Measuring Gas Velocity with the D-FL 100 Differential Pressure Bar

D−FL 100 Seite 73.1. Differential Pressure BarThe special shape of the differential pressure bar provides a defined separation point for the gas flowaround its edges. This effects constant conditions for measurement in a large speed range.The separation point of the gas flow normally depends on the shape of the differential pressure bar’sedges and on the speed of the gas. If the position of the separation point of the gas flow on thedifferential pressure bar’s edges changes, a whirl will be created behind the differential pressure bar.The area behind the differential pressure bar thus has different pressure areas due to the differentwhirls. However, a constant flow pattern across the entire speed range on the back of the differentialpressure bar is required for a constant reference pressure. This is ensured by the following profile (Fig.6).Fig. 6:Cross section of the differential pressure barThe differential pressure bar has the shape of a tilted square divided into two chambers by an integrateddivider plate. The tips of the integrated divider plate create the differential pressure bar’s edges. Thedouble chamber system transfers the two pressures (impact pressure and reference pressure) outsideto the differential pressure transducer.The correction factor (c-factor) of the differential pressure bar (see Eq. 2) must be calculated taking intoaccount the geometry of the differential pressure bar and the dimensional relation of the differentialpressure bar to the duct/stack cross section. The c-factor is between 0.5 and 0.7, depending on thediameter of the duct/stack.The application of the differential pressure bar and the gas to be measured will determine what itscomposition should be. The standard differential pressure bar is made of stainless steel (material No.1.4571). Other materials, such as stainless steel alloys, are available for extreme conditions (stainlesssteel alloy material No. 1.4539, Hastelloy C 276 material No. 2.4819, Inconel 602 material o. 2.4816).GmbH