Q: Why is Section 5.3(b) important?
A: No measurement is perfect, there is always some doubt (uncertainty) about every measurement, it’s a gray area. This uncertainty isn’t a problem until you need to call a measurement In-tolerance/Out-of-Tolerance; Good/Bad; Pass/Fail. When the observed value lies close to the test limit, the laboratory’s uncertainty may prevent a definitive answer. There are many ways to handle the calibration process uncertainty when making a statement of compliance. The industry standard practice is to use a Simple Acceptance / Simple Rejection Decision rule, this decision rule allows for a PFA up to 50%. It is important to communicate your desired decision rule to the calibration provider, and/or to know which decision rule the calibration provider will use as a default. Z540.3 Section 5.3(b) defines a maximum allowable PFA of 2% for any decision rule in order to mitigate the risk associate with the calibration process uncertainty.
Q: What can be done is a PFA is greater than 2%?
A: There are several ways to improve the PFA and mitigate risk as identified in Z540.3
Measurement reliability: The measurement reliability of both the M&TE being calibrated and those used as calibration standards affect the estimated PFA. If either or both are increased, the PFA would be expected to decrease. The increase in measurement reliability may be accomplished by shortening calibration intervals, using measurement control processes, etc.
Calibration intervals: Calibration intervals are established to achieve measurement reliability and/or uncertainty goals. Shortening an interval for a given set of M&TE, such as through raising the measurement reliability target, will result in a reduced PFA.
Measurement uncertainty: Calibration process uncertainty. If the calibration process uncertainty is reduced, the PFA is reduced. Reducing the calibration process uncertainty also increases the TUR. The reduction of calibration process uncertainty may be accomplished by use of shortened calibration intervals, using measurement control processes, etc.
Calibration tolerances: If the test tolerances of the M&TE being calibrated are increased, both PFA and TUR are affected. An increase in an M&TE test tolerance would reduce PFA and increase TUR. This improvement may be at the expense of added controls and limiting the application of the M&TE.
Guard bands: If a guard band is applied to an M&TE test tolerance to create acceptance limits inside the tolerance, the PFA is reduced. This reduction can be at the expense of an increase in the number of measurement quantities rejected and additional costs for increases in adjustment and repair actions.
Q: What does Transcat do to address this Z540.3 requirement?
A: Transcat has developed methods, policies and procedures to mitigate the calibration process uncertainty impact on the PFA for Accredited calibration by:
- Transcat routinely monitors the reliability of the laboratory standards and adjusts calibration intervals, implements process control charts, uses guard-banding techniques on our standards and strives to improve the calibration uncertainty available to our customers as part of our dedication to process improvement.
- Selecting Laboratory Standards, methods and test points which prevent TUR’s <4:1, thus avoiding a < 2% PFA as per Z540.3 5.3(b) entirely
- Transcat standard practice to identify all TUR< 4:1 and to provide the Calibration Process Uncertainty. This information allows the customer to compute the actual PFA of the reported value if they so desire.
- Transcat also has a service level available that will calculate the Probability of Compliance to the Specification (PCS) for each test point for the customer.
- Transcat can also develop custom datasheet using any specification or tolerance a client chooses to use. The use of custom tolerances can further mitigate or eliminate risks associated with TUR’s <4:1 or high PFA’s.
- Transcat is developing the capability to implement alternate decision rules based upon several of the Guardbanding techniques identified in ANSI Z540.3, ILAC G8 and ASME B188.8.131.52.