The sensors are the basic elements in any controller configuration. In a typical process control scenario, it is a device which measures and transmits the process value to the controller (e.g. RTD - temperature, Diaphragm - Pressure, Loadcell-Mass, Orifice - Flow, household security sensors, proximity card readers etc.). The sensors must be connected to a controller. The type of connection and communication methods differ based on the controller. For example in the proximity card reader the sensor need to be connected by a serial cable. This indirectly limits the distance between sensor and respective controller to maximum of 100 to 200 meters.
In the case of hydrocarbon handling area the proximity type sensors need to be intrinsically safe type and only few vendors are available (like Honeywell , HID). For Oil & Gas industry application take extracare while selecting sensors.
The key points while selection of sensors,
1. Type of industry (intrinsic safe or not)
2. Sensor sensitivity
3. method of communication between sensor and the controller
For more information on proximity type sensors/ cards etc ---http://www.hidglobal.com/technology.php?tech_cat=4&subcat_id=9
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