How should you select the instrument's full-scale output range for a given control loop relative to the process variable range?

• A. Set full-scale to PV maximum.
• B. Use the minimum PV for range.
• C. Choose a range that spans the PV with headroom and has full-scale near the upper bound.
• D. Set full-scale well above PV.

Answer: (C)

The main idea is to map the process variable into the instrument’s output range in a way that makes full use of the available signal span while leaving some room for surprises. You want an output range that can cover the entire expected PV range, plus a little headroom so that occasional spikes or drift don’t push the signal beyond the end of scale. Placing the PV range so that the highest values of PV map near the upper end of the output scale ensures you’re using most of the output span during normal operation, which gives better resolution and accuracy where you actually measure.

If you tied the full-scale to the PV maximum, you’d waste resolution at the lower end of the PV range and may not have enough room for excursions. If you set the full-scale well above the PV range, most of the output span stays unused, reducing sensitivity. Using a range that spans the PV with headroom and places full-scale near the upper bound strikes the right balance: you get good resolution across the operating PV range and a margin to handle unexpected changes without saturating.