Pressure gauges and barometers are used to measure pressure. Barometers are used to measure atmospheric pressure. For other measurements, pressure gauges are used. The word pressure gauge comes from two Greek words: manos - loose, metreo - measuring.
Tubular metal pressure gauge
Exist Various types pressure gauges. Let's take a closer look at two of them. The following picture shows a tubular metal pressure gauge.
It was invented in 1848 by the Frenchman E. Bourdon. The following figure shows its design.
The main components are: a hollow tube bent into an arc (1), an arrow (2), gears (3), a tap (4), a lever (5).
Operating principle of a tubular pressure gauge
One end of the tube is sealed. At the other end of the tube, using a tap, it is connected to the vessel in which the pressure needs to be measured. If the pressure begins to increase, the tube will unbend, thereby acting on the lever. The lever is connected to the arrow through a gear, so as the pressure increases, the arrow will deflect, indicating the pressure.
If the pressure decreases, the tube will bend and the arrow will move in the opposite direction.
Liquid pressure gauge
Now let's look at another type of pressure gauge. The following picture shows a liquid pressure gauge. It is shaped like a U.
It consists of a glass tube in the shape of the letter U. Liquid is poured into this tube. One of the ends of the tube is connected using a rubber tube to a round flat box, which is covered with rubber film.
Operating principle of a liquid pressure gauge
In the initial position, the water in the tubes will be at the same level. If pressure is applied to the rubber film, the liquid level in one elbow of the pressure gauge will decrease, and in the other, therefore, it will increase.
This is shown in the picture above. We press on the film with our finger.
When we press on the film, the air pressure in the box increases. Pressure is transmitted through the tube and reaches the liquid, displacing it. As the level in this elbow decreases, the fluid level in the other elbow of the tube will increase.
By the difference in liquid levels, it will be possible to judge the difference between atmospheric pressure and the pressure exerted on the film.
The following figure shows how to use a liquid pressure gauge to measure the pressure in a liquid at various depths.
When designing and operating heating systems, the most important indicator and parameter is coolant pressure. At normal pressure, located within the hydraulic schedule, the worker the process is underway without disturbance, the coolant reaches the most remote points of the heating system. If the pressure exceeds the critical point, there is a danger of pipeline rupture. When the pressure drops below the permissible level, there is a threat of cavitation - the formation of air bubbles, leading to corrosion and destruction of pipelines. In order to keep pressure levels at the required level, you need to constantly monitor them. This is precisely what pressure gauges are used for - devices that measure this very pressure.
Pressure is the ratio of the force acting perpendicular to a surface to the area of that surface. Pressure largely determines the stroke technological process, the state of technological devices and their operating modes.
TYPES OF PRESSURE:
- Atmospheric (barometric) pressure is the pressure created by the mass of the air column of the earth's atmosphere.
- Absolute pressure is the total pressure taking into account atmospheric pressure, measured from absolute zero.
- Excess pressure is the difference between absolute and barometric pressure.
- Vacuum (rarefaction) is the difference between barometric and absolute pressure.
- Differential pressure is the difference between two measured pressures, neither of which is ambient pressure.
Based on the type of pressure measured, pressure gauges are divided into:
- overpressure gauges,
- absolute pressure gauges,
- barometers,
- vacuum gauges,
- pressure and vacuum gauges – for measuring excess and vacuum pressure;
- pressure gauges – pressure gauges for low excess pressures (up to 40 kPa);
- draft gauges – vacuum gauges with an upper measurement limit of up to 40 kPa;
- differential pressure gauges – means of measuring pressure difference.
The general principle of operation of pressure gauges is based on balancing the measured pressure by some known force. According to the principle of operation, pressure gauges are divided into:
- liquid pressure gauges;
- spring pressure gauges;
- membrane pressure gauges;
- electrical contact pressure gauges (ECM);
- differential pressure gauges.
