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Main performance indicator of instrument
DATE:2014-04-11 11:58 CLICK:times
Main performance indicator of instrument


The accuracy of instrument also called precision.Accuracy and error can be said to be the twin brother, because of the existence of error, there comes the concept of accuracy.In short, Instrument precision is accurate extent of measured value close to the true value, usually expressed by a relative percentage error (also known as the relative equivalent error).
Instruments accuracy is not only relevent to the absolute error, but also to the measure range. If the value of the absolute error is big, the relative percentage error is also big, the accuracy of instrument is low. If absolute error of the two instruments are same,but the measure range are different, the relative percentage error of instrument with the bigger measure range is small, instrument accuracy is high.
Accuracy is an important quality indicator for instrument, usually formed and expressed by accuracy class. As the the unified state regulations, levels are rank to 0.005,0.02,0.05,0.1, 0.2,0.35,1.0,1.5,2.5,4 etc., instrument accuracy class are generally signed on the ruler or board on the meter scale, eg, 0.5, etc., the smaller the number is, the higher precision of instrument described.
To improve the accuracy of the instrument, it is necessary to analyze the errors. Errors can usually be divided into negligence error, slowly varying error, system error and random error.
Neglecterror is the error of huamn in the measurement process, it can be can overcomed, and nothing to do with the instrument itself. 
Slowly Varying  error is due to the aging process of internal components, it can be overcomed by  replace 
components, parts, or constant correction and elimination. 
System error is in the repeated measurement of one parameter, appeared value and signal are same or change as
some rule.It caused by casual factors which have been not recognised, and value and nature is not stead, difficult to 
estimate. But we can estimate its influence on the test results in the theory via the statistical method.
The main sources of error refers to systematic errors and random errors. When accuracy expressed by the error, it refers to the random and systematic errors combined.
Instrument performance is usually described by accuracy(also known as precision),variation and sensitivity or so in engineer.
Variation is when the meter measured by variation (also input signal) reaches the same value several times from different 
directions, meter indicates the maximum difference between the values??, or meter under external conditions remain unchanged, the degree of inconsistency that measured parameters changes from small to large(forward characteristics and the measured parameters changes from large to Small(reverse characteristics). 
Variation is determined by  a percentage of the maximum absolute error and scale range of the instrument. 
The main reason of variation is that interval between the moving parts of the instrument, the friction between components, 
and the lag of elastic components.
With the continuous improvement on instrument manufacturing technology, particularly the introduction of microelectronics technology, many electronic instruments are fully electroniced, no moving parts, analog meters change to digital meters, etc. 
so this indicator deteriorated in smart meter does not seem so important and impressive.
Sensitivity refers to the degree of the instrument sensitive to the measured parameter's changes, or reaction to the measured value's changes, is a ration of Incremental changes in the output and input  in steady state.Sensitivity is sometimes called also called "zoom ratio", also apt line slope meter static characteristics of each point.
Increasing magnification times can increase the sensitivity of the instrument, simply increase the sensitivity of does not change the instrument's performance, that is to say the sensitivity does not increase and on the contrary, shocks may appear sometimes, lead to output instable.  Instrument sensitivity should maintain an appropriate amount. However, for instrument users, such as chemical companies in terms of instrumentation engineering, precision is certainly an important indicator, but more emphasis on the stability and reliability of the  instrument in actual use, because there is no much application on the measurement in the chemical industry for test andprocess control. And a large number are used for detection.