Pressure gauges. Purpose and classification

A liquid thermometer is a device for measuring the temperature of technological processes using a liquid that reacts to changes in temperature. Liquid thermometers are well known to everyone in everyday life: for measuring room temperature or human body temperature.

Liquid thermometers consist of five main parts, these are: the thermometer ball, liquid, capillary tube, bypass chamber, and scale.

The thermometer ball is the part where the liquid is placed. The liquid responds to changes in temperature by rising or falling through the capillary tube. A capillary tube is a narrow cylinder through which liquid moves. Often the capillary tube is equipped with a bypass chamber, which is a cavity into which excess liquid flows. If there is no bypass chamber, once the capillary tube is filled, enough pressure will build up to destroy the tube if the temperature continues to rise. The scale is part liquid thermometer, with which readings are taken. The scale is calibrated in degrees. The scale can be fixed to the capillary tube, or it can be movable. The moving scale makes it possible to adjust it.

Working principle of a liquid thermometer

The operating principle of liquid thermometers is based on the ability of liquids to compress and expand. When a liquid is heated, it usually expands; The liquid in the thermometer bulb expands and moves up the capillary tube, thereby indicating an increase in temperature. Conversely, when a liquid cools, it usually contracts; the liquid in the capillary tube of a liquid thermometer decreases and thereby indicates a decrease in temperature. In the case when there is a change in the measured temperature of a substance, heat transfer occurs: first from the substance whose temperature is measured to the thermometer ball, and then from the ball to the liquid. The liquid reacts to changes in temperature by moving up or down the capillary tube.

The type of liquid used in a liquid thermometer depends on the range of temperatures the thermometer measures.

Mercury, -39-600 °C (-38-1100 °F);
Mercury alloys, -60-120 °C (-76-250 °F);
Alcohol, -80-100 °C (-112-212 °F).

Partial Immersion Liquid Thermometers

Many liquid thermometers are designed to hang on a wall, with the entire surface of the thermometer in contact with the substance whose temperature is being measured. However, some types of industrial and laboratory liquid thermometers are designed and calibrated to be immersed in liquid.

Of the thermometers used in this way, the most widely used are partial immersion thermometers. To obtain an accurate reading with a partial immersion thermometer, immerse the bulb and capillary tube only to this line.

Partial immersion thermometers are immersed to a mark to compensate for changes in ambient temperature that may affect the liquid inside the capillary tube. If changes in ambient temperature (changes in the temperature of the air around the thermometer) are likely, they can cause the liquid inside the capillary tube to expand or contract. As a result, the readings will be affected not only by the temperature of the substance that is being measured, but also by the temperature of the surrounding air. Immersing the capillary tube to the marked line removes the effect of ambient temperature on the accuracy of the readings.

In conditions industrial production It is often necessary to measure the temperatures of substances passing through pipes or contained in containers. Measuring temperature under these conditions creates two problems for instrument technicians: how to measure the temperature of a substance if there is no direct access to this substance or liquid, and how to remove a liquid thermometer for inspection, verification or replacement without stopping the process. Both of these problems are eliminated if measuring channels are used to insert thermometers.

The measuring channel for inserting the thermometer is a pipe-shaped channel that is closed at one end and open at the other. The measuring channel is designed to accommodate the ball of a liquid thermometer and thus protect it from substances that can cause corrosion, toxic substances, or under high pressure. When measuring channels are used to insert thermometers, heat exchange occurs in the form of indirect contact (through the measuring channel) of the substance whose temperature is measured and the thermometer ball. The measuring channels are a seal for increased pressure and prevent the liquid, the temperature by which is measured, from escaping.

Measuring channels are made standard sizes, so they can be used with various types thermometers. When the thermometer is installed in the measuring channel, its ball is inserted into the channel, and a nut is screwed on top of the thermometer to secure the thermometer.

Liquid (pipe) pressure gauges operate on the principle of communicating vessels - by balancing the fixed pressure with the weight of the filler liquid: the liquid column shifts to a height that is proportional to the applied load.

