How is hydrostatic pressure in a liquid measured?

Description of atmospheric pressure measuring instruments

06/19/2015
Air pressure varies widely. If it is more than 760 millimeters of mercury, then it is considered elevated

, if less, then
reduced
.

Observations of changes in atmospheric pressure make it possible to predict the weather. For example, when pressure increases in winter, the weather becomes frostier, and in summer it becomes hot.

Low atmospheric pressure contributes to cloudiness and precipitation.

Therefore, constantly knowing the value of atmospheric pressure and monitoring its changes is necessary not only for scientists and doctors, but for all of us.

Atmosphere pressure

Atmospheric pressure is measured in millimeters of mercury, as well as in Pascals and hectoPascals. Normal blood pressure is considered to be 760 mmHg. Art. (1013.25 hPa) .

Atmospheric pressure generally changes with changes in weather conditions. Often, pressure drops before inclement weather and rises before good weather. Keeping records of pressure changes allows you to determine the movement of cyclones and the direction of winds.

A change in characteristic pressure often does not affect the well-being of a person living for a long time in a certain area. In cases where non-periodic fluctuations in atmospheric pressure occur, even healthy people experience headaches, decreased performance and a feeling of heaviness in the body.

Changes in atmospheric pressure also affect many technological processes.

For example, when processing petroleum products, where pressure is one of the main controlled technical parameters; bakery production, where pressure readings greatly affect the moisture content of semi-finished dough products; in the aviation industry this is a very important parameter that affects the terms and conditions of operation.

Instruments for measuring atmospheric pressure

Today, there are several types of barometers that are used to measure air pressure:

• Mercury siphon barometer - represents a Y-shaped tube filled with mercury with an open and sealed end.
• Mercury cup barometer - consists of a vertical tube filled with mercury, the upper end of which is sealed, and the lower end is in a special cup with mercury.
• An aneroid barometer is an airless metal box with wavy walls.
• A barograph is a self-recording instrument that is used to monitor barometric pressure at certain periods of time.
• An electronic barometer is a digital device that operates on the principle of a conventional aneroid or on the principle of measuring air pressure on a sensitive crystal.

Mercury barometers are more accurate and reliable compared to aneroids; they are used to check the operation of other types of barometers. The height of pressure in them is determined by the height of the mercury column. Meteorological stations are equipped with cup barometers.

Measuring atmospheric pressure using a thermohygrometer

Atmospheric pressure is measured not only using various types of barometers, but also with such universal digital instruments as thermohygrometers.

Despite the fact that the main task of these devices is to determine relative humidity and temperature, they also do an excellent job of measuring air pressure, showing the most accurate values.

Therefore, such multifunctional devices are much more profitable to purchase than outdated barometers and psychrometers.

JSC "EXIS" brings to your attention a huge range of electronic pressure meters and other high-quality control and measuring instruments and always at affordable prices.

In particular, in our company you can purchase the following models of thermohygrometers:

• Thermohygrometer IVTM-7 M 2-D-V
  . The device, in addition to measuring and recording temperature and relative humidity of air and other non-aggressive gases, measures atmospheric pressure in millimeters of mercury and hPa, can record data in non-volatile memory, recalculate measurement results into various units (percentage of relative humidity, g/m3), carry out simultaneous indication of measured values. IVTM-7 M 2-D-V has a high degree of dust and moisture protection (IP65), making it possible to use it in rooms with high humidity.
• Thermohygrometer IVTM-7 K-1
  . The device measures atmospheric pressure in kPa, can recalculate the values ​​of various humidity units, carry out simultaneous indication of measured values, record data on microSD, it is possible to connect various types of primary transducers.
• Thermohygrometer IVTM-7 R-03-I-D
  . The device is equipped with a liquid crystal indicator designed for visual monitoring of relative humidity, temperature and pressure values. It has small dimensions and an ergonomic body.
• Thermohygrometer IVTM-7 M 6-D (in an ergonomic housing).
  The device measures atmospheric pressure in kPa, can record data on a non-volatile memory card, recalculate measurement results into various units, and simultaneously display the measured values. It has an ergonomic body and a large and convenient display.
• Thermohygrometer IVTM-7 M 3-D-V
  . The device, in addition to measuring and recording temperature and relative humidity of air and other non-aggressive gases, measures atmospheric pressure in millimeters of mercury and hPa, can record data in non-volatile memory, recalculate measurement results into various units (percentage of relative humidity, g/m3), carry out simultaneous indication of measured values. Model IVTM-7 M3-D-V is intended for creating a measuring network. The degree of moisture protection of the housing and sensor is IP65, which makes it possible to use it in rooms with high humidity.
• Thermohygrometer IVTM-7 M 6-D.
  The device measures atmospheric pressure in kPa, can record data on a non-volatile memory card (microSD), recalculate measurement results into various units, and simultaneously display the measured values.

