Piezometer opening

Local Static Pressure In a moving fluid, the local static pressure is equal to the pressure on a surface which moves with the fluid or to the normal pressure (for newtonian fluids) on a stationary surface which parallels the flow. The pressure on such a surface is measured by making a small hole perpendicular to the surface and connecting the opening to a pressure-sensing element (Fig. 10-8a). The hole is known as a piezometer opening or pressure tap. 

Occasionally, the precise calibration of a venturi tube has given a value of C greater than 1. Such an abnormal result is sometimes due to improper piezometer openings. Another explanation is that we may apply it to the venturi tube with pt/w -p-Jw = h. The equation then may be written as... 

Specifications for Piezometer Taps The size of a static opening should be small compared with the diameter of the pipe and yet large compared with the scale of surface irregularities. For rehable results, it is essential that (1) the surface in which the hole is made be substantially smooth and parallel to the flow for some distance on either side of the opening, and (2) the opening be flush with the surface and possess no burr or other irregularity around its edge. 

Piezometer tube An open-ended calibrated glass or plastic tube that measures the pressure in a pipe or vessel full of a fluid. 

For the flow not to be appreciably disturbed, the diameter of the instrument must not exceed about one-fiftieth of the diameter of the pipe the standard instrument (diameter 7.94 mm) should therefore not be used in pipes of less than 0.4 m diameter. An accurate measurement of the impact pressure can be obtained using a tube of very small diameter with its open end at right angles to the direction of flow hypodermic tubing is convenient for this purpose. The static pressure is measured using a single piezometer tube or a piezometer ring upstream at a distance equal approximately to the diameter of the pipe measurement should be made at least 50 diameters from any bend or obstruction. 

It is quite evident that in the case of an open channel the hydraulic gradient coincides with the water surface, because if a piezometer tube is attached to the side of the channel, the water will rise in it until its surface is level with that of the water in the channel. 

The sample is enclosed in a glass piezometer and separated from the pressure transmitting fluid by mercury. As the sample volume decreases with increasing pressure the level of the mercury, which is contained in a calibrated glass tube, sinks and thus opens the electric contact to a tungsten tip. By means of a screw spindle the contact of the tungsten tip with the mercury level is made again. The number of revolutions needed for it is a measure for the decrease in volume of the sample and can be measured by an electronic altimeter outside the autoclave. The reproducibility of the density is 0.0003 g cm . ... 

An open standpipe piezometer consists of a vertical pipe, which is perforated over the bottom part such that ground water can flow into the pipe. This monitoring... 

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