If the instruments has no drift it is perfectly reproducible. No drift means that with a given input the measured values do not vary with time.
Drift may be classified into three categories:
(1) Zero Drift: If the whole calibration gradually shifts due to slippage permanent set or due to undue warming up of electronic tube circuits, zero characteristics with zero drift are shown in fig(a).
(2) Span Drift or Sensitivity Drift: If there is proportional change in the indication all along the upward scale, the drift is called span drift or sensitivity drift. The characteristics with span drift are shown.
(3) Zonal Drift: In case the drift occurs only over a portion of span of an instrument, it is called zonal drift. There are many environmental factors which cause drift. They may be stray electric and magnetic fields, thermal EMFs changes in temperature, mechanical vibration, wear and tear and high mechanical stresses developed in some parts of the instruments and systems.
Drift is undesirable quality in industrial instruments because it is rarely apparent and cannot be easily compensated for. Thus it must be carefully example, stray electrostatic and electromagnetic fields can be prevented from affecting the measurements by proper shielding. Effect of mechanical vibrating can be minimized by having proper mountings. Temperature changes during the measurement process should be preferably avoided or otherwise be properly compensated for.