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%.