In liquid pressure gauges, the measured pressure or pressure difference is balanced hydrostatic pressure column of liquid. The devices use the principle of communicating vessels, in which the levels of the working fluid coincide when the pressures above them are equal, and when the pressures above them are unequal, they occupy a position where the excess pressure in one of the vessels is balanced by the hydrostatic pressure of the excess liquid column in the other. Most liquid pressure gauges have a visible level of working fluid, the position of which determines the value of the measured pressure. These devices are used in laboratory practice and in some industries.
There is a group of liquid differential pressure gauges in which the level of the working fluid is not directly observed. Changing the latter causes the float to move or the characteristics of another device to change, providing either a direct indication of the measured value using a reading device, or conversion and transmission of its value over a distance.
Most wide application Among the instruments for measuring pressure, spring pressure gauges were found. Their advantages are that they are simple in design, reliable and suitable for measuring medium pressure in a wide range from 0.01 to 400 MPa (0.1 to 4000 bar).
Elastic sensitive elements of deformation pressure gauges:
a - tubular springs;
b - bellows;
c, d - flat and corrugated membranes;
d - membrane boxes;
e - flaccid membranes with a hard center
The sensitive element of a spring pressure gauge is a hollow, curved tube of ellipsoidal or oval cross-section, which deforms under pressure. One end of the tube is sealed, and the other is connected to a fitting, through which it is connected to the medium in which the pressure is measured. The closed end of the tube is connected to a transmission mechanism mounted on a stand, which consists of a driver, a gear sector, a gear with an axis and a pressure gauge pointer. To eliminate backlash between the teeth of the sector and the gear, a spiral spring is used. The scale is graduated in pressure units (pascal or bar) and the arrow shows the direct value of the excess pressure of the measured medium. The pressure gauge mechanism is housed in the housing. The measured pressure enters the tube, which, under the influence of this pressure, tends to straighten, since the outer surface area more area internal surface. The movement of the free end of the tube is transmitted through a transmission mechanism to the arrow, which rotates at a certain angle. There is a linear relationship between the measured pressure and the deformation of the tube, and the arrow, deviating relative to the pressure gauge scale, shows the pressure value.
The operating principle of a membrane pressure gauge is based on pneumatic compensation, where the force developed by the measured pressure is balanced by the elastic force of the membrane box.
The sensitive element of the device consists of two membranes welded together, forming a membrane box 1. The measured pressure is supplied through a fitting to the internal cavity of the box. Under the influence of the difference between atmospheric and measured pressure, the box changes its volume, which causes the rigid center of the upper membrane to move, which moves the needle of the device 4 through the leash 2 and the lever 3.
Electrical contact pressure gauges (ECM) are used in automatic control, regulation and alarm systems. Two special arrows, set to the minimum and maximum pressure within the scale, have electrical circuit contacts built into them. When the moving arrow reaches one of the contacts, the circuit closes, which causes a signal to be sent or a corresponding action of the system to which the pressure gauge is connected.
1 — index arrow; 2 and 3 - electrical contact settings; 4 and 5 - zones of closed and open contacts, respectively; 6 and 7 - objects of influence.
Version 1 - single-contact for short circuit;
Version 2 - single-contact opening;
Version 3 - two-contact open-open;
Version 4 - two-contact for short-circuit;
Version 5 - two-contact open-short;
Version 6 - two-contact for short-circuiting.
Electric pressure gauge have standard diagram functioning, which can be illustrated in Fig. a). When the pressure increases and reaches a certain value, indicator arrow 1 with an electrical contact enters zone 4 and closes using base contact 2 electrical circuit device. Closing the circuit, in turn, leads to the commissioning of impact object 6.
Types of ECM:
- Electrical contact pressure gauges on microswitches: vibration-resistant (liquid-filled), industrial, in a stainless steel case, corrosion-resistant with a flat membrane or tubular spring.
- Electrical contact pressure gauges with magnetomechanical contacts: corrosion-resistant with flat or tubular membrane, industrial.
- Explosion-proof electrical contact pressure gauges: with an explosion-proof shell made of of stainless steel or aluminum alloy, and also used for low pressures.
