Fig.B-5 Basic types of liquid-filled manometers.
- one arm is replaced by a vessel, in which the surface level decrease is negligible (as documented by the example of diameters shown in Fig. B-5, it may be up to three decimal orders of magnitude smaller than the read difference ). In aerodynamic laboratories, there are in common use arrangements according to C, Fig. B-5, where the inclination (usually adjustable) of the arm with the scale makes possible to increase sensitivity. These micromanometers are commonly manufacured with inclinations up to 1:50, where on a millimetre scale one scale division corresponds to height difference = 0,02 mm, - so that with alcohol the corresponding pressure difference is = 0,157 Pa. In other commonly used variants, the accuracy of reading the column height is increased by optical means, e.g. in the u Betz micromanometer the displacement is read on a scale carried by a float, etc..

Most often, colculations of pressure in fluid is performed in order to evaluate some force effect on some object, e.g. on submersed body or some cover covering a hole in the vessel wall. Pressure was actually introduced (in chap. [A]) as the derivative ; force is therefore found by the inverse mathematical operation,

Fig. B-6 Force acting on a piston located in the out-
put cross section, generated by pressure which it is pos-
sible to assume to be constant across the whole cross-
-sectional area.
integration of pressure across the investigated surface:
- because in hydrostatic problems pressure increases with depth, the integration in case of complex surfaces may represent mathematically interesting and demanding problem. Let us, however, turn our attention first to importand cases in which the positional dependence may be neglected - either because the fluid is gas or because the pressure is so high that its variations between the highest and lowest position of the surface represents an unimportand contribution to the overall effect:
Piston actually represents a movable wall supported from outside. Pressure action on this wall may be thus converted into mechanical force, which is then transmitted by the supporting component - the piston rod in Fig.B-8, where there is also some indication how the force is calculated. This conversion is usually described as = fluido / mechanical transformation. The same device may, on the other hand, also perform the (= mechano / fluidic ) conversion of a mechanical action into a fluidic one, realised by moving fluid. The term "fluidics" used in this context is a description involving both hydraulics (systems using liquids as working fluids) as well as pneumatics (working with gas, usually air). It originated from latin word "fluo" = to flow. Piston travel is limited to finite lengths, and that is why its motion as well as the fluid flow have to be alternating ones. This limitation may be removed on the fluid side by valves (usually placed in the cylinder head) - or motion of the whole device reative to inlet and outlet openings - which make possible to admit (or to generate) continuous one-direction flow.



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This is page Nr. B02 from textbook Vaclav TESAR : "BASIC FLUID MECHANICS"
Any comments and suggestions concerning this text may be mailed to the author to his address tesar@fsid.cvut.cz

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