Measurements based on the hydrostatic method are attractive due to their combination of simplicity, reliability, cost-effectiveness and high accuracy. A pressure gauge with liquid inside is optimal for measuring pressure differences within 7 kPa (v special options execution - up to 500 kPa).

Types and types of devices

For laboratory measurements or industrial applications are used various options pressure gauges with pipe structure. The following types of devices are most in demand:

• U-shaped. The basis of the design is communicating vessels in which pressure is determined by one or several liquid levels at once. One part of the tube connects to pipeline system to carry out the measurement. At the same time, the other end can be hermetically sealed or have free communication with the atmosphere.
• Cupped. A single-tube liquid pressure gauge is in many ways similar to the design of classic U-shaped instruments, but instead of a second tube, it uses a wide reservoir, the area of ​​​​which is 500-700 times larger than the cross-sectional area of ​​the main tube.
• Ring. In devices of this type, the liquid column is enclosed in an annular channel. When the pressure changes, the center of gravity moves, which in turn leads to the movement of the indicator arrow. Thus, the pressure measuring device records the angle of inclination of the axis of the annular channel. These pressure gauges attract high accuracy of results that do not depend on the density of the liquid and the gaseous medium on it. At the same time, the scope of application of such products is limited by their high cost and complexity of maintenance.
• Liquid piston. The measured pressure displaces the extraneous rod and balances its position with calibrated weights. By selecting the optimal parameters for the mass of the rod with weights, it is possible to ensure its ejection by an amount proportional to the measured pressure, and, therefore, convenient for control.

What does a liquid pressure gauge consist of?

The device of a liquid pressure gauge can be seen in the photo:

Application of Liquid Pressure Gauge

The simplicity and reliability of measurements based on the hydrostatic method explain wide application device with liquid filler. Such pressure gauges are indispensable when conducting laboratory research or solving various technical problems. In particular, the instruments are used for the following types of measurements:

• Slight overpressure.
• Pressure difference.
• Atmosphere pressure.
• Underpressure.

An important area of ​​application for pipe pressure gauges with liquid filler is verification of control measuring instruments: draft gauges, pressure gauges, vacuum gauges, barometers, differential pressure gauges and some types of pressure gauges.

Liquid pressure gauge: principle of operation

The most common device design is a U-shaped tube. The operating principle of the pressure gauge is shown in the figure:

One end of the tube has communication with the atmosphere - it is affected by Atmosphere pressure Patm. The other end of the tube is connected to the target pipeline using supply devices - it is exposed to the pressure of the measured medium Rab. If the Rabs indicator is higher than Patm, then the liquid is displaced into a tube communicating with the atmosphere.

Calculation instructions

The height difference between liquid levels is calculated by the formula:

h = (Rabs – Ratm)/((rl – ratm)g)
Where:
Abs – absolute measured pressure.
Ratm – atmospheric pressure.
rzh – density of the working fluid.
ratm – density of the surrounding atmosphere.
g – gravitational acceleration (9.8 m/s2)
The working fluid height indicator H consists of two components:
1. h1 – decrease in column compared to the original value.
2. h2 – increase in the column in another part of the tube compared to the initial level.
The ratm indicator is often not taken into account in calculations, since rl >> ratm. Thus, the dependence can be represented as:
h = Rizb/(rzh g)
Where:
Rizb is the excess pressure of the measured medium.
Based on the above formula, Rizb = hrж g.

If it is necessary to measure the pressure of discharged gases, measuring instruments are used in which one of the ends is hermetically sealed, and vacuum pressure is connected to the other using supply devices. The design is shown in the diagram:

For such devices the formula is used:
h = (Ratm – Rabs)/(rzh g).

The pressure at the sealed end of the tube is zero. If there is air in it, calculations of vacuum gauge pressure are performed as follows:
Ratm – Rabs = Rizb – hrzh g.

If the air in the sealed end is evacuated and the counter pressure Ratm = 0, then:
Rab = hrzh g.

