Switches are no longer the blind, ‘dumb’ electromechanical devices of a generation ago. Signals from sensors in critical safety loops (e.g. protecting against high temperature or pressure) are typically integrated with the plant control system via a discrete or analog input channel.
A switch provides a binary (on/off) input, while transmitters provide a continuous (4 to 20 mA) analog input. Both signal types have a role to play in Safety Controls, Alarms and Interlocks (SCAI) including emergency shutdown.
Respective control output
Switches have been replaced in many cases with all solid state 2-wire ‘smart’ switches for pressure and temperature control. Like the process transmitter, these hazardous area and general purpose instruments have a piezo-resistive strain gauge sensor (or RTD in the case of temperature). This sensor is combined with a microprocessor which drives the respective control output.
These transistors have a special niche where protection of equipment or piping is concerned
In the case of solid state switches, an embedded photo-MOSFET (Metal Oxide, Semi-conductor Field Effect Transistor) is used. This is the ‘switch’ of the modern digital age. These transistors have a special niche where protection of equipment or piping is concerned (e.g. maximum allowable working pressure limits). Pumps, compressors and flow lines are three examples of equipment that benefit from cost-effective, solid state ‘smart’ switches.
General purpose instruments
These devices are easy to program and deploy, are highly repeatable and give the automation engineer different options. These options range from changing modes, widening or narrowing on/off deadband, to adding a delay to the output signal. These devices also monitor their health with diagnostic coverage unheard in legacy products.
Best of all, these instruments use just a trickle of power (VDC or VAC) to allow deployment on two wires like an analog transmitter. The difference is they provide binary, on/off, go/no go control which is often just what’s needed for safety control, alarm or interlock, or emergency shutdown.