TABLE OF CONTENTS Sensors in a safety instrumented system measure process variable conditions in order to recognize a potential hazard. Usually these are the same process variables that are used for control. So the first and perhaps most important consideration when selecting sensors for safety applications is that they accurately and reliably measure the process variable. Another key parameter is that any process wetted materials must be compatible with the chemicals of the process. These are two of the key principles required in a "well designed system."
Do not select instruments only because they are safety "certified." Actual performance is the first consideration. This is especially true in level and flow where many different types of measurement technology exist. Of course, if the right technology exists and one manufacturer has done a complete IEC 61508 (Ref. [1]) assessment, then that product might have a considerable advantage over another equally able to measure the process.
Many different types of sensors are used in safety instrumented functions. An informal survey was done as part of a market study (Ref. [2]). Results regarding sensor usage are shown in Figure 9-1.
[1]IEC 61508, Functional Safety of electrical / electronic / programmable electronic safety-related systems, 2000.
[2] Sensor Market Study. Exida, 2003.
Sensors designed for safety instrumented system applications typically have excellent built-in automatic diagnostics. That attribute is one of the main advantages of sensors designed per IEC...