Designs in which the air at the sealed end is evacuated and evacuated before filling are suitable for use as barometers. Recording the difference in column height in the sealed part allows for accurate calculations of barometric pressure.

Advantages and disadvantages

Liquid pressure gauges have both strong and weak sides. When using them, it is possible to optimize capital and operating costs for control and measurement activities. At the same time, one should remember possible risks and vulnerable areas of such structures.

Key advantages of liquid-filled measuring instruments include:

• High measurement accuracy. Devices with low level errors can be used as exemplary ones for checking various control measuring equipment.
• Ease of use. The instructions for using the device are extremely simple and do not contain any complex or specific actions.
• Low cost. The price of liquid pressure gauges is significantly lower compared to other types of equipment.
• Quick installation. Connection to the target pipelines is made using supply devices. Installation/disassembly does not require special equipment.

When using liquid-filled pressure gauges, some weaknesses of such designs should be taken into account:

• A sudden increase in pressure can lead to the release of working fluid.
• The possibility of automatic recording and transmission of measurement results is not provided.
• The internal structure of liquid pressure gauges determines their increased fragility
• The devices are characterized by a fairly narrow measurement range.
• The correctness of measurements can be impaired by poor cleaning of the internal surfaces of the tubes.

The operating principle is based on balancing the measured pressure or pressure difference with the pressure of a liquid column. They have a simple structure and high accuracy measurements are widely used as laboratory and calibration instruments. Liquid pressure gauges are divided into: U-shaped, bell and ring.

U-shaped. The principle of operation is based on the law of communicating vessels. They come in two-pipe (1) and single-pipe cups (2).

1) are a glass tube 1 mounted on a board 3 with a scale and filled with a barrier liquid 2. The difference in levels in the elbows is proportional to the measured pressure drop. “-” 1. series of errors: due to inaccuracy in measuring the position of the meniscus, changes in T surrounding. environment, capillarity phenomena (eliminates by introducing corrections). 2. the need for two readings, which leads to an increase in error.

2) rep. is a modification of two-pipe ones, but one elbow is replaced with a wide vessel (cup). Under the influence of excess pressure, the liquid level in the vessel decreases and in the tube increases.

Float U-shaped Differential pressure gauges are similar in principle to cup gauges, but to measure pressure they use the movement of a float placed in a cup when the liquid level changes. By means of a transmission device, the movement of the float is converted into the movement of the indicating arrow. “+” wide measurement range. Operating principle liquid pressure gauges are based on Pascal's law - the measured pressure is balanced by the weight of the column of working fluid: P = ρgh. Consist of a reservoir and a capillary. The working fluids used are distilled water, mercury, ethanol. They are used for measuring small excess pressures and vacuum, barometric pressure. They are simple in design, but there is no remote data transmission.

Sometimes, to increase sensitivity, the capillary is placed at a certain angle to the horizon. Then: P = ρgL Sinα.

IN deformation pressure gauges are used to counter the elastic deformation of the sensing element (SE) or the force developed by it. There are three main forms of SE that have become widespread in measurement practice: tubular springs, bellows and membranes.

Tubular spring(gauge spring, Bourdon tube) - an elastic metal tube, one of the ends of which is sealed and has the ability to move, and the other is rigidly fixed. Tubular springs are mainly used to convert the measured pressure applied to inner space spring, into proportional movement of its free end.

The most common is a single-turn tubular spring, which is a 270° bent tube with an oval or elliptical cross-section. Under the influence of the supplied excess pressure, the tube unwinds, and under the influence of vacuum it twists. This direction of movement of the tube is explained by the fact that, under the influence of internal excess pressure, the minor axis of the ellipse increases, while the length of the tube remains constant.

The main disadvantage of the springs considered is their small angle of rotation, which requires the use of transmission mechanisms. With their help, moving the free end of a tubular spring by several degrees or millimeters is converted into an angular movement of the arrow by 270 - 300°.

