• Threshold: If the instrument input is increased very gradually from zero there will be some minimum value below which no output change can be detected. This minimum value defines the threshold of the instrument.

• Resolution: If the input is slowly increased from some arbitrary (non-zero) input value, it will again be found that output does not change at all until a certain increment is exceeded. Thus, the smallest increment in input (the quantity being measured) which can be detected with certainty by an instrument is its resolution.

• Sensitivity: The sensitivity of an instrument is the ratio of magnitude of the output quantity (response) to the magnitude of input (quantity being measured). Its units are mm/mA, counts per volt, etc. depending upon type of input & output.

• Drift: All calibrations and specifications of an instrument are only valid under controlled conditions of pressure, temperature, etc. As variations occur in ambient temperature, etc. certain static characteristics change. Such environmental conditions affect the output and the instrument and can be called as Drift. No drift means with the given input the measured values do not change with time. It is classified in 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 drift sets in. This can be prevented by the setting.

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.

3) Zonal Drift: The drift occurs only over a portion of span of an instrument, it is called zonal drift. Drift is undesirable quantity in instruments and can be caused because of electric and magnetic fields, thermal emfs, changes in temperature, wear & tear, vibrations, etc.

4) Hysteresis: It is a phenomenon which gives different output effects when loading and unloading whether it is mechanical system or an electrical system or any system. This effect arises due to the fact that all energy is put into the stressed part when loading is not recoverable upon unloading.

Input on both positive and negative sides

5) Accuracy: It is the closeness with which an instrument reading approaches the true value of the quantity being measured. Thus accuracy of a measurement means conformity to truth.

6) Span & Range: Span is the difference between the highest reading & the lowest reading. Span = Xmax – Xmin

The scale range is defined as the difference between the largest and the smallest reading of the instrument. Suppose the highest point is Xmax units lowest is Xmin units. Therefore, the range of calibration is between Xmax and Xmin or we can say that the range is Xmax.

Example: Suppose a thermometer having a calibration of -40˚C to 400˚C Therefore, the range is 400˚C Span = 400 – (-40) =440˚C