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Measurement uncertainty of static differential pressure measurement in mine ventilation networks usi |
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Measurement uncertainty of static differential pressure measurement in mine ventilation networks using the barometric method Author: A. Krach
Measurements of the static differential pressure in mine ventilation networks by means of barometric methods are often performed to determine pressure losses in ventilation side-branches and to calculate their aerodynamic resistance coefficient. As experience shows, the results of these measurements are distorted by a significant level of measurement error (Biernacki and Gumihski, 1999). According to guidelines published in "Wyrazanie Niepewnosci Pomiaru, Przewodnik" [Guide to the Presentation of Measuring Uncertainty] (GUM 1999), the accuracy of measurement is expressed by the so-called measuring uncertainty. In Part 2, concepts of standard uncertainty and composite standard uncertainty are discussed (1). In Part 3, three methods of measuring differential pressures are outlined: * with the use of two different instruments at a certain time interval (Section 3.1); * with the use of one instrument at different times (Section 3.2); * with the use of two instruments at the same time (Section 3.3); and for each of these methods the measurement modelling function is determined (5), (14), (16). Partial derivatives of the measurement modelling function are calculated for all input values (both measured and not measured). Furthermore, sensitivity coefficients and formulas expressing their composite variances are presented for those values. In addition, variances and covarianccs of coefficients of the measuring instruments' static characteristics are calculated by means of the minimum chi-square method in the process of calibrating instruments. Using these relationships of the sensitivity coefficients and the variances of input values, two sample calculations are presented in Part 4, performed with the help of a measurement database prepared by KWK "Anna" Coal Mine and the results of recording pressure values measured in a cross heading of this mine. Data for calculating the uncertainty of coefficients of measuring instruments' static characteristics were obtained from a collection of calibration certificates of anemometers and pressure gauges issued by the Institute of Strata Mechanics, Polish Academy of Sciences (IMG PAN). The results of calculations of composite variances for differential pressures are presented in_tabular form to demonstrate the impact of uncertainty of individual input values on the measuring uncertainty of the differential pressure. Conclusions from the analysis of data listed in the tables are presented in Part 5.
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2009