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This adjustment shifts the instrument reading up or down scale by an equal amount at every point on the measurement scale. Figure 2–17 illustrates the effect of zero adjustment. It can be seen that the error caused by zero is constant at all points. Correction for zero error can usually be made at any point on the scale as this adjustment will change the reading the same amount at every point. Zero adjustment on mechanical instruments is done by slipping a link on a hub, a pointer on a shaft or by some other means of establishing a starting or reference point for measurement.

Figure 2–12 illustrates dead band and hysteresis. Linearity is the closeness with which an instrument calibration curve approximates a straight line. It is usually measured as nonlinearity and expressed as linearity. Appendix A defines the terms associated with linearity: independent, terminal-based and zero-based. The terms hysteresis, dead band and linearity are introduced here and will be reviewed later in the chapter. 8 Figure 2–11 Examples of Repeatability and Accuracy HYSTERETIC ERROR ONLY DEAD BAND ONLY HYSTERESIS (HYSTERETIC ERROR PLUS DEAD BAND) Figure 2–12 Dead Band and Hysteresis 34 Industrial Pressure, Level & Density Measurement EXAMPLE 2-14 Problem: Two pressure instruments with ranges of 0 to 100 psi produce the following data.

The procedure just given for instrument calibration is valid for any calibration situation but is probably not applicable to more modern digital/microprocessor-based instruments. Such instruments are very accurate, repeatable, and have few or no moving parts; therefore, there is little chance of error introduced by friction, hysteresis, component wear, and so on. Calibration is much less involved and is usually performed less frequently than with mechanical or analog electronic devices. Calibration checks should, however, be performed after maintenance or parts replacement.