Gas Detectors for use in Safety Instrumented Systems

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I often hear/read about the following query from users and designers who have to incorporate gas detectors in their Safety Instrumented Systems, "which gas detector should I use in my SIS?". There are of course gas detectors that are rated for use in meeting SIL requirements, but the main objective of using a gas detector tends to be forgotten while splitting hairs about SIL 1 and SIL 2, PFD, demand, proof testing and other aspects about Safety Instrumented Systems-which is the actual detection of a gas by the gas detector. First the user or system designer must ensure that the selected gas detector can

1. Measure the target gas correctly
2. Respond to it in a predictable way
3. Is physically mounted in the plant in such a way that it WILL detect any and all gas leakages within its area
4. Is calibrated to the gas sought to be detected

Unless these points are considered, it is pointless to talk about SIL rating and PFD values and other aspects, isn't it?