The advantage is a static characteristic close to linear. The main application is indicating instruments. Measurement ranges of pressure gauges from 0 to 10 3 MPa; vacuum gauges - from 0.1 to 0 MPa. Instrument accuracy classes: from 0.15 (exemplary) to 4.

Tubular springs are made of brass, bronze, of stainless steel.

Bellows. Bellows is a thin-walled metal cup with transverse corrugations. The bottom of the glass moves under pressure or force.

Within the linearity of the static characteristics of the bellows, the ratio of the force acting on it to the deformation caused by it remains constant. and is called the rigidity of the bellows. Bellows are made from various grades of bronze, carbon steel, stainless steel, aluminum alloys etc. Bellows with a diameter of 8–10 to 80–100 mm and a wall thickness of 0.1–0.3 mm are mass-produced.

Membranes. There are elastic and elastic membranes. An elastic membrane is a flexible round flat or corrugated plate that can bend under pressure.

Static characteristic flat membranes changes nonlinearly with increasing pressure, therefore a small part of the possible stroke is used as the working area. Corrugated membranes can be used for larger deflections than flat ones, since they have significantly less nonlinearity of the characteristic. Membranes are made from various grades of steel: bronze, brass, etc.

A pressure gauge is a compact mechanical device for measuring pressure. Depending on the modification, it can work with air, gas, steam or liquid. There are many types of pressure gauges, based on the principle of taking pressure readings in the medium being measured, each of which has its own application.

Scope of use

Pressure gauges are one of the most common instruments that can be found in various systems:

• Heating boilers.
• Gas pipelines.
• Water pipelines.
• Compressors.
• Autoclaves.
• Cylinders.
• Balloon air rifles, etc.

Externally, the pressure gauge resembles a low cylinder various diameters, most often 50 mm, which consists of a metal body with a glass lid. Through the glass part you can see a scale with marks in pressure units (Bar or Pa). On the side of the housing there is a tube with an external thread for screwing into the hole of the system in which it is necessary to measure the pressure.

When pressure is injected into the medium being measured, the gas or liquid through the tube presses the internal mechanism of the pressure gauge, which leads to a deflection of the angle of the arrow that points to the scale. The higher the pressure created, the more the needle deflects. The number on the scale where the pointer stops will correspond to the pressure in the system being measured.

Pressure that a pressure gauge can measure

Pressure gauges are universal mechanisms, which can be used to measure various values:

• Excess pressure.
• Vacuum pressure.
• Pressure differences.
• Atmospheric pressure.

The use of these devices allows you to control various technological processes and prevent emergency situations. Pressure gauges intended for use in special conditions may have additional modifications to the body. This may be explosion protection, resistance to corrosion or increased vibration.

Types of pressure gauges

Pressure gauges are used in many systems where there is pressure, which must be at a clearly defined level. The use of the device allows you to monitor it, since insufficient or excessive exposure can harm various technological processes. In addition, excess pressure causes rupture of containers and pipes. In this regard, several types of pressure gauges designed for specific operating conditions have been created.

They are:

• Exemplary.
• General technical.
• Electric contact.
• Special.
• Self-recording.
• Ship's.
• Railway.

Exemplary pressure gauge intended for verification of other similar measuring equipment. Such devices determine the level of excess pressure in various environments. Such devices are equipped with a particularly precise mechanism that gives minimal error. Their accuracy class ranges from 0.05 to 0.2.

General technical are used in general environments that do not freeze into ice. Such devices have an accuracy class from 1.0 to 2.5. They are resistant to vibration, so they can be installed on transport and heating systems.

Electric contact are designed specifically for monitoring and warning of reaching the upper limit of a dangerous load that can destroy the system. Such devices are used with various media such as liquids, gases and vapors. This equipment has a built-in electrical circuit control mechanism. When excess pressure appears, the pressure gauge gives a signal or mechanically turns off the supply equipment that pumps pressure. Electrical contact pressure gauges may also include special valve, which relieves pressure to safe level. Such devices prevent accidents and explosions in boiler rooms.

