Sensor and Controller Catalog

Think Safety

Designing Safe Thermal Systems Most heated thermal systems include: • A heat source, either fuel fired or electrical resistance • A temperature controller • A temperature sensor The sensor produces a signal value based on the temperature to which it is exposed. The temperature controller interprets the signal into a value that is either above, below or at a then predetermined set point. The controller will then create an output signal to command a device to turn the heat source on or off. As with any heated thermal system, failure in the sensor, temperature controller or heat-source controlling device could create an over- or under-temperature condition. These conditions can ruin product in process or pose a danger to personnel and property. Limit controllers are used to prevent injury and property loss that could occur if a process variable’s value were to cross outside the safe operating range for the system. In systems where temperature is controlled, a limit controller should be used to respond to an over- or under-temperature condition before it becomes a hazard. A properly applied limit controller senses when the temperature or other process variable is outside the limit and shuts the system down. The limit controller must be independent of the primary control system and able to interrupt the flow of energy that could cause a hazard. For example, in a system where temperature is controlled with an electric heater, the limit controller must be able to disconnect the heater from the power source when the temperature exceeds the limit. A proper design requires user intervention to correct the cause of the failure before the system is restored to normal operation. Usually, the sensor used in a limit control system does not need to be as accurate as the primary sensor. Its only function is to create a temperature signal that allows a controller to determine if a pre programmed over-or under-temperature condition exists. Sensor longevity and aging must be considered if the sensor will have an unacceptable impact on the limit control system’s ability to accurately determine an over- or under-temperature condition. As with enclosures, there are agency standards for the design and construction of limit control systems, and their suitability for use exist.

Designing Intrinsically Safe Circuits When installing temperature sensors in hazardous areas, circuits should be made intrinsically safe with “barriers” to prevent sparks and excessive heat on the “safe” side from reaching the hazardous area and causing sparks. All barrier device parameters affect sensor performance and the sensor circuit. Parameters include: • Polarity rated for AC or DC signals • Rated voltage or working voltage of the signal the device is designed to carry before it senses a fault • Internal resistance, as the amount of resistance inherent to the barrier device, affects the strength of the current signal it is allowing to pass

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