Hydrostatic head pressure

The figures in Table XXVII show that a 0.001°C depression in the melting temperature corresponds approximately to a 10-cm. change in hydrostatic pressure head in the osmotic pressure. With appropriate pains, osmotic pressures may be measured within 0.01 cm. of liquid... 

In order for solvent and solution to be in equilibrium in an apparatus such as that shown in Figure 3.2, the solution side must be at a higher pressure than the solvent side. This excess pressure is what is known as the osmotic pressure of the solution. If no external pressure difference is imposed, solvent will diffuse across the membrane until an equilibrium hydrostatic pressure head has developed on the solution side. In practice, to prevent too much dilution of the solution as a result of the solvent flow into it, the column in which the pressure head develops is generally of a very narrow diameter. We return to the details of osmotic pressure experiments in the next section. First, however, the theoretical connection between this pressure and the concentration of the solution must be established. 

It has been nearly a century and a half since Boussingault (1868) presented the hypothesis that the accumulation of assimilates in an illuminated leaf may be responsible for a reduction in the net photosynthetic rate of that leaf. According to the Munch hypothesis for phloem transport, the greater the sink strength, the greater the depression in solute concentration in the phloem at the sink. This increases the concentration differential between the source and sink, creating the hydrostatic pressure head that drives the system. 

As described in the Introduction, the process of diffusion of a solvent through a semipermeable membrane from a less-concentrated solution into a more-concentrated solution is osmosis. This results in the development of a hydrostatic pressure head on the more-concentrated solution side of the membrane. Alternatively, pressure may be applied to the moreconcentrated solution side of the semipermeable membrane to prevent the diffusion of solvent. This applied pressure on the concentrated solution is identical to the hydrostatic pressure head that may develop owing to osmosis. It is known as the osmotic pressure and is directly proportional to the solute concentration in an ideal solution. A semipermeable membrane is one that allows the movement of only solvent molecules, and if the membrane is not semipermeable, osmosis may not be observed because the solute will diffuse quickly through the membrane to equalize the concentration on two sides of the membrane. 

In the method of the Jailing meniscus a liquid-wetted tapering tube is placed vertically in a reservoir, as in fig. 1.26. Inside the tube liquid is held by the capillary pressure. The tube is now moved upwards - or the liquid in the vessel downwards - to increase the hydrostatic pressure head, and this is continued until the liquid in the capillary collapses. From the hydrostatic head the Laplace pressure is obtained and from that the surface tension. The method is very simple and may be considered as the counterpart of the maximum bubble pressure technique there are also similarities to the situation sketched in fig. 1.8a. The idea is rather old... 

Election of standard conditions and state is comparable with the selection of horizontal surface for the comparison of hydrostatic pressure heads. Certainly, such selection is tentative and there are other approaches to the determination of standard conditions and states, which is mostly rooted in the convenience of solving specific tasks. 

Examples of linear relationships in engineering include the volume of an ideal gas in a closed vessel and the temperature (at constant pressure), V = P T (where = nR/P) the hydrostatic pressure head in the bottom of a tank and the height of the liquid, AP = P h (where = pg) the heat flux through a solid slab of a fixed thickness and the temperature differential, q/A = PiAT (where /3i = k/Ax). 

For other applications such as the simultaneous sedimentation at large co (Eq. 17), the overall minimization of the flow rate Q may be required. Resorting again to Eq. 9, we can also reduce the quotient Ar to throttle the flow rate. This geometric factor reflects the ratio of the hydrostatic pressure head ( Ar) to the hydrodynamic resistance ( Uct). So a radially shallow channel with a high tilt angle between the channel axis and the radial direction, i.e., Ar I, diminishes Q (Fig. 1). A further reduction of Q can be accomplished by increasing the flow resistance, e.g., by a narrow, meander-like flow channel with an embedded stationary phase. 

The membrane is placed between an aqueous salt-free compartment containing unpolymerized crown ether and an aqueous compartment containing a higher BaCh, with the latter compartment having a higher osmotic pressure. When a hydrostatic pressure head is created with a column of BaCl2 solution in the latter compartment, pressure-mediated flow of solution across the membrane is punctuated by short and small amplitude quasiperiodic reversals. Such oscillations do not occur when BaCh is replaced with CaCh, presumably because Ca does not bind to the crown ether. 

As the pumping process continues, the liquid level falls reducing the hydrostatic pressure head and associated pressure subcooling, at the entry to the pump. Eventually, the total pressure head falls to the NPSH or below the pump begins to stall and wiU eventually no longer remove further liquid from the tank. (However, if the outlet valve of a centrifugal pump is throttled when the Uquid level falls to near this stalling point, flie flow reduction in effect reduces the NPSH and enables the liquid to be pumped further. This can be important if the tank is to be emptied completely.)... 

Pressurising by use of a pressure raising coil or vaporiser connected between bottom and top of the vacuum-insulated tank. The flow is driven by the difference in hydrostatic pressure heads between liquid in the tank and vapour in the line above the vaporiser, and may be simply controlled by a valve in the vapour line to the top of the tank. Automatic control via a pressure regulator enables the tank ullage pressure to be maintained independently of the rate of liquid removal. 

Waterproofness is an indication of a particular membrane material s ability to withstand a 60 cm hydrostatic pressure head. The apparatus employed in this test is shown in Fig. 8-34. The membrane film is affixed at the bottom end of the J-tube and water is then carefully introduced to an overall height of 60 cm above the level of the membrane. 

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