Selection of Sensors for Safety Instrumented Systems - ISA EXPO 2000

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Implementation of ANSI/ISA S84.01, IEC 61508 or IEC 61511 requires an informed approach when selecting sensors for use in Safety Instrumented Systems (SIS). Placing primary emphasis on Risk Assessment methodologies and determination of required Safety Integrity Levels (SIL), these standards provide few guidelines for sensor selection. Initially, this may seem perplexing given the sensor’s critical role in the SIS; however, this is consistent with industry’s lack of consensus on a benchmark sensor technology. Therefore, it is essential that design engineers are fully cognizant of both the capabilities and the limitations of various sensor technologies for proper integration in their systems.

Electromechanical Switches, Analog Transmitters, Smart Transmitters, Safety Certified Transmitters and recently introduced hybrid electronic devices often referred to as Electronic Switches are all suitable for process variable monitoring in a SIS. Each has unique characteristics that must be considered in the SIS design phase. Sensor performance parameters (e.g. accuracy, response time, etc.), impact on system reliability (e.g. self-diagnostics, security, fail-safe capabilities) and economics (e.g. purchase price, peripheral equipment costs, periodic testing requirements) must be considered to verify the suitability of an individual sensor selection.

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