Several organizations and publications reference or insist on maintaining a 4 to 1 TUR (Test Uncertainty Ratio) without understanding the level of risk that they may be subjecting themselves to. The general thought is if the lab performing the calibrations has standards at least four times better than what they are calibrating that everything is good. This paper discusses TUR, PFA Risk, and why the location of the measurement matters. We will discuss two managed risk guard banding methods (5 & 6) found in the ANSI/NCSL Z540.3 Handbook. We will show that a 4 to 1 TUR is not enough and can result in a 50 % risk.
