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.

RTDs determine changes in the electrical resistance of materials in relation to temperature. RTDs deliver very precise readings (typically to 2–3 decimal places) and are manufactured in a variety of form factors. Though they are sometimes composed of nickel, copper, or other metals, historically, RTDs have been made of platinum--largely due to the fact that platinum's resistance-temperature relationship is maintained in a very linear fashion across very broad temperature ranges. RTDs' platinum composition also makes them somewhat expensive and unsuitable for applications involving temperatures above 660 °C, as temperatures above this range compromise the inertness of the platinum and may cause it to become contaminated and deliver inaccurate readings.

This white paper describes various temperature sensors such as RTDs, thermocouples, ICTDs, thermistors, and infrared sensors, and the Opto 22 solutions for using them.

Security measures aim at protecting the confidentiality, integrity, and availability of a computer system from being compromised through deliberate or accidental attacks. Similar to process and safety improvements, security improvement needs to be a continuous activity.

This white paper provides background and a general overview of different elements of information system security, with specific emphasis on how it applies to industrial automation and process control. Different security measures that should be considered when an automation system is connected to external networks of different kinds are discussed, including connections to general purpose IS and corporate networks, remote connections, and wireless connections.