All models of thermohygrometers have a communication interface with a PC via USB, RS-232 and can be mounted on a wall.

Liquid pressure gauges

Liquid pressure gauges have a fairly simple operating principle. The pressure in them will be measured precisely by the height of the liquid in the column. Typically, such devices are made in the form of two communicating vessels in which the working fluid is present.

One end of the vessel must be connected to the medium where the pressure measurement will take place. The other end should remain open. Under the influence of the pressure of the measured medium, the liquid will gradually begin to move from one elbow to another. When the fluid level in the knee is level, measurements can be taken and the readings can be recorded. The fluid difference will be proportional to the excess pressure (PH). Its calculation is carried out according to the formula:

PH=Hpg, where:

• H – height of the liquid column.
• P is the density of the liquid.
• G – acceleration due to gravity.

Thanks to this pressure gauge, you will also have an excellent opportunity to measure vacuum and pressure difference.

Of course, such a pressure gauge is actively used, but it may also have certain disadvantages. The main disadvantage is that it is almost impossible to measure pulsating pressure.

Pressure measuring instruments

A tonometer is a medical device for measuring blood pressure (BP) and pulse rate. It is used in healthcare institutions and at home to determine the state of the cardiovascular system. In the absence of problems, blood pressure levels remain stable.

• Small pressure fluctuations up to 20 mm. Hg Art. for systolic or upper value and up to 10 mm. Hg Art. for diastolic or lower value within one day are possible and not dangerous. The reasons for fluctuations are varied: from stress and anxiety to physical activity and heavy meals.
• A prolonged deviation of blood pressure levels from standard values, especially if it goes beyond normal limits, is a possible signal of the development of the disease and requires an early visit to the doctor.

It is important to note that each person has their own optimal blood pressure range to feel their best. Knowing the optimal blood pressure range helps your doctor identify changes in your health.

To determine your pressure range at home, you need to purchase a wrist-mounted device for measuring human blood pressure "Omron". Remember: pressure different from the norm does not work for a healthy person. Even feeling normal in this case is an additional reason to seek medical attention. consultation.

Why measure blood pressure?

Heart attack, stroke, kidney failure, blindness are all precursors to hypertension. And there is only one way to avoid serious complications - to maintain normal blood pressure levels with the help of medications.

Hypertensive patients need a home blood pressure monitor to prevent the risk of possible complications. It is very important to measure blood pressure in a quiet environment to obtain the most accurate data.

The pressure readings of sick and healthy people are influenced not only by external factors and various diseases; age and gender are of particular importance.

According to the data shown in the table, blood pressure increases with age and this is normal, as the body ages and age-related changes occur that provoke disorders.

We remind you! The parameters shown in the table are average values. To determine the exact individual blood pressure level, you must regularly use the Omron home blood pressure monitor and consult a specialist.

Typical elements of devices for measuring pressure levels

The main components of mechanical and semi-automatic blood pressure meters:

• pressure gauge with scale / electronic monitor;
• shoulder cuff (pneumatic chamber in a fabric “sleeve” with fixing Velcro);
• rubber bulb with an adjustable deflation valve for pumping air into the cuff;
• phonendoscope;
• rubber tubes for air supply.

The main components of automatic blood pressure meters:

• electronic unit with display;
• cuff on the shoulder or wrist (pneumatic chamber in a fabric “sleeve” with Velcro fasteners);
• rubber tubes;
• batteries type AA (“finger”) or type AAA (“little finger”);
• network adapter.

Types of Omron tonometers

Mechanical pressure meters CS Medica (CS 106 without phonendoscope / with phonendoscope, CS 105, CS 107, CS 110 Premium).

A mechanical device determines blood pressure by heart beats on the arteries, and the results are displayed on a pressure gauge. Air is pumped into the cuff manually using a special bulb.

