Standard Process Signals for Industrial Instrumentation

Industrial measurement and control processes employ standard process signals that are used throughout all the industries. For instance, one of the most popular forms of signal transmission in modern industrial instrumentation systems is the 4-20 mA DC standard. The signal standards employed in industrial instrumentation are unique. Processes are referred to or measured as 0% to 100% of the process range. These signals typically use a live zero for 0% of process measurement (referred to as live zero in the sense that, the range starts with a non-zero value i.e. 3 PSI in the case of the 3-15 standard and 4 mA in the case of 4-20 mA standard).

The live zero gives insight as to whether the process loop is working within given parameters or if the control loop is open or malfunctioning. If the loop is functioning in a proper way, the signal will indicate a measurement greater than live zero for 0% of the process.

In the representation of the process, 3 PSI represents 0% of the process and 15 PSI represents 100% of the process. The span of the process is 12 PSI. The span is where the process takes place in a range of measurement.

Upper Range Value (URV) – Lower Range Value (LRV) = Span

From the figure above, it shows that if the process signal is 0 PSI or -25%, the loop is open or malfunctioning. This is very helpful for troubleshooting measurement and control loops and the identification and removal of elements that are not associated with the source of the problem.

The table below shows some of the various signals used in industrial instrumentation systems.

Table 1.0: Standard Signals for Measurement and Control

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