## 2 The Subject of Study and Measurement Result  
The study was conducted in laboratory conditions in the Fluid Mechanics Laboratory in Faculty of Technology at The President Stanislaw Wojciechowski Academy in Kalisz. For these experimental studies, a four-opening orifice was designed, taking into consideration all the recommendations used when designing a centrical orifice [5; 6]. The orifice with this design was made of brass. Figure 1 shows a picture and production drawing of the orifice. The orifice was placed in housing to allow the measurement of the build-up pressure (the measurement of the difference in pressure before and behind the orifice) nearby. The orifice was mounted in the housing in a standard orientation, where the inflow was directed at the sharp edges of flow openings, and the pressure measurement pulse openings were located on an axis between them. The water testing station was built with two straight and smooth pipeline segments--DN50 and DN15, connected with a bend forming a 180\({}^{\circ}\) angle so that the pipelines would be parallel. The test orifice (Figure 1) was installed in the DN50 straight pipeline with a total length of 82\(\cdot\)_D_. To shape the inlet speed profile, the straight segment before the orifice was 46\(\cdot\)_D_ (2.3 m) long, and 36\(\cdot\)_D_ (1.8 m) long behind the orifice. An Endress + Hauser electromagnetic flowmeter was installed in the DN15 straight pipeline, according to the manual. It functions as the standard/model flow meter in the measuring system where straight segments were taken: 100\(\cdot\)_D_ before the flow meter and 50\(\cdot\)_D_ behind it. Figure 2 presents the diagram of the water hydraulic flow system. The water stream produced by the centrifugal pump (1) flows through the vent (2) built into the hydraulic system. When designing it, the continuous flow condition was used, by which it appears that within a large cross-section of the vent (DN 150), the average flow velocity inside decreases. It has been made in the form of a vertical sealed cylindrical tank, in which any air bubbles spontaneously separate from the flowing stream generated by the stream pump. Since air may also build up near the upper cover, an additional outlet was installed (side bleeder) which also allows for bleeding adjustment of water stream in the measurement system. Air bubbles saturated from the stream are carried away with the bleeding stream through replaceable choking glands (3), which allow for discrete adjustment of stream in the hydraulic measurement pipeline (4). In the measurement system (4), the diameter of the pipeline \(D\) and the flow-through openings \(d_{4}\) in the orifice were measured using a slide caliper with digital display (with resolving power at 0.01 mm) for which the limiting error is 0.05 mm. Differential pressure on the tested orifice was measured in close proximity with an APR-2000/ALW (APLISENS SA, Warsaw, Morel  
Figure 1: Multi-opening orifice model: (**a**) production drawing; (**b**) brass orifice. Figure 2: Block scheme of the hydraulic testing station. pressure transducer (5). It was programmed for pressure difference measuring range of \(\Delta p\) = 0-2.4 kPa with a measuring error of 0.15% and time constant \(t\) = 5 s with an output current signal of 4-20 mA. The water stream in the system also flows through the PROMAG 30 AT15 magnetic flow meter (6) (used as standard/model in the measuring system). It was set for stream volume in the measuring range of \(q_{v}\) = 0-1.0 dm\({}^{3}\)/s with measuring error at \(\Delta q_{v}\) = \(\pm\)(0.2%-\(q_{v\text{-}measured}\)\(\pm\) 0.05%-\(q_{v}\)) with time constant \(t\) = 5 s and an output current signal of 0-20 mA. In the presented measuring system, both the volume stream and water temperature were measured simultaneously for each measurement series, the latter measured with a 0.35 m\({}^{3}\) tank thermometer (7).