Advantages: high reliability, long service life and reasonable price of the pressure meter.

Disadvantages: requires the direct participation of a third party at all stages of measurement, but the human factor introduces some error into the measurement results, the magnitude of which depends on the professionalism of the person measuring.

Automatic shoulder pressure meters (Omron M2 Basic, M2 Basic with adapter, M2 Basic with universal cuff and adapter, Omron M2 Classic, M2 Classic with adapter, M3 Eco, Omron M3 Expert, M3 Comfort, M3 Family, M6 Comfort, Omron M6 , Mit Elite, Mit Elite Plus, M10-IT) Automatic wrist pressure meters (Omron R1, R2, R3 Opti, Omron R5 Prestige)

The operating principle of automatic devices: using a compressor, the device automatically pumps air into the cuff. The result of systolic, diastolic pressure and pulse rate are displayed.

Advantages: accuracy is not affected by the human factor (artificial intelligence pumps air into the cuff, adapting to the individual characteristics of a person), simple use without specific skills.

Disadvantages: error in determining blood pressure in patients with cardiovascular problems (take two or three measurements to avoid inaccuracies); high price, especially of those digital devices for measuring human blood pressure that have the maximum range of consumer functions. The possible error is 3 – 10 mmHg.

Semi-automatic pressure meters (Omron S1, M1 Compact, M1 Eco)

Blood pulsation in the artery is determined automatically, the results are displayed on a liquid crystal display, but the cuff must be filled with air using a rubber bulb.

Advantages: no need to independently calculate pressure readings, all results are in electronic form, excellent functionality at a low cost.

Disadvantages: small errors in blood pressure calculations; require human intervention to pump air into the cuff.

You can buy Omron muscle stimulators with delivery in Moscow and Russia from the official Omron online store in the Russian Federation. Free call within Russia: 8 (800) 333-53-39.

Measurement methods

Blood pressure is measured in two ways:

• Auscultatory (Korotkoff method) – listening to the pulse using a phonendoscope. The method is typical for mechanical devices.
• Oscillometric – the result is immediately displayed on the screen of an automatic device.

However, in both cases the principle of operation of tonometers is the same.

How to measure blood pressure correctly?

When measuring with mechanical instruments, you must follow the instructions:

1. The first measurement is carried out in the morning, the second or third measurement is taken in the afternoon and evening (or only in the evening); 1-2 hours after eating and no earlier than 1 hour after smoking or drinking coffee.
2. It is advisable to take 2-3 measurements and calculate the average blood pressure value.
3. It is correct to take the measurement on your non-working hand (on your left if you are right-handed, and on your right if you are left-handed).
4. When applying the cuff, its lower edge should be 2.5 cm above the cubital fossa. The measuring tube extending from the cuff is located in the middle of the elbow.
5. The stethoscope should not touch the tonometer tubes. It should be located at the level of the 4th rib or heart.
6. The air is pumped vigorously (slow results in pain).
7. Air inlet from the cuff should flow slowly - 2 mmHg. per second (the slower the release, the higher the quality of the measurement).
8. You should sit at the table, leaning on the back of the chair, with your elbow and forearm on the table so that the cuffs are level with the heart line.

When measuring blood pressure with an automatic device, you should also follow points 1-4 from the instructions above:

1. You should sit at the table, calmly leaning on the back of the chair, with your elbow and forearm lying on the table so that the cuff is level with the line of the heart.
2. Then press the Star/Stop button and the device will automatically measure your blood pressure, but at this time you should not talk or move.

Blood pressure cuff and its size

The cuffs for the blood pressure meter must be the right size for you; the accuracy of the readings directly depends on this (measure the circumference of your arm above the elbow).

The set of Omron pressure measuring devices includes various cuffs, so it is necessary to check the size and the possibility of connecting additional cuffs.

with mechanical devices:

• Enlarged nylon without fixing ring for shoulder circumference 24-42 cm.
• Nylon with a metal fixing ring for shoulder circumference 24-38 cm.
• Nylon with a metal fixing ring for shoulder circumference from 22-38 cm.
• Enlarged without a fixing bracket for shoulder circumference 22-39 cm.