- Differential diaphragm pressure gauges are used to measure pressure drop in gas filters or in flow meter restriction devices.
In most pressure gauges, the technology for determining and calculating data is based on deformation processes in special measuring units, for example, in a bellows unit. This element acts as an indicator that senses pressure changes. The block also becomes a difference converter in pressure indicators - the user receives information in the form of moving the pointer arrow on the device. In addition, data can be presented in Pascals, covering the entire measurement spectrum. This method of displaying information, for example, is provided by the Testo 510 differential pressure gauge, which during the measurement process eliminates the need for the user to hold it in his hand, since back side The device has special magnets.
Bellows differential pressure gauge type DS:
a - diagram of the bellows block; b - appearance; 1 - working bellows; 2 - silicon organic liquid; 3 - internal cavity of the bellows; 4 - rod; 5 - springs; 6 - fixed glass; 7 - lever; 8 - torn; 9 - axis; 10 - rubber rings; 11 - corrugations; 12, 13 - shut-off and equalization valves
In mechanical devices, the main indicator is the location of the arrow, controlled by a lever system. The pointer moves until the changes in the system cease to have an impact certain strength. A classic example of this system is the DM 3538M series differential pressure gauge, which provides proportional delta (pressure difference) conversion and provides the result to the operator in the form of a unified signal.
In liquid pressure gauges, or differential pressure gauges, the measured pressure or pressure difference is balanced by the pressure of a liquid column. Pressure measurement using liquid pressure gauges is based on the change in the height of the column (level) of the working fluid in a glass measuring tube depending on the applied pressure. The most commonly used manometric (working) fluids are ethyl alcohol, distilled water, and mercury. The use of these substances is related to their stability physical properties, low viscosity, non-wetting of the walls.
The pressure measurement process can be carried out with a high degree of accuracy. The simplicity of the device and ease of measurement are the reason for the widespread use of liquid pressure gauges.
Devices of this type include two-pipe (U-aboutdifferent, Figure 6.1 A) and single-tube cup (Fig. 6.1 b) pressure gauges, as well as micromanometers.
a) b)
Figure 6.1 - U-shaped diagram ( A) and single-tube cup pressure gauge ( b)
A two-pipe pressure gauge is designed to measure excess pressure or pressure difference. The instrument scale is usually movable. Before starting measurements, check the zero by connecting both elbows of the U-shaped pressure gauge to the atmosphere. In this case, the working fluid levels are set on the same horizontal ab. By moving the instrument scale, align the zero mark of the scale with the established liquid level. When one bend of a tube is connected to a container in which pressure must be measured, the liquid moves until the measured pressure is balanced by the pressure of a liquid column of height H. Since the liquid level in one tube rises and in the other decreases, the height of the column H is determined as the difference of two readings. This disadvantage of U-shaped pressure gauges is partially eliminated in the cup pressure gauge consisting of vessels different diameters. The measured pressure is supplied to the positive (wide) vessel, and the level difference is determined by taking one reading along the negative thin tube.
For section a-b(Figure 6.1 A) the following equality of forces is true:
Where R a and R b
- absolute and atmospheric pressure, Pa; f is the area of the measuring tube opening, m2; H is the height of the liquid column, m;
- density of the working fluid, kg/m 3 ; g
- free fall acceleration, m/s 2.
By transforming expression (6.2) we obtain:
It is obvious that when measuring excess pressure, the height of the rise of the working fluid does not depend on the cross-sectional area of the tubes. Based on the conditions of convenience of working with the device (to limit the height of the pressure gauge tubes), when measuring excess pressure of 0.15–0.2 MPa, it is recommended to use mercury as the working fluid; at lower pressures, water or alcohol.
Cup and U-shaped pressure gauges cannot be used when measuring small excess pressures and vacuums, since the measurement error becomes excessively large. In these cases, special cup pressure gauges with an inclined tube (micromanometers) are used.
