## 1 Introduction  
Fluid mechanics face problems with the measurement of volume flow on a micro-scale, which include, among others, biological systems, especially blood flow measurement [1], and on a macro-scale, in industrial flows. Several different flow meters are used for measuring, mainly for the purpose of controlling the technological process where constant measurement of fluid flow is required. The most common, cheapest, and most reliable devices to do the job are wedge meters. They utilize a sudden constriction of the pipeline which the fluid comes across. Fluid speed rapidly rises, passing through the constriction, which results in a difference in static pressure before and behind the constricting segment. The following devices are normally used as flow build-up constriction devices: Venturi tubes, nozzles, and measuring orifices [2; 3; 4]. Orifices are popular for measurement, despite their weaknesses, which include permanent loss of pressure with fluid flow and low rangeability (4:1). Among commonly known measuring orifices, the standard (central) orifice, characterized by accuracy at 1.5-2%, was accepted as a basic model [5; 6]. Many researchers around the globe are looking for new solutions for metering orifices to measure fluid flow, which would be characterized by a constant value of discharge coefficient \(C\) in a wide range of changes in Reynolds numbers [7; 8]. Such a solution seems to come in the form of a flow meter with a flow choking element--a multi-opening orifice. It features more than one opening, usually cylindrical, in the orifice acting as the flow choking element. The conducted experimental study for such multi-opening orifices of various designs, compared to the standard orifice, allowed easier observation due to lesser distortion of the stream behind the orifice [9; 10]. This results in a more stable distribution of velocity field, which in turn causes a lesser decrease in build-up pressure at the orifice. Such a design gives the orifice higher ability to counteract the distortions, compared to the standard orifice (as it requires smaller straight segments of the pipeline before and behind the orifice). Such an advantage has been used when building a cut-off ball valve, where the valve ball has a multi-opening orifice built into it for measuring the stream of the flowing liquid [11]. In the literature concerning multi-opening orifices of such type, one can also come across a comparison of results from the experiment and numerical modelling for a progressed turbulent flow [12; 13]. The literature on this topic normally presents results of numerical simulation studies of such orifices, with a different number of holes and different position in the flow plane [14; 15; 16]. When estimating uncertainty based on numerical simulations, we often come across very complex models and resulting complications with determining uncertainty sources [17; 18]. That is why it is much easier and simpler to estimate the uncertainty of measurement by using experimental studies with assumed flow conditions. With the traditional approach to calculating uncertainty, we use the rules from GUM international standard [19], where uncertainty is defined as a parameter of distribution of measurement error probability. With such an approach to the problem of uncertainty assessment, two types of standard uncertainties are determined: type A and type B [20]. To assess type A standard uncertainty, statistical analysis is taken from a series of observations conducted in the same measuring conditions. It is estimated as the experimental standard deviation of mean resulting from averaging procedure, and presents a measure of the dispersion of measurement values around arithmetic mean [21]. Assessment of type B uncertainty results from familiarity with measuring instruments used in the experiment. This is why it is most often estimated on the basis of the accuracy of the measuring instruments used (information from the manufacturer or past familiarity with the instrument).