A flow controller's accuracy is typically expressed as a percent of Full-Scale flow (%FS). This means that the flow controller is most accurate when operating at its rated full-scale flow rate. This also means that its accuracy, expressed as a percent of Setpoint, drops off significantly when operating at lower flow rates.
For example, a 1000 cc/min flow controller with a +/- 1%FS accuracy, operating at a setpoint of 1000cc/min can be expected to be accurate to +/- 1% of the set point. However, when operating at a setpoint of 100 cc/min, the accuracy expressed as a percent of setpoint drops to +/- 10% of the setpoint.
Environics instruments use flow controllers calibrated to an accuracy of +/- 1% of SETPOINT, NOT FULL-SCALE. This means that the accuracy of the flow controller is the same, regardless of the operating point of the flow controller. This is achieved through the use of an 11 point calibration of each flow controller in the instrument.
Download the article to see the graph which compares the % setpoint error of a standard uncalibrated flow controller, with that of an Environics instrument using calibrated flow controllers