Instruments for pressure measurement

I.Y.Haciyeva

Laboratory by

Hydraulics

Laboratory work №1
INSTRUMENTS FOR PRESSURE MEASUREMENT
1. General information
The total or absolute hydrostatic pressure (slave) at any point of the fluid at rest is determined by the formula:

where р_о is the absolute pressure on the surface of the liquid;
γ - is the specific gravity of the liquid;
 h - is the depth of immersion of the point under the liquid level.
Manometric or excess (р) is the pressure above atmospheric (р_аtm), i.e.
or .
If the pressure on the surface of the liquid is equal to atmospheric pressure (in open tanks), the excess pressure will be:

The total pressure may be less than atmospheric pressure . In this case, the difference between р_аtm and р_аbs called vacuum pressure р_vac:
or
The pressure units used in hydraulics and their interrelations are as follows:
1 кQ/sm² = 1 атм = 10⁵ N/м² = 10 м wat.с. = 736 мм hg.с.
To measure the pressure of a liquid, manometers of various types are used. In determining the pressure difference, differential pressure gauges are used, and when measuring the vacuum value, vacuum gauges are used. In addition, the manometers used are liquid and mechanical.
Figure 2.1 shows a diagram of a liquid manometer in the form of an open glass tube with a diameter usually equal to 10-15 mm, called a piezometer, which indicates excess or gauge pressure at the point of attachment of the tube. The height in the piezometer is called the piezometric height or
The distance z to point A, measured from the selected horizontal reference plane (0-0), is called the height of the position or the geometric head.
The sum of two heads - geometric and piezometric - is called the hydrostatic head. A liquid manometer, which determines the pressure difference at two points, is called differential..

Fig.2.1
Figure 2.2 shows a differential pressure gauge attached to two points of a horizontally located pipeline. The working fluid in the knee of the differential pressure gauge has a specific gravity different from the specific gravity of the liquid in which the pressure is measured (in accordance with the scheme ).
Obviously, the pressure difference will be equal to
where h – the difference in the levels in both knees of the manometer.
If the knee of the differential pressure gauge is opened from above, the pressure p_o will be equal р_атм and the tubes of the manometer will turn into piezometers, along which the absolute values ​​of the pressures at the points can be determined. In this case, the device will be called a differential piezometer.

Fig.2.2
In laboratory practice, sometimes use two-fluid differential gauges, which look like an inverted U-shaped tube.
The upper part of such a manometer (Fig. 2.3) is filled with a liquid that does not mix with the liquid and is in the pipeline, the pressure difference in the two sections of which is measured:


Fig.2.3
It follows from this formula that the smaller the specific gravities of liquids, the higher the sensitivity of the manometer will be.
The main advantages of liquid devices for measuring pressure are the simplicity of the device and high accuracy, and the lack of liquid devices is the narrow range of measured pressures not exceeding 0.3 ÷ 0.4 MPa for mercury manometers. At high pressures, the instruments become too bulky.
In cases where it is necessary to measure high pressures, usually use not liquid, but metal (mechanical) manometers. They come in different types.

Fig.2.4
Figure 2.4 shows a spring manometer. The liquid enters through the fitting 1 into a curved brass tube - a spring 2 of elliptical cross section. The end of the spring is connected by a chain 3 with a gear which, when the spring is sprung, drives the arrow 4. The manometer has a scale with the marks indicating the pressure in MPa.
Other types of metal manometers include membrane manometers, vacuum gauges and monovacuummeters. Versatility, simplicity of the device, portability and a huge range of measured pressures are valuable qualities of metal manometers.
The main drawback of spring devices is the inconstancy of their readings (for the same value of the measured pressure), caused by a number of reasons: a gradual change in the elastic properties of the deformed element, wear of the transmission mechanism, etc.