Mechanical tonometers (except for the CS Medics CS 107 model) have the ability to connect 5 different additional cuffs:

• No. 1, type H (9-14 cm).
• No. 2, type D (13-22 cm).
• Medica No. 3, type P (18-27 cm).
• Medica No. 4, type S (24-42 cm).
• Medica No. 5, type B (34-50 cm).

The Omron Fan-Shaped fan-shaped cuff (22-32 cm) is supplied with semi-automatic However, these tonometers have the ability to connect additional cuffs, which are purchased separately:

• Small + small “pear” (17-22 cm).
• Large for arm circumference (32-42 cm).

included with automatic devices:

• Compression standard CM, repeating the shape of the hand, medium size, (22-32 cm).
• Large CL (32-42 cm).
• Children's CS2 (17-22 cm).
• Universal CW (22-42 cm).
• Innovative Omron Intelli Wrap cuff (22-42 cm).
• Compression, new generation Easy Cuff, repeating the shape of the hand (22-42 cm).

Professional automatic models HBP-1100, HBP-1300 are supplied with two cuffs: a medium compression cuff Omron GS Cuff M (22-32cm) and a large compression cuff Omron GS Cuff L (32-42cm). It is possible to additionally purchase cuffs in the following sizes:

• GS Cuff SS, extra small (12-18 cm).
• GS Cuff S, small (17-22 cm).
• Omron GS Cuff M (22-32 cm).
• GS Cuff XL, extra large (42-50 cm).

Classification by type of pressure measured

Instruments used to obtain data on gas pressure parameters in gas tanks, transport lines, gas cylinders and other reservoirs are classified according to several criteria. They differ in their structure and principle of operation.

Devices used to measure pressure are divided into classes according to:

• type of pressure being measured;
• purpose;
• operating principle;
• accuracy class.

Based on the type of pressure being measured, instruments designed to determine accurate indicators are divided into pressure gauges, vacuum gauges, draft gauges, pressure gauges, barometers and others.

Depending on the degree of protection from the influence of the external environment, the following devices are produced:

• standard;
• protected from dust;
• waterproof;
• protected from aggressive environments;
• explosion-proof.

One product can combine several types of protection.

The diagram shows the division of measuring devices according to operating principle, type of pressure, application and display. Liquid and deadweight instruments are rarely used to obtain data on gas pressure

A pressure gauge is a small device that is used to measure pressure or pressure difference. The operating principle of this instrument depends on its internal structure. Within one class, they are further divided into groups depending on the accuracy class.

To measure absolute pressure, measured from absolute zero (vacuum), absolute pressure gauges are used. Excess pressure is determined using an excess pressure gauge. In general, all varieties of such devices are called in one word: “pressure gauge”.

Most types of pressure gauges are designed to measure excess pressure values. Their peculiarity is that they show pressure, representing the difference between absolute and atmospheric.

Vacuum gauges are devices that indicate the pressure value of a rarefied gas. Using pressure and vacuum gauges, excess pressure and rarefied gas pressure are measured. Information is displayed on a single scale.

Using pressure meters, excess pressure parameters are determined with values ​​up to 40 kPa. Traction meters, on the contrary, make it possible to measure rarefaction down to – 40 kPa. Thrust pressure meters measure rarefaction and excess pressure in the range from – 20 to + 20 kPa.

Pressure gauges are used in a wide variety of industries. Working with gas involves a high risk, so it is important to monitor all system indicators. Pressure information gives users information about the current state of the object being measured

Differential pressure gauges can be used to determine the pressure difference at two arbitrary points to be studied. A micromanometer is a differential pressure gauge that allows you to measure pressure differences within 40 kPa.

What device measures excess pressure?

The characteristic of pressure is the force that uniformly acts on a unit surface area of ​​the body. This force influences various technological processes. Pressure is measured in pascals. One pascal is equal to a force of one newton applied to a surface area of ​​1 m2. Instruments are used to measure pressure.

Types of pressure

• Atmospheric pressure is generated by the Earth's atmosphere.
• Vacuum pressure is pressure that does not reach atmospheric pressure.
• Excess pressure is a pressure value that exceeds atmospheric pressure.
• Absolute pressure is determined from the value of absolute zero (vacuum).

Types and work

Devices that measure pressure are called pressure gauges. In technology, it is most often necessary to determine excess pressure.

