Document: NRC Regulatory Guide
Document ID: bcbe26a0-004f-4616-b7f4-b32b2bfe4909
Document Type: regulatory_guide
Title: Setpoints for Safety-Related Instrumentation (Rev. 4)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML2005/ML20055G823.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.105
CFR Part: 
CFR Title: 

Content:
use this bounding value as a 95/95 tolerance interval term in the uncertainty analysis. DG-1363, Page 7 According to ANSI/ISA 67.04.01-2018, the uncertainty terms contributing to the estimate of total instrument channel random uncertainties can be made up of deterministic, statistical, or bounding estimate terms. The revised standard also states that the individual random uncertainty tolerance interval terms are combined at the same number of standard deviations and that the result of the combination represents a value of the random uncertainty performance of the instrument channel at a 95-percent probability at a 95-percent confidence level. In addition, uncertainty terms that are dependent, not random, or not normally distributed may be added using algebraic or other statistically appropriate methods. The total instrument channel uncertainty includes the algebraic combination of the total of the resulting estimate of random uncertainties and the estimate of nonrandom uncertainties. Section 4.5.3 of ANSI/ISA 67.04.01-2018 provides a recommended method of combining all instrument channel uncertainties. 2.2.3 Evaluation of the Allowance for Drift The 1994 revision of ANSI/ISA S67.04 provided limited guidance on drift evaluations and uncertainty term development for the evaluation of an instrument surveillance interval. The NRC staff has generally accepted drift evaluations based on statistical prediction techniques. Historically, the NRC staff and external stakeholders have had discussions regarding how to appropriately account for drift that occurs over multiple increments of vendor-specified drift estimates (e.g., a transmitter vendor drift specification of 0.25 percent of calibrated span per 6 months but the instrument is expected to be in continual service for 24 months between calibrations). These discussions considered whether the estimate of total drift that occurs between successive surveillances should be estimated through linear extrapolation of the vendor