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In describing the characteristics andoperation of process control instruments (such as process refractometers), itis very important to understand some common terms used in the industry. Thedefinitions of some of the more common terms are provided below: Accuracy: The closeness of an indicator orreading of a measurement device to the actual value of the quantity beingmeasured; usually expressed as ± percent of the full scale output or reading. Drift: The change in output or set pointvalue over long periods of time due to such factors as temperature, voltage,and time. Hysteresis: The difference in output aftera full cycle in which the input value approaches the reference point(conditions) with increasing, then decreasing values or vice versa; it ismeasured by decreasing the input to one extreme (minimum or maximum value),then to the other extreme, then returning the input to the reference (starting)value. Linearity: How closely the output of asensor approximates a straight line when the applied input is linear. Noise: An unwanted electrical interferenceon signal wires. Nonlinearity: The difference between theactual deflection curve of a unit and a straight line drawn between the upperand lower range terminal values of the deflection, expressed as a percentage offull range deflection. Precision: The degree of agreement betweena number of independent observations of the same physical quantity obtainedunder the same conditions. Repeatability: The ability of a sensor toreproduce output readings when the same input value is applied to itconsecutively under the same conditions. Resolution: The smallest detectableincrement of measurement. Sensitivity: The minimum change in inputsignal to which an instrument can respond. Stability: The ability of an instrument toprovide consistent output over an extended period during which a constant input is applied. Zero balance: The ability of the transducerto output a value of zero at the electronic null point.
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