A significant range of measured pressure values ​​and special conditions for measuring them in various technological processes determine the variety of types of pressure gauges, which have their own differences in design features and operating principles. Let's consider the main types used.

Barometers

A barometer is a device that measures air pressure in the atmosphere. There are several types of barometers.

A mercury barometer works by moving mercury in a tube along a specific scale.

A liquid barometer works on the principle of balancing a liquid with atmospheric pressure.

An aneroid barometer works by changing the dimensions of a sealed metal box with a vacuum inside, under the influence of atmospheric pressure.

An electronic barometer is a more modern device. It converts the parameters of a conventional aneroid into a digital signal, which is displayed on a liquid crystal display.

Liquid pressure gauges

In these models of devices, the pressure is determined by the height of the liquid column, which equalizes this pressure. Liquid devices for measuring pressure are most often made in the form of 2 glass vessels connected to each other, into which liquid (water, mercury, alcohol) is poured.

Fig-1

One end of the container is connected to the medium being measured, and the other is open. Under the pressure of the medium, the liquid flows from one vessel to another until the pressure equalizes. The difference in fluid levels determines the excess pressure. Such devices measure pressure difference and vacuum.

Figure 1a shows a 2-pipe pressure gauge that measures vacuum, gauge and atmospheric pressure. The disadvantage is the significant error in measuring pressures that have pulsation. For such cases, 1-pipe pressure gauges are used (Figure 1b). They contain one edge of a larger vessel. The cup is connected to the cavity being measured, the pressure of which moves the liquid into the narrow part of the vessel.

When measuring, only the height of the liquid in the narrow elbow is taken into account, since the liquid changes its level in the cup insignificantly, and this is neglected. To measure small excess pressures, 1-pipe micromanometers with a tube inclined at an angle are used (Figure 1c). The greater the inclination of the tube, the more accurate the readings of the device, due to the increase in the length of the liquid level.

A special group is considered to be devices for measuring pressure, in which the movement of liquid in a container acts on a sensitive element - a float (1) in Figure 2a, a ring (3) (Figure 2c) or a bell (2) (Figure 2b), which are connected to an arrow, which is a pressure indicator.
Fig-2
The advantages of such devices are remote transmission and recording of values.

Strain gauges

In the technical field, strain gauges for measuring pressure have gained popularity. Their operating principle is to deform the sensing element. This deformation occurs under the influence of pressure. The elastic component is connected to a reading device having a scale graduated in pressure units. Deformation pressure gauges are divided into:

• Spring.
• Bellows.
• Membrane.

Fig-3

Spring pressure gauges

In these devices, the sensitive element is a spring connected to the pointer by a transmission mechanism. Pressure acts inside the tube, the cross-section tries to take a round shape, the spring (1) tries to unwind, as a result the pointer moves along the scale (Figure 3a).

Diaphragm pressure gauges

In these devices, the elastic component is the membrane (2). It bends under pressure and acts on the arrow using a transmission mechanism. The membrane is made like a box (3). This increases the accuracy and sensitivity of the device due to greater deflection at equal pressure (Figure 3b).

Bellows pressure gauges

In bellows-type devices (Figure 3c), the elastic element is a bellows (4), which is made in the form of a corrugated thin-walled tube. Pressure is applied to this tube. At the same time, the bellows increases in length and, with the help of a transmission mechanism, moves the pressure gauge needle.

Bellows and membrane types of pressure gauges are used for measuring minor excess pressures and vacuum, since the elastic component has little rigidity.

When such devices are used to measure vacuum, they are called draft gauges .

The device that measures excess pressure is a pressure gauge pressure gauges are used to measure excess pressure and vacuum .

Devices for measuring pressure of the deformation type have an advantage over liquid models. They allow readings to be transmitted remotely and recorded automatically.

This occurs due to the conversion of the deformation of the elastic component into an electrical current output signal. The signal is recorded by measuring instruments that are calibrated in pressure units. Such devices are called strain-electric manometers. Strain gauge, differential transformer and magnetic modulation converters are widely used.

Differential transformer converter

Fig-4

The principle of operation of such a converter is to change the induction current depending on the pressure value.

Devices with such a converter have a tubular spring (1), which moves the steel core (2) of the transformer, and not the arrow. As a result, the strength of the induction current supplied through the amplifier (4) to the measuring device (3) changes.

