Instrument Ranges Used in Process Control System Design

In process control design process variables such as temperature, pressure, flow, level, valve signals, analyzers etc are ranged in such a manner to be proportional to the primary signal as much as possible for control purposes. The table below shows the typical instrument scales employed in process control system design:

Instrument Variable	Scale
Temperature	Direct Reading
Pressure	Direct Reading
Flow (differential)	0 – 100 linear
Flow (Linearized)	0 – 100 linear
Level	0 – 100 linear
Analyzer	Direct Reading
Signal to Valves	0 – 100%

As far as possible, the instrument ranges will be selected such that the normal value will be between 50 and 75% of the adjusted range taking into account the specified minimum and maximum values and trip settings will lie between 10% and 90% of the adjusted range.

Typical accuracy of the pressure and differential pressure Smart transmitters will be +/- 0.25% of the calibrated span.

Typical accuracy of temperature Smart transmitters will be +/- 0.3% of the calibrated span.

Typical accuracy of the other transmitters will be as far as possible better than +/- 0.5%.