What is Guardbanding?
Guardbanding is a technique used to ensure that measurements remain accurate and reliable, even when there is a level of uncertainty or error in the measurement process. A guardband, which narrows the tolerance range of acceptable values for a measured quantity, is utilized to prevent false acceptances or false rejections, which can occur when the measured value is near the edge of the tolerance boundary and the uncertainty in the measurement is high. Guardbanding is a critical practice in many industries, such as manufacturing, engineering, and scientific research, to ensure the quality and reliability of products or systems being designed or manufactured using these measurements.
Calibration Guardbanding Width
The width of the guardband is typically related to the uncertainty of the measurement. However, while it may be desirable to set the guardband to the actual uncertainty value, this may not always be possible or practical. This is because the tolerance range for many measurements is often very narrow and setting the guardband to the uncertainty value would result in an overly restrictive range.
In practice, the width of the guardband is often determined by a balance between the desire to prevent false acceptances or false rejections and the need to maintain a reasonable tolerance range. The guardband width also depends on the specific application, industry standards and regulations, and the specific measurement process being used. In some cases, a larger guardband may be needed to account for known sources of error or uncertainty, while in other cases a smaller guardband may be sufficient.
There are multiple guardbanding methods that were devised by various authorities and regulatory bodies over the years. Most common methods used by the ISO/IEC 17025 accredited laboratories can be grouped into 4 distinct categories, listed in the order of complexity:
- Constant guardbanding based on percentage of tolerance: This method is used to prevent calibrations that have a passing result close to the tolerance limits and may potentially require a re-calibration soon. As an example, a unit that has the accuracy of more than 70% of its tolerance range may require and adjustment to improve the accuracy despite it being in the passing range.
- Taking measurement uncertainty directly into account: With this method, the guardband width is determined by the product of the uncertainty value itself. This method is useful for situations where the tolerance range is wide and uncertainties are small. These methods are described in ILAC G8:09/2019, ISO 14253-1.
- Guardbands based on the Test Uncertainty Ratio (TUR): This method takes into account the TUR and adjusts the guardbands for each specific situation. It is used where the tolerance range is very narrow and the uncertainty in the measurement process is high. It allows for a certain amount of error to be accounted for, while still maintaining a reasonable tolerance range. These methods are described in NCSLi RP10 and ANSI Z540.3 rulebook.
- Statistical guardbanding based on probability: These are the most sophisticated methods of calculating the guardband width based on the desired PFA (Probability of False Accept) or desired confidence level. These include Fluke™ RSS method, UKAS M3003, ANSI/NCSL Z540.3-2006 Method 6 and involve statistical tables and more complex math.
All methods have their own advantages and disadvantages, and the choice of which method to use depends on the specific application, industry standards and regulations, and the specific measurement process being used.
Qualer’s Approach to Guardbanding Calibration
When selecting a calibration management software or computerized maintenance management system (CMMS) for your operation, it is important to confirm how guardbanding is addressed within the platform. While some software solutions claim to support guardbands by simply allowing to enter the value manually, it could be cumbersome to calculate and implement. It would be best if the CMMS could determine the guardbands automatically based on the enforced regulations and desired outcomes.
Qualer has simplified guardbanding to a single dropdown, where you select the guardbanding method to be applied and the metrology engine does the rest. It automatically calculates the uncertainty, determines the applicability of the guardband based on the TUR (Test Uncertainty Ratio) and calculates the proper guardband width. If the measured value falls within the guardband, the measurement result can be configured to fail altogether or be reported as an ambiguous pass or fail. There is also a special case for units that are significantly out of tolerance, which may require a different calibration approach.
This guardbanding feature in Qualer is born of the overall Qualer design methodology: rather than simply provide the tools, intelligence is implemented into the software so that redundant tasks are automated and the chance of error is significantly reduced.