Experience has shown that considerable misunderstanding can occur when engineers discuss the performance of process instrumentation. Clearly, this situation is likely to impede education; in addition, it can cause serious errors in specifying and purchasing instrumentation to satisfy requirements. Purchasers and suppliers must share an unequivocal understanding of the performance specifications of equipment, including instrumentation. Thus, professional organizations have prepared standard definitions of key terms (e.g., ISA, 1979); practicing engineers are well advised to use these standards.
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A few of the most frequently used terms are presented here.
Accuracy: Degree of conformance of a value to a recognized, accepted standard value. Accuracy is expressed as the maximum positive and negative deviation from the standard for specific conditions, usually expressed as the inaccuracy as a percent of the value, instrument range, or full-scale value. It includes causes of inaccuracy, including linearity, repeatability and hysteresis.
Dead Band : The range through which an input can be varied, upon reversal of direction, without causing an effect in the output signal.
Drift : An undesired change in output over a period of time that is unrelated to the input and operating conditions.
Hysteresis: The property that the output depends on the history of the input and current direction of change.
Linearity : The closeness of the calibration curve to a straight line. It is expressed as the maximum deviation of the calibration curve and the specified straight-line characteristic.
Range : The region within which a value is measured, received or transmitted, expressed as lower and upper range values.
Repeatability: The closeness of agreement among consecutive measurements of the output for the same value of the input under the same operating conditions and approaching from the same direction. It is usually reported as non-repeatability in percent of span and does not include hysteresis.
Reproducibility: The closeness of the output reading for the same value of input over a period of time approaching in both directions. It is usually reported as non-reproducibility as a percent of span and includes hysteresis, drift, repeatability and dead band.
Response Time : Time required for an output to reach a specified percentage of its final value as a result of a step change in input.
Span : The difference between the upper and lower limits of range.