncertainty.  Likewise, you should also review the Calibration and Measurement Capability (CMCs) of the National Metrology Institute in the same way that you would an accredited laboratory.  You can find the CMCs for each National Metrology Institute at the BIPM Website in the Key Comparison Database (KCDB). 

If you have the ability in your calibration laboratory to do a direct realization of the SI, you must ensure that there are documented direct or indirect comparisons to national or international organizations that can make the same realization such as an interlaboratory comparison. 

ILAC P10:07/2020, the ILAC Policy on measurement results, provides additional guidance on ensuring that your measurements are metrologically traceable.  For example, if it is not possible to find an accredited calibration laboratory to calibrate your equipment, you may still use a non-accredited laboratory.  The burden of proving that they are metrologically traceable is now on you and you will have to apply the requirements of your accreditation body’s policy about using non-accredited laboratories.

While we are still focused on building a new test station, these requirements for equipment applies to all the equipment in your laboratory.  Next week we will discuss your facility and environment. #MetrologyMonday #FlukeMetrology