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Velocity Measuring Devices
Published in Gregory D. Wight, Fundamentals of Air Sampling, 2018
The pitot tube actually measures the difference between the static pressure of the flowing fluid and the pressure due to the velocity or momentum of the fluid molecules. The static pressure, Pstat, is defined as the pressure without regard to motion, or the pressure that would be measured if the pressure sensor were moving with the fluid. This static pressure can be measured by inserting an open-tube probe into the fluid so that the opening is parallel to the direction of flow. See Figure 5.2b. In contrast, the pressure due to fluid momentum — called total pressure or static-plus-dynamic pressure — can be measured by inserting an open-tube probe facing directly upstream, as in Figure 5.2a. This pressure is also called “stagnation” pressure because at least some fluid molecules are brought to a complete standstill at the front opening of the probe. If both of the probes are connected to two sides of a differential pressure gauge, as in Figure 5.2c, the gauge reading is the difference in pressures and is called “velocity head”: () ΔP=velocityhead=Ptot−Pstat
Instrumentation and Pigging
Published in Henry Liu, Pipeline Engineering, 2017
The most fundamental and accurate method to measure the velocity of fluid, be it a liquid or a gas, is by using a Pitot tube. Two common types of Pitot tubes are the simple Pitot tube (also called stagnation tube) and the Prandtl-type Pitot tube. The simple Pitot tube is simply an L-shaped small tube, usually made of stainless steel, which can be inserted into the pipe for velocity measurements. As shown in Figure 10.8a, the tube is inserted into the pipe at the location (desired radial distance from the centerline) where the velocity is to be determined. The tube must be aligned with the flow, with the tube opening facing the flow and becoming the stagnation point. The end of the stagnation tube must be connected to either a manometer or a pressure transducer to determine the pressure at the stagnation point ps. The wall pressure (static pressure po) must be measured separately with a pressure gage, transducer, or manometer. Knowing the difference between these two pressures, Δp = ps – po, the velocity of the flow at the location of the tube, undisturbed by the presence of the tube, is u=2(Δp)/ρ
Equations of motion
Published in Mohammad H. Sadraey, Aircraft Performance, 2017
Speed and its measurement have a significant position in aircraft performance analysis. Although the Global Positioning System (GPS) is a powerful tool in the measurement of several flight variables including speed, but, due to safety reasons and FAA regulations, all aircraft use a device called Pitot tube to measure the aircraft speed. In addition, the GPS only measures the ground speed; not the airspeed. Airspeed is measured by comparing the difference between the pitot and static pressures (Figure 2.18) and, through mechanical linkages, displaying the resultant on an airspeed indicator. A static port (tube) measures only the static pressure, since the hole is perpendicular to the air flow, so the flow must turn 90° to enter into the tube. In contrast, a pitot tube measures the dynamic pressure, since the hole faces the airflow. When a pitot tube has a static port, it is often referred to as the pitot-static tube.
Narrow stack emissions: Errors in flow rate measurement due to disturbances and swirl
Published in Journal of the Air & Waste Management Association, 2021
Stanislav Knotek, Marcel Workamp, Jan Geršl, Menne D. Schakel
The first type of error that can occur when flow velocity is measured using pitot tubes is the error due to an angular misalignment of the pitot tube with respect to the actual flow direction in the measurement point. This error depends on how the pitot tube reacts on the angular deviations of the flow velocity vector and is different for various pitot tube types. Here we consider one representative L-type and one representative S-type pitot tube, and we discuss their possible velocity measurement errors.