At its most basic, a switch acts in a binary fashion, changing state when a pressure, temperature, level or other process variable crosses over some predefined threshold. If the process variable in question can be allowed to vary in the course of normal operation, a simple mechanical switch linked to an on/off valve or pump can effectively and reliably control the process at hand, keeping a tank from running dry or a temperature from climbing too high.
Transmitters, on the other hand, continuously measure and communicate their assigned process variables over a range of values. A transmitter can facilitate on/off control actions similar to those of a switch through configurable discrete outputs within an associated controller. But a continuous transmitter teamed with a modulating control valve or pump with variable speed drive also can be used to implement more subtle (albeit more expensive) control strategies -- such as a proportional‐integral‐derivative (PID) algorithm ‐‐ to maintain the process variable at a specific value or setpoint.
A third option is the integrated or hybrid switch‐transmitter, which combines a continuous transmitter and solid‐state switch within a single instrument housing. This approach effectively combines a number of the advantages of both.]]>