Probabilistic Modeling of Sensors

Failure rate data for sensors is often first generated using functional failure modes as it is often not possible to know which failures are safe versus dangerous at the product level. As an example, consider the pressure transmitter failure data in terms of functional failure modes per the data in Table 9-1. 

Pressure Transmitter Failure Mode Failure Rate  

When the transmitter fails with its output saturated under-range ( 3.6 mA), the safety PLC will automatically detect this as a failure and send an alarm. No false trip will occur. As above, the transmitter is not capable of responding to a demand during this time so the failure should be classified as Dangerous Detected. 

When the transmitter fails with its output drifting, the drift may go in either direction. So this failure mode could be classified as either safe or dangerous. However, since the drift is usually unpredictable it must be classified in worst-case mode. Since this is not detected by any internal transmitter diagnostics and the current will be in the active range, it is classified as Dangerous Undetected. 

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