My PFDavg is in the SIL 2 band. Is the function SIL 2? Only if fault tolerance and systematic capability also meet SIL 2, and the PFDavg also meets the required value from the risk analysis.
Why does First Out AI give a slightly different PFDavg from my hand calculation? It averages PFD(t) over the mission time with each part of λDU cleared by the test that reveals it, including repair times. Hand formulas are approximations; the results table also shows the simplified-equation value for comparison.
Should detected dangerous failures trip the function? It depends on the spurious trip consequences and on whether repairs are quick. Trip (or vote as tripped) keeps the function safe; alarm and repair keeps production but needs compensating measures. Decide and write it in the SRS.
What proof test coverage should I use for a valve? The coverage your actual test achieves, estimated from the failure modes it reveals. See "What each proof test reveals: coverage by device type".
Can I use 1oo2 transmitters with β = 0? No reviewer will accept it. Use a justified β, typically a few percent for well-separated field devices.
How long a mission time should I use? The planned service life or the renewal interval of the devices, whichever applies; it matters most when coverage is below 100 %.
The logic solver certificate gives a PFDavg. How do I enter it? Enter the failure rates if the safety manual gives them; otherwise choose λDU so that λDU · T1 / 2 matches the stated PFDavg for its test interval, and note the source.
Is high demand mode worse? It is different: PFH is a frequency, and the proof test interval no longer protects you. Choose the mode from the demand rate, not from the result you prefer.
Written for First Out AI as engineering guidance. It supports engineering work and does not replace it; check every value against your standards, certificates and company rules with a competent functional safety engineer.