If an instrument is mounted below a vessel it sense a total head but the head due to the distance from the instrument location to the bottom of the vessel does not represent level in the vessel.This head must be must be cancelled by a range suppression calibration,using the zero adjustment provided in the instrument.
Level Measurement – Zero Elevation
If an instrument is mounted above a vessel , the distance from the instrument location to the zero level of the level must be added by a range elevation calibration again using the zero adjustment provided in the instrument.
Once zero calibration is complete for either zero suppression or elevation, the span adjustment provided in the instrument is used to calibrate the actual range of level change measured in the vessel.
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after tempering with zero adjustment you double check the span because zero adjustment also affects the span