What devices can measure pressure?
Pressure is characterized by force operating on the surface of the body. It is the important characteristic rendering, as well as the temperatuyora, influence on many technology and other processes. In nayostoyashchy time apply several units of pressure. In the International System of Units (SI) unit of pressure is the pascal (Pa) equal to force pressure in 1 N (newton) upon the area of 1 sq.m where N - force reporting to weight of 1 kg acceleration of 1 m/s. For high pressures sometimes use kgf/sq.m and bar; for low - millimeter of the mercury or water column (mm Hg. and mm w.g.).
Distinguish pressure atmospheric, excessive, vacuumetric, absolute. Devices for measurement of pressure call mayonometram. Depending on the look and value of the measured pressure, use kinds of manometers: barometers, vakuumyometra, etc.
The atmospheric pressure (barometric) put atmosfeyory Earth is measured by barometers. The excessive pressure which is exceeding of pressure of the environment over atmospheric - manometrayom, vacuum pressure gages, differential manometers. Vacuum gage pressure (the vacuum, the depression) lacking to atmospheric - vacuum meters. The absolute pressure of the environment counted from absolute zero, i.e. from the absolute vacuum can be measured by one of the above-stated kinds of manometers depending on that, it is higher atmospheric, below or equally to it. In most cases measure excessive pressure in the equipment. Manometers for its measurement separate on liquid and deformation.
Pressure is measured in liquid column manometers on height of the fluid column counterbalancing this pressure. Devices are usually carried out in the form of two reported glass vessels which are filled in with working fluid (mercury, the distilled water, ethyl alcohol, etc.).
One end of the vessel connects with Wednesday where pressure is measured, the second - remains open. Under the influence of pressure of the measured environment liquid begins to move from one knee to another until pressure of the environment and the fluid column in the knee are not counterbalanced. The difference of levels counted on the scale is proportional to the measured excessive pressure (Рн) which is equal
Рн = Hpg
where N - fluid column height; р - liquid density; g - acceleration due to gravity. It is possible to measure by such manometers also depression and the difference of pressure.
In fig. 1 and the double-column manometer by which it is possible to measure atmospheric, excessive and vacuum gage pressure is shown. Its shortcoming is the big error at measurement of pulsating pressures. In this case use single-column manometers (fig. 1) in which one knee of the communicating vessel has much bigger diameter in comparison with another. The wide knee (cup) connects to the measured environment, davleyony with which moves liquid to the narrow knee. At measurement only height of the column of the narrow knee as change of the fluid level in the cup is so insignificant that it can be neglected is considered. For measurement of small excessive pressures priyomenyat one-pipe micromanometers with the inclined tube (fig. 1 in). Reduction of slope angle of the tube allows to increase the counting accuracy as at the same time fluid column length increases.
The separate group is represented by liquid column manometers, movement of liquid in the vessel is transferred to kotoyory to the sensing device - the float 1 (fig. 2 and), to the bell 2 (fig. 2) or to the ring 3 (fig. 2), connected with the arrow - the pressure indicator. Advantages of these devices - the possibility of registration of indications on the chart and their remote transfer.
In the equipment deformation manometers which principle of action is based on the deformayotion of the elastic sensing device arising under vliyayoniy the measured pressure are more widely used. The elastic part is connected with the reading device graduated directly in terms of the davyoleniye. By the form the elastic element distinguish spring, membranyony and silphon manometers.
In spring manometers the elastic part is the metal tube of the oval section (pruzhiyon) bent on the circle and connected through the transfer meyokhanizm to the arrow.
The measured pressure moves in the tube therefore section aims to take the form of the circle, the pruzhiyona 1 is untwisted - and the pointer moves on the scale (fig. 3a).
In membrane manometers the elastic part is the flat membrane 2 which under the influence of pressure caves in and via the transmission gear affects the arrow. The membrane can be executed also in the form of the box 3 that increases sensitivity and instrument accuracy at the expense of its bigger deflection with the identical pressure (fig. 3b).
In silphon manometers (fig. 3v) the elastic part is the sylphon 4, predstavlyayuyoshchiya the thin-walled corrugated tube in which подастся the measured pressure. Under its influence the sylphon stretches in length and via the transmission gear moves the arrow of the priyobor.
Membrane and silphon manometers apply to measurement of small excessive pressures and depression thanks to small rigidity of the elastic element. And when using for measurement of depression (vacuum) they are called draft gages, excessive pressure - naporomer, and depression and izbytochyony pressure - tyagonaporomer.
One of important advantages of deformation manometers in comparison with liquid - the possibility of automatic recording and remote transfer of indications.
It is carried out by the preobyorazovaniye of deformation of the elastic element in electric vykhodyony the signal which is fixed by the secondary elektroizmeritelyyony devices graduated in terms of pressure (milliampermetrayom, potentiometers, etc.).
Such manometers are called deformation electric. Most rasprostraneyona differential and transformer, magnetomodulation and tensometric converters.
The principle of operation differentsialno of the Yotransformatorny converter is based on change of force индукционного current depending on pressure.
In manometers with such converter (fig. 4) the tubular spring 1 moves not the arrow, and the steel core 2 in the transformer coil therefore force of induction current which through the electronic amplifier of the signal 4 moves on electrical measuring instrument 3 changes.
In magnitoyomodulyatsionny converters pressure will be transformed to the direct current signal as a result of movement of the magnet connected with the elastic sensing device. At the movement the magnet okazyyovat influence on the magnetomodulation converter (MMP) and after strengthening in the semiconductor intensifying device sigyonat moves on secondary devices of the direct current.
Tensometric converters are based on dependence of the elektrosoprotivyoleniye of metal or the semiconductor of tensoresistors on deformation. Tensoresistors 1 (fig. 5) are fixed directly on the upruyogy sensing device of the manometer. Change their soprotivleyoniya will be transformed to the electric output signal fixed by secondary devices.
For measurement of high and extreme pressures in laboratoryony practice and also for check of industrial deformation manometers use deadweight manometers.
Their action is based on equilibration of pressure of the measured environment by force created by the calibrated load of 1 (fig. 6) operating on the piston 2.Top