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1 Archives of Mining Sciences

1 2004

1 Krach A.

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Źródła niepewności w kopalnianych pomiarach wentylacyjnych

100%

Krach A.

tom Vol. 49, nr spec

151-178

W artykule przedstawiono sposoby określania niepewności pomiaru wybranych parametrów wentylacyjnych, mierzonych w ramach ciągłego monitoringu lub okresowych kontroli procesu wentylacji kopalni. W kolejnych rozdziałach artykułu zaprezentowano funkcje modelujące pomiar i wariancje złożone następujących wielkości: -średnia prędkość i strumień objętości powietrza w wyrobisku; -stężenie niebezpiecznych gazów w strumieniu powietrza; -absolutne ciśnienia statyczne w węzłach sieci wentylacyjnej, różnice ciśnień statycznych i całkowitych, straty naporu; -gęstość powietrza w bocznicach, niezbędna przy pomiarach prędkości powietrza przyrządami spiętrzającymi, przy wyznaczaniu oporów aerodynamicznych, depresji naturalnej i sporządzaniu schematów potencjalnych; -opory aerodynamiczne wyrobisk, od których zależy rozpływ powietrza w sieci wentylacyjnej.

Monitoring and control of ventilation in mines requires the measurements of basic parameters on the regular or periodic basis. The main parameters of the ventilation process include: -average velocity and stream of air in the mine headings; -concentration of hazardous gases in the air stream; -absolute static pressure in the nodes of the ventilation network, differences in absolute and total pressure, pressure loss; -air density in branches required in airflow velocity measurements using impact devices, necessary to determine aerodynamic drag, natural depression and to graph potential diagrams; -aerodynamic resistance in mine headings which affects air propagation in the ventilation network. Of particular interest is reliability of measurements. In accordance with the guidelines issued by the International Committee of Measures, the quality of measurement is measured by its uncertainty. The standard value of uncertainty of measurements is expressed as the standard deviation. When the measurement result is derived from other quantities, the total uncertainty equals the positive square root of the complex variation, expressed by the formula (1). A major determinant of measurement result precision in complex measurements is the function modelling the measurements. It should involve as many correlations between input quantities as possible, even though some of these quantities are negligible while computing the final results. Further sections outline the methods of estimating the uncertainty of measurements of selected ventilation parameters. Chapter 4 reviews the major sources of uncertainty of measurements of the average airflow velocity in the gallery cross-section, using a vane anemometer. The method of continuous traversing is employed. The modelling function is provided (3) and so is the complex variance of the measurement result (4). Component variations of the two input parameters in the model are: -accuracy of readouts of average airflow velocity from the anemometer; -coefficients of the anemometer characteristics; -fluctuations of airflow velocity; -shape of the traversing path; -the rate of anemometer movement; -velocity profile in the gallery cross-section; -presence of people in the gallery cross-section. Chapter 5 investigates the uncertainty of measurements of the surface area of the gallery cross-section, which comprises the uncertainty of air volume measurements expressed by (20) in chapter 6. Chapter 7 focuses on measurements of local concentrations of hazardous gases and volume fractions of those gases in the stream of flowing air. Variances of streams of hazardous gases in measurements using the summation of elementary streams or measurements of local flow intensity are given as (22) and (25). Measurements of pressure and pressure differences are of key importance in mines. They are required for flow velocity measurements using impact devices, in measurements of pressure loss, fan characteristics, aerodynamic drags and while preparing socalled depression images of the mine. Measurements of pressure differences at two distant points of the ventilation network are taken using the barometric method whereby the difference between absolute pressures at those points is measured. Chapter 8 deals with uncertainty involved in measurements of barometric pressure and pressure difference, taking into account such sources of uncertainty as random fluctuations of absolute pressure in the mine headings, characteristics of the measuring equipment and resolution of the readout systems. In the case of measurements of pressure differences three cases are considered: -uncertainty of the difference of two absolute pressures measured at the beginning and at the end of the gallery with two different instruments, the pressure measurements taken at certain .time intervals (39); -uncertainty of the difference of two absolute pressures measured at the beginning and at the end of the gallery using the same instrument, the pressure measurements taken at certain time intervals; -uncertainty of the difference of two absolute pressures measured at the beginning and at the end of the gallery with two different instruments, the pressure measurements taken at the same time (43). The knowledge of air density is requisite for measurements of airflow velocity using impact devices, for determining aerodynamic resistance, natural depressions and potential diagrams. Uncertainty involved in air density measurements is investigated in chapter 9. Results of measurements of absolute pressure difference, air density, airflow velocity and gallery cross-section and the difference in altitude of control points are input values to the mathematical model of measurements of aerodynamic resistance, expressed as (59) or (60). The uncertainty of resistance measurement is the complex one. Individual components of the complex variance of the drag are investigated in chapter 10. For each input value the sensitivity factor is also provided.