Figure 6.2 – Diagram of a micromanometer with an inclined tube
Using an inclined tube (Figure 6.2) allows, by reducing the angle
, at the same height of rise of the liquid column h
increase its length, which exceeds the accuracy of the count. The measurement of the length and height of a liquid column is related by the relation h
=
l
sin
.
From here
. Changing the angle of the tube
, you can change the measurement limits of the device. Minimum angle tube tilt 8-10°. The instrument error does not exceed ±0.5% of the final scale value.
In liquid pressure gauges, or differential pressure gauges, the measured pressure or pressure difference is balanced by the pressure of a liquid column. Pressure measurement using liquid pressure gauges is based on the change in the height of the column (level) of the working fluid in a glass measuring tube depending on the applied pressure. The most commonly used manometric (working) fluids are ethyl alcohol, distilled water, and mercury. The use of these substances is associated with the stability of their physical properties, low viscosity, and non-wetting of the walls.
The pressure measurement process can be carried out with high degree accuracy. The simplicity of the device and ease of measurement are the reason for the widespread use of liquid pressure gauges.
Devices of this type include two-pipe ( U-shaped, fig. 15.1) and single-tube (cup, Fig. 15.2) pressure gauges, as well as micromanometers.
U ab
|
|
The two-pipe pressure gauge (GOST 9933-75) is designed to measure excess pressure or pressure difference. The instrument scale is usually movable. Before starting measurements, check the zero by connecting both elbows to the atmosphere U-shaped pressure gauge. In this case, the working fluid levels are set on the same horizontal ab. By moving the instrument scale, align the zero mark of the scale with the established liquid level.
When one bend of the tube is connected to a container in which the pressure needs to be measured, the liquid moves until the measured pressure is balanced by the pressure of a liquid column height N. Since the liquid level in one tube increases and in the other decreases, the height of the column N is defined as the difference between two readings. This disadvantage U-shaped pressure gauges are partially eliminated in the cup pressure gauge, consisting of vessels of different diameters. The measured pressure is fed into the positive (wide) vessel, and the level difference is determined by taking one reading along the negative thin tube.
For section 1-1 (Fig. 15.1), the following equality of forces is true:
Where p a And r b - absolute and atmospheric pressure, Pa;
f - hole area of the measuring tube, m 2 ;
N - height of rise of the liquid column, m;
R - density of the working fluid, kg/m 3 ;
g - free fall acceleration, m/s 2.
By transforming expression (15.2) we obtain:
P ex =P a -P b =Hpg. (15.3)
It is obvious that when measuring excess pressure, the height of the rise of the working fluid does not depend on the cross-sectional area of the tubes. Based on the conditions of ease of use of the device (to limit the height of the pressure gauge tubes), when measuring excess pressure, 0.15-0.2 MPa is recommended as a working fluid use mercury, at lower pressures - water or alcohol.
Cup and U-shaped pressure gauges cannot be used when measuring small excess pressures and vacuums, since the measurement error becomes excessively large. In these cases, special cup pressure gauges with an inclined tube (micromanometers) are used. The use of an inclined tube (Fig. 15.3) allows, by reducing the angle φ, at the same height of rise of the liquid column h, to increase its length, which increases the accuracy of the count. The measurement of the length and height of a liquid column is related by the relation. From here Changing the angle of the tube φ
, you can change the measurement limits of the device. The minimum angle of inclination of the tube is 8-10°. The instrument error does not exceed ±0.5% of the final scale value.
Principle of operation
The principle of operation of the pressure gauge is based on balancing the measured pressure by the force of elastic deformation of a tubular spring or a more sensitive two-plate membrane, one end of which is sealed in a holder, and the other is connected through a rod to a tribic-sector mechanism that converts the linear movement of the elastic sensing element into a circular movement of the indicating arrow.
Varieties
The group of instruments measuring excess pressure includes:
Pressure gauges - instruments with measurements from 0.06 to 1000 MPa (Measure excess pressure - the positive difference between absolute and barometric pressure)
Vacuum gauges are devices that measure vacuum (pressure below atmospheric) (up to minus 100 kPa).
