Accuracy and Precision Accuracy of a measurement describes how close the measurement approaches the true value of the process variable. Accuracy is often expressed as a % error over a range or an absolute error over a range. % Error Over a Range Accuracy may be specified as ± a percentage over a range, span or full scale of an instrument. The uncertainty in your measurement will be the percentage times the range specified by the manufacturer. For example, Manufacturer A specifies that their pressure instrument has an accuracy of ±0.4% of full scale. The full scale of their instrument is 500 psig. We can expect the measurement signal from this instrument to be accurate to 2 psig for all pressures in the instruments range. 0.4% x 500 psig = 0.004 x 500 psig = 2 psig Even if we span the instrument for the application to 0 to 100 psig the instrument is still only accurate to 2 psig. Absolute Over a Range Accuracy may also be specified as ± an absolute value over a range. Manufacturer B specifies that their pressure instrument has an accuracy of ±1 psig over the full operating range. The full scale of their instrument is also 500 psig. We can expect the measurement signal from this instrument to be accurate to 1 psig for all pressures in the instruments range. In any case, the sensors we select must be more accurate then the degree to which we want to control the process. There is no amount of control loop tuning you can do on a process to maintain 30 psig ± 1 psig pressure instrument is only accurate to ± 2 psig. Precision is the reproducibility with which repeated measurements can be made under identical conditions. Precision may also be referred to as stability or drift. Precision is always required for good control, even when accuracy is not required. Accuracy vs. Precision The distinction between accuracy and precision is illustrated in Figure 3-3. The dashed line represents the actual temperature being measured. The upper line represents a precise but inaccurate value from an instrument; the lower line represents an accurate but imprecise measurement from an instrument Precision is the more important characteristic of an instrument. Instrumentation Dynamics Instruments have dynamic properties just as process do. The dynamic properties instruments posses are identical to process dynamics: gain, time constants and dead time. All of these instrument dynamics contribute to the process dynamics that the controller sees. Instrument Gain The gain of an instrument is often call sensitivity. The sensitivity of a sensor is the ratio of the output signal to the change in process variable. For a thermocouple the typical sensitivity is 5 mV per °C. This means for every °C change in the process variable the thermocouple will change its output by 5 mV. Instrument Time Constants As for processes, one time constant for an instrument is the time it takes to provide a signal that represents 63.2% of the value of variable it is measuring after a step change in the variable. Instrument manufacturers may sometimes specify the rise time instead of the time constant. Rise time is the time it takes for an instrument to provide a signal that represents 100% of the value of the variable it is measuring after a step change in the variable. The rise time of an instrument is equal to 5 time constants. Instrument Dead Time The dead time of an instrument is the time it takes for an instrument to start reacting to process change.
Posted on: Sat, 29 Mar 2014 15:55:52 +0000
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