Special Pressure gauges are designed to work with a specific gas. Such devices usually have colored cases rather than the classic black ones. The color corresponds to the gas with which this device can work. Also, special markings are used on the scale. For example, pressure gauges for measuring ammonia pressure, which are usually installed in industrial refrigeration units, are colored yellow. Such equipment has an accuracy class from 1.0 to 2.5.

Self-recording are used in areas where it is required not only to visually monitor the system pressure, but also to record indicators. They write a chart that can be used to view pressure dynamics over any period of time. Such devices can be found in laboratories, as well as at thermal power plants, canneries and other food enterprises.

Ship's include a wide the lineup pressure gauges that have a weatherproof housing. They can work with liquid, gas or steam. Their names can be found on street gas distributors.

Railway pressure gauges are designed to control overpressure in mechanisms that serve electric rail transport. In particular, they are used on hydraulic systems, moving the rails when extending the boom. Such devices have increased resistance to vibration. Not only do they withstand shock, but the pointer on the scale does not respond to mechanical impact on the housing, accurately displaying the pressure level in the system.

Types of pressure gauges based on the mechanism for taking readings of pressure in the medium

Pressure gauges also differ in the internal mechanism that results in taking pressure readings in the system to which they are connected. Depending on the device they are:

• Liquid.
• Spring.
• Membrane.
• Electric contact.
• Differential.

Liquid The pressure gauge is designed to measure the pressure of a liquid column. Such devices operate on the physical principle of communicating vessels. Most devices have a visible level of the working fluid from which they take readings. These devices are one of the rarely used. Due to contact with liquid, they inner part gets dirty, so transparency is gradually lost, and it becomes difficult to visually determine the readings. Liquid pressure gauges were one of the very first ones invented, but they are still found.

Spring pressure gauges are the most common. They have simple design which is suitable for repair. Their measurement limits usually range from 0.1 to 4000 Bar. Directly myself sensing element Such a mechanism is a tube of oval cross-section, which shrinks under pressure. The force pressing on the tube is transmitted through a special mechanism to a pointer, which rotates at a certain angle, pointing to a scale with markings.

Membrane The pressure gauge operates on the physical principle of pneumatic compensation. Inside the device there is a special membrane, the level of deflection of which depends on the effect of the pressure created. Typically, two membranes are soldered together to form a box. As the volume of the box changes, the sensitive mechanism deflects the arrow.

Electric contact Pressure gauges can be found in systems that automatically monitor pressure and adjust it or signal when a critical level has been reached. The device has two arrows that can be moved. One is set to minimum pressure, and the second to maximum. The electrical circuit contacts are mounted inside the device. When the pressure reaches one of the critical levels, the electrical circuit is closed. As a result, a signal is generated on the control panel or an automatic mechanism is triggered for an emergency reset.

Differential Pressure gauges are one of the most complex mechanisms. They work on the principle of measuring deformation inside special blocks. These pressure gauge elements are pressure sensitive. As the block deforms, a special mechanism transmits the changes to an arrow pointing to the scale. The pointer moves until the changes in the system stop and stop at a certain level.

Accuracy class and measurement range

Any pressure gauge has a technical passport, which indicates its accuracy class. The indicator has a numerical expression. The lower the number, the more accurate the device. For most instruments, the norm is an accuracy class of 1.0 to 2.5. They are used in cases where a small deviation is not of particular importance. The biggest error is usually caused by the devices that motorists use to measure air pressure in tires. Their class often drops to 4.0. Best class Exemplary pressure gauges have precision, the most advanced of which operate with an error of 0.05.

Each pressure gauge is designed to operate over a specific pressure range. Massive models that are too powerful will not be able to record minimal fluctuations. Very sensitive devices, when exposed to excess, fail or are destroyed, leading to depressurization of the system. In this regard, when choosing a pressure gauge, you should pay attention to this indicator. Typically, you can find models on the market that are capable of recording pressure differences ranging from 0.06 to 1000 mPa. There are also special modifications, so-called draft meters, which are designed to measure vacuum pressure down to a level of -40 kPa.