Before we go any further it would be wise to define what we mean by IAC.  We are referring to a scale feature whereby the scale uses an internal weight that can be placed upon a location that is equivalent to placing it upon the weighing pan.  This action is performed under the control of the scale.  This process is done automatically when certain predefined, but user controlled, conditions are met.  These conditions, such as temperature changes or predefined time intervals, are user defined in a setup procedure..

The polished stainless steal internal weight is weighed upon a high precision scale and the weight (usually close to the weight of what would be used for an external standard) is stored in the scales memory.  The scale waits for one of the defined preconditions for calibration to be met.  Then it waits for the scale to be idle for a predefined period of time with its weigh pan empty (which is interpreted as the scale not being in use), and automatically starts a calibration cycle.  The display warns the user that a calibration cycle is in progress and the weigh pan must not be disturbed.  The calibration cycle is fairly time-consuming as the internal weight is placed, weighed, and removed two times to be sure that there is no discrepancy between calibration cycles.  When the calibration cycle is complete the scale returns to whatever mode it had been in prior to the start of the calibration cycle.

These machines keep themselves calibrated at all times and relieve the user of both the chore and the worry associated with bad weighing.  Temperature changes, which can cause unacceptable drift, are hard to discern when they are happening, and can produce weighing results that are misleading and wrong.  The scale uses an internal temperature sensor to detect changes where they create reluctance changes in the magnetic motor that cause the sensitivity (gain) of the motor to vary.  Temperature changes can also cause or relieve stresses in the magnetic materials used in the motor with similar effects.  Magnetic circuits are also subject to drifting with time when they are used in the very high accuracy circuits of laboratory and analytical scales.  Most of the drift with time is attributed to stress related changes in the high energy permanent magnets used in these devices.

This is one area where a relatively small investment can preserve the integrity of your project and provide some built in security against neglect.