Pressure and vacuum gauges are pressure gauges that measure both excess (from 60 to 240,000 kPa) and vacuum (up to minus 100 kPa) pressure.
Pressure meters - pressure gauges for small excess pressures up to 40 kPa
Traction meters - vacuum gauges with a limit of up to minus 40 kPa
Thrust pressure and vacuum gauges with extreme limits not exceeding ±20 kPa
Data are given in accordance with GOST 2405-88
Most domestic and imported pressure gauges are manufactured in accordance with generally accepted standards; therefore, pressure gauges various brands replace each other. When choosing a pressure gauge, you need to know: the measurement limit, the diameter of the body, the accuracy class of the device. The location and thread of the fitting are also important. These data are the same for all devices produced in our country and Europe.
There are also pressure gauges that measure absolute pressure, that is, excess pressure + atmospheric
A device that measures atmospheric pressure is called a barometer.
Types of pressure gauges
Depending on the design and sensitivity of the element, there are liquid, deadweight, and deformation pressure gauges (with a tubular spring or membrane). Pressure gauges are divided into accuracy classes: 0.15; 0.25; 0.4; 0.6; 1.0; 1.5; 2.5; 4.0 (the lower the number, the more accurate the device).
Types of pressure gauges
By purpose, pressure gauges can be divided into technical - general technical, electrical contact, special, self-recording, railway, vibration-resistant (glycerin-filled), ship and reference (model).
General technical: designed for measuring liquids, gases and vapors that are not aggressive to copper alloys.
Electric contact: have the ability to adjust the measured medium, due to the presence of an electric contact mechanism. A particularly popular device in this group can be called EKM 1U, although it has long been discontinued.
Special: oxygen - must be degreased, since sometimes even slight contamination of the mechanism in contact with pure oxygen can lead to an explosion. Often available in cases blue color with the designation on the dial O2 (oxygen); acetylene - copper alloys are not allowed in the manufacture of the measuring mechanism, since upon contact with acetylene there is a danger of the formation of explosive acetylene copper; ammonia - must be corrosion-resistant.
Reference: having more high class accuracy (0.15;0.25;0.4) these devices are used for checking other pressure gauges. In most cases, such devices are installed on deadweight piston pressure gauges or some other installations capable of developing the required pressure.
Ship pressure gauges are intended for use in river and marine fleets.
Railway: intended for use in railway transport.
Self-recording: pressure gauges in a housing, with a mechanism that allows you to reproduce the operating graph of the pressure gauge on chart paper.
Thermal conductivity
Thermal conductivity pressure gauges are based on the decrease in thermal conductivity of a gas with pressure. These pressure gauges have a built-in filament that heats up when current is passed through it. A thermocouple or resistive temperature sensor (DOTS) can be used to measure the temperature of the filament. This temperature depends on the rate at which the filament transfers heat to the surrounding gas and thus on thermal conductivity. A Pirani gauge is often used, which uses a single platinum filament at the same time as a heating element and like DOTS. These pressure gauges give accurate readings between 10 and 10−3 mmHg. Art., but they are quite sensitive to chemical composition measured gases.
[edit]Two filaments
One wire coil is used as a heater, while the other is used to measure temperature through convection.
Pirani pressure gauge (one thread)
The Pirani pressure gauge consists of a metal wire exposed to the pressure being measured. The wire is heated by the current flowing through it and cooled by the surrounding gas. As the gas pressure decreases, the cooling effect also decreases and the equilibrium temperature of the wire increases. The resistance of a wire is a function of temperature: by measuring the voltage across the wire and the current flowing through it, the resistance (and thus the gas pressure) can be determined. This type of pressure gauge was first designed by Marcello Pirani.
Thermocouple and thermistor gauges work in a similar way. The difference is that a thermocouple and thermistor are used to measure the temperature of the filament.
