Most liquids, gases, or both are used in the industrial process, indicating measurement is required to control the pressure of liquid and gas, pressure is an important factor of measurement like Pressure gauge, Manometer, Inclined Tube Manometer, etc.
When pressure is measured, it is measured in terms of pressure, absolute pressure, and vacuum pressure, different types of pressure are written below.
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Pressure Measuring Instruments
All pressure-measuring instruments work on these principles
1. Manometer
Manometer is the oldest instrument for measuring pressure, it is the simplest, most direct, and most accurate instrument of all pressure measuring instruments. A manometer is used to measure pressure differences of less than 30 psi. They are not reliable at high pressure.Â
The commonly used manometers are as follows.
i). U-Tube ManometerÂ
It consists of a U-shaped transparent glass tube partially filled with a liquid. Mainly this liquid is mercury (Hg) because its specific weight is known to us at different temperatures and it does not even stick to the tube. One end of this tube is connected to the medium whose pressure we have to know, and its other end is opened in the atmosphere.
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ii). Enlarged Lag Manometer
The enlarged lag manometer is used in recording type manometer as it shows the pressure as well as the accompanying change.
iii). Well Type ManometerÂ
This is another form of enlarged manometer, it is used to measure very micro pressure. In this, the pressure can be read directly with the help of a scale, while in the other manometer, different values ​​of pressure are measured by subtracting each other.
iv). Inclined Tube Manometer
The purpose of the inclined tube is to expand the scale so that lower pressures can be read more easily. Its angle of inclination is of the order of 10°. The orientation spreads across the scale, which greatly increases its sensitivity.
v). Ring Tube ManometerÂ
In this, a tube is made in the shape of a ring, which is supported by a point at the centre. The tube is partially filled with a manometer. The tube rotates due to pressure. The rotation of the ping is controlled by a counterweight.
Measuring Vaccum Instruments
The measuring instruments used to measure pressure below 2000 psi are listed below.
i). Pressure Spring GaugeÂ
These pressure measuring instruments are used in the form of a bourdon tube spiral and helix kr. A bronze pressure spiral is used to measure pressure up to 600 psi and a beryllium copper spring is used to measure pressure up to 1000 psi. Steel or alloy steel pressure springs are used to measure the pressure of 1000 psi or more.
ii). The Bellows GaugeÂ
Bellow tube gauges ​​are used in the measurement of absolute pressure, they are somewhat more sensitive than bounden tube gauges, and they are used to measure the pressure of less than 3 psi main range. Bellows are made of alloys that are ductile and of high strength and retain their properties for a long time.
iii). The Diaphragm GuageÂ
It is made of metals like brass or bronze which are malleable. A pointer is attached to the diaphragm. When pressure is applied to the effective area of ​​the diaphragm, a deflection occurs in the diaphragm. This movement of the diaphragm operates the indicator or recording device, thus the gauge can operate in any position and be moved from one place to another.Â
Their range depends on the manufacturer and they are used to measure the vacuum of normal micro pressure.
Types of PressureÂ
1. Gauge Pressure
When the pressure is measured above the atmospheric pressure i.e. considering the atmospheric pressure as zero, then it is called gauge pressure. In other words, the difference between actual pressure and atmospheric pressure is called gauge pressure.
2. Absolute PressureÂ
The sum of the ideal actual pressure and atmospheric pressure applied above a surface is called absolute pressure.
Absolute Pressure = Actual pressure + atmospheric pressure
3. Vaccum or Differential Pressure
When the gauge pressure is less than atmospheric pressure it is negative but it is written in positive rather than negative and is called a vacuum.
Vacuum pressure = atm pressure - absolute pressure