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4-20mA signal magnitude comparator and alarm

Rob Eagle

I have a safety system with 3 independent fan sets operating in a duty/standby/back-up configuration, each fan set has an associated pressure differential transmitters (PDT), the PDT control the fan speed to maintain a depression in a common area.

The PDTs also collectively initiate a duty/standby/back-up switching operation on detection of excessive depression based on a 2oo3 voted decision. 

For diagnostic purposes I want to compare the output of each of these PDTs with each other and raise an alarm if one is out of whack with respect to the other two, whether it be reading higher or lower, by a predetermined margin.

So basically I am looking for a 4-20mA signal magnitude comparator with trip function, anyone know of a suitable device to do this, ideally able to support a SIL 2 safety function?

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