B31.3 sets no rules for this. Haven't read this section of B31.1 but suspect it's no different.
The 1.5x to 4x is the requirement for the range of the gauge relative to the calculated hydrotest pressure per ASME VIII. Calibration is mandatory; see ASME B31.1 Appendix J-1.2.10.
The 1.5X to 4X limits only applies to analog gauges, and that's because they reach their best accuracy (as a percentage of full scale) within this range. That's a feature of the Bourdon tube. The code does have different rules for digital gauges, or at least Section III does: "Digital type pressure gages may be used without range restriction provided the combined error due to calibration and readability does not exceed 1% of the test pressure."
QCRobert
