(Range) The algebraic difference between the upper and lower values of a range. In laymen’s terms – Think of it as establishing two walls. When the instrument reads in between the values established by the walls, all is good. A reading outside of either wall (too high or low) and the unit will alarm. Over time these walls drift apart and allow a wider range to be considered safe by the unit. This phenomenon (rate of span drift) is why manufacturers have different calibration intervals. A calibration will set the walls back in place