Pressure difference measurement

The absolute, barometric pressure is not normally required in ventilation measurements. The air density determination is based on barometric pressure, but other applications are sufficiently rare. On the other hand, the measurement of pressure difference is a frequent requirement, as so many other quantities are based on pressure difference. In mass flow or volume flow measurement using orifice, nozzle, and venturi, the measured quantity is the pressure difference. Also, velocity measurement with the Pitot-static tube is basically a pressure difference measurement. Other applications for pressure difference measurement are the determination of the performance of fans and air and gas supply and e. -haust devices, the measurement of ductwork tightness or building envelope leakage rate, as well as different types of ventilation control applications. 

It is essential to appreciate that the pressure difference measured by. a manometer automatically eliminates the static head difference. This is shown in Figure 8.1(b). The static head pg(zi — z2) in the pipe is exactly balanced by the extra static head above the right hand limb of the manometer. Consequently, if Ah is calculated from Azm using equation 8.4, no further correction for the static head should be made. 

A typical in-line cartridge filtration application is illustrated in Figure 7.13. A pump forces liquid through the filter, and the pressure across the filter is measured by a pressure gauge. Initially, the pressure difference measured by the gauge is small, but as retained particles block the filter, the pressure difference increases until a predetermined limiting pressure is reached, and the filter is changed. 

The pressure difference measured by pipe taps, where the downstream tap is eight pipe diameters below the orifice, is really a measurement of permanent loss rather than of the orifice differential. 

Indeed, even nonequilibrium systems do not necessarily show measurable excess noise and, thus, deviate from relation 1. An appropriate example that is relevant to the subject is a capillary channel that contains a stream of electrolyte maintained by an external pressure difference. Measurements on several aqueous polymer solutions with added electrolytes performed at up to 5000 dyn/cm2 shear stresses and zero external voltage showed that measurable excess noise can be observed only for non-Newtonian solutions exhibiting elasticity (19, 20). Similar results were obtained for colloid suspensions... 

Some kind of pressure-difference measuring device, e.g., manometer... 

Figure 5.8 shows a horizontal venturi meter. It consists of a truncated cone in which the cross-sectional area perpendicular to flow decreases, a short cylindrical section, and a truncated cone in which the cross-sectional area increases to its original value. There are pressure taps both upstream and in the short cylindrical section (the throat ) they are connected to some pressure-difference-measuring device, usually a manometer. Applying Bernoulli s equation between locations 1 and 2 and neglecting friction, we find... 

Tang, C., and Heindel, T.J. (2006a), Estimating gas holdup via pressure difference measurements in a cocurrent bubble column, International Journal of Multiphase Flow, 32(7) 850-863. 

However, neither the original nor the initial resistance factor can be measured at a given distance from the injection face. The pressure difference measured between two points in the system is determined by the integral average flow resistance between these two points. Scrupulously the integral average resistance factor calculated from pressures measured at two distances does not belong to the midpoint of the finite distance difference in question. 

Coarse dust filter 13. Restriction with pressure difference measurement (Ap)... 

For porous and fibrous materials, it is often most convenient to measure electroosmosis across a packed bed or plug (9). In this case, bulk electroosmotic fluid flow must be measured directly in an open circuit, or the pressure difference measured across a closed cell. 

Pressure measurements, flow measurement through pressure difference measurement and level measurements by electrostatic head are closely related. They all use a pressure difference measurement. An example is the level measurement in which a diaphragm is used. A strain gauge could be used to detect the deflection of the diaphragm due to the pressure differential at its two sides. 

Schematic of axial annular flow die with wall pressure difference measurement...

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