Measuring range: 10−3 - 10 mmHg. Art. (roughly 10−1 - 1000 Pa)
Ionization pressure gauge
Ionization pressure gauges are the most sensitive measuring instruments for very low pressures. They measure pressure indirectly by measuring the ions produced when the gas is bombarded with electrons. The lower the gas density, the fewer ions will be formed. Calibration of an ion pressure gauge is unstable and depends on the nature of the measured gases, which is not always known. They can be calibrated by comparison with the McLeod pressure gauge readings, which are much more stable and independent of chemistry.
Thermionic electrons collide with gas atoms and generate ions. The ions are attracted to the electrode at a suitable voltage, known as a collector. The collector current is proportional to the ionization rate, which is a function of system pressure. Thus, measuring the collector current allows one to determine the gas pressure. There are several subtypes of ionization pressure gauges.
Measuring range: 10−10 - 10−3 mmHg. Art. (roughly 10−8 - 10−1 Pa)
Most ion gauges come in two types: hot cathode and cold cathode. The third type - a pressure gauge with a rotating rotor - is more sensitive and expensive than the first two and is not discussed here. In the case of a hot cathode, an electrically heated filament creates electron beam. The electrons pass through the pressure gauge and ionize the gas molecules around them. The resulting ions collect on the negatively charged electrode. The current depends on the number of ions, which in turn depends on the gas pressure. Hot cathode pressure gauges accurately measure pressure in the range of 10−3 mmHg. Art. up to 10−10 mm Hg. Art. The principle of a cold cathode pressure gauge is the same, except that electrons are produced in a discharge created by a high-voltage electrical discharge. Cold cathode pressure gauges accurately measure pressure in the range of 10−2 mmHg. Art. up to 10−9 mm Hg. Art. Calibration of ionization pressure gauges is very sensitive to structural geometry, chemical composition of the measured gases, corrosion and surface deposits. Their calibration may become unusable when turned on at atmospheric and very low pressure. The composition of vacuum at low pressures is usually unpredictable, so a mass spectrometer must be used in conjunction with an ionization pressure gauge for accurate measurements.
Hot cathode
A Bayard-Alpert hot cathode ionization pressure gauge typically consists of three electrodes operating in triode mode, where the cathode is a filament. The three electrodes are the collector, filament and grid. The collector current is measured in picoamps by an electrometer. The potential difference between the filament and ground is typically 30 volts, while the grid voltage under constant voltage is 180-210 volts unless there is optional electronic bombardment through heating the grid, which can have a high potential of approximately 565 volts. The most common ion gauge is a Bayard-Alpert hot cathode with a small ion collector inside the grid. A glass casing with a hole to the vacuum can surround the electrodes, but usually it is not used and the pressure gauge is built directly into the vacuum device and the contacts are routed through a ceramic plate in the wall of the vacuum device. Hot cathode ionization gauges can be damaged or lose calibration if they are turned on when atmospheric pressure or even at low vacuum. The measurements of hot cathode ionization pressure gauges are always logarithmic.
The electrons emitted by the filament move several times in forward and reverse directions around the grid until they hit it. During these movements, some electrons collide with gas molecules and form electron-ion pairs (electron ionization). The number of such ions is proportional to the density of gas molecules multiplied by the thermionic current, and these ions fly to the collector, forming an ion current. Since the density of gas molecules is proportional to pressure, pressure is estimated by measuring the ion current.
Sensitivity to low pressure Hot cathode pressure gauges are limited by the photoelectric effect. Electrons striking the grid produce X-rays, which produce photoelectric noise in the ion collector. This limits the range of older hot cathode gauges to 10−8 mmHg. Art. and Bayard-Alpert to approximately 10−10 mmHg. Art. Additional wires at cathode potential in the sight line between the ion collector and the grid prevent this effect. In the extraction type, the ions are attracted not by a wire, but by an open cone. Since the ions cannot decide which part of the cone to hit, they pass through the hole and form an ion beam. This ion beam can be transmitted to a Faraday cup.