Magnetomodulation devices for measuring pressure

In such devices, the force is converted into an electric current signal due to the movement of a magnet associated with an elastic component. When moving, the magnet acts on the magnetic modulation converter.

The electrical signal is amplified in a semiconductor amplifier and sent to secondary electrical measuring devices.

Strain gauges

Converters based on a strain gauge operate on the basis of the dependence of the electrical resistance of the strain gauge on the amount of deformation.

Fig-5

Strain gauges (1) (Figure 5) are fixed on the elastic element of the device. The electrical signal at the output arises due to a change in the resistance of the strain gauge, and is recorded by secondary measuring devices.

Electric contact pressure gauges

Electrical contact pressure gauges have become popular in alarm circuits, automatic process control systems, and thermal protection devices. The figure shows a diagram and view of the device.

Fig-6

The elastic component in the device is a tubular single-turn spring. Contacts (1) and (2) are made for any marks on the instrument scale by rotating the screw in the head (3), which is located on the outside of the glass.

Pressure measuring instruments

Design of pressure transducers

Figure 4 - Design diagram of pressure transducers
The figure below shows the general design diagram of pressure transducers. Depending on the type of sensor, device manufacturer and application features, the design may vary. This diagram is intended to familiarize you with the main elements of a typical pressure transmitter.

1. Cable entry: This part of the pressure transmitter is used to seal the electrical cable into the sensor. As a rule, a gland type PG9 is used, but other connection options are also available (for example PG16, M20x1.5).
2. Terminals: Terminals are required to physically connect the electrical wires to the sensor. Today, the vast majority of pressure transducers use a 2-wire connection circuit with an output signal of 4...20 mA.
3. Power / spark protection board: This board distributes electrical energy between the electronic components of the sensor. For explosion-proof converters, this board implements the spark protection function. Inexpensive pressure sensors (for example, PTE5000) usually have a combined power supply and conversion board.
4. Electronics housing: The part of the pressure sensor that houses the power supply and conversion board. For low-price converters (WIKA, BD Sensors), the electronics housing and the sensor housing itself are one unit. The presence of a separate housing for electronics is typical only for high-quality pressure transducers (for example, KLAY-INSTRUMETNS, EMERSON, VALCOM, YOKOGAWA).
5. Converter Board: This is one of the most important parts of pressure transducers. This board converts the signal from the primary sensor into a unified electrical signal in terms of current or voltage.
6. Sensor body: The main mechanical part that represents the actual body of the transducer.
7. Wires and Atmospheric Tube: Wires are typically a cable harness that connects the sensor leads to the converter board. An atmospheric tube is used in gauge and vacuum pressure sensors to communicate the sensitive element (pressure sensor) with atmospheric pressure.
8. Process connection: This part of the pressure transducers is used to physically connect the sensor to the process (pipeline, tank, apparatus). The most common connection is a threaded pressure gauge connection G1/2″ according to DIN 16288 and M20x1.5 thread. Connections G1/4″, G1″, and flange connections are also widely available. In the food industry, special sanitary connections are common, for example milk nut DIN 11851, DRD flange, Tri-clamp clamps. The assortment of KIP-Service LLC includes special pressure transducers for use in the food (dairy, brewing) industry. These are devices manufactured by KLAY-INSTRUMENTS BV - pressure sensors of the 8000-SAN series and intelligent pressure sensors of the 2000-SAN series, which fully meet all the requirements of the food industry in terms of hygiene, measurement accuracy and temperature conditions.

   
   Figure 5 - Process connections

9. Pressure sensor (primary transducer): The pressure sensor is one of the key elements of any pressure transducer. This element directly converts the pressure acting on it into an electrical signal, which is then unified on the conversion board. Today, there are several ways to convert pressure into an electrical signal. Inductive, capacitive and strain-resistive conversion methods are used in industry. The most common is strain-resistive. This method is based on the phenomenon of the strain effect in metals and semiconductors. Strain gauges connected in a bridge circuit (Wheatstone bridge) change their resistance under pressure, which leads to imbalance of the bridge. The unbalance depends directly on the degree of deformation of the resistors and, therefore, on the applied pressure.

   
   Figure 6 - Wheatstone Bridge

There are 4 main types of sensors on the market, based on the strain-resistive conversion method, which are used by all existing manufacturers of pressure transducers. Let's look at each type separately.