A change in an instrument’s reading or set point value over extended periods due to factors such as time, line voltage, or ambient temperature effects.
Drift is an indication of the loss of perfect repeatability or reproduction of a measured value by an instrument.
Drift may be classified into three types:
Zero Drift – This is caused due to a shift in the whole calibration due to slippage or undue warming of the electronic circuits. It can be prevented by zero setting.
Span drift or sensitivity drift – This is observed when there is proportional change in the indication of an instrument all along the upward scale.
Zonal Drift – When the drift occurs only over a portion of the full scale or span of an instrument, it is called zonal drift.
Drift is an undesirable quantity that is rarely apparent and is hard to compensate for. Thus, it needs to be carefully guarded against by continuous prevention, inspection and maintenance.