Pressure gauges for measuring gas pressure: types, design features and operation of the meters
There is often a need to measure the pressure created by a gas.
For example, in cylinders, in gas pipelines, in various containers and vessels. To control and monitor indicators, pressure gauges are used to measure gas pressure. These devices serve in various spheres of life, from medicine to heavy industry. To ensure that the purchase of the device is not in vain, and that the purchased pressure gauge meets the requirements of production processes, it is worth familiarizing yourself with the classification.
We will introduce you to the types of gas pressure meters. Let's talk about their design features and operating principles.
Rules for installing the device
The pressure gauge cannot be installed if:
- There is no seal or inspection mark.
- The verification period has expired.
- There is visible damage, such as cracks.
- The arrow does not return to zero when disconnected.
- Installation at a height of more than 3 m from the site is prohibited.
The device is installed in such a way that the readings are clearly visible. The scale must be vertical or inclined at 30°.
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.
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.
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.
Pressure gauge installation
A three-way valve is placed between the pipe and the pressure gauge.
So that the gas meter can measure and regulate pressure correctly, it is placed in areas where it will be as easy as possible to take readings, carry out maintenance and repair of the device. There are maximum intervals between the regulator and the walls that must be observed during installation. If the device is placed at a height of up to 2-3 meters, the diameter of its body must be at least 160 mm.
In addition to the mounting structure of the pressure gauge, a three-way valve is installed between the pipe and the regulator itself. If the unit is operated in conditions due to which its functionality may be affected by high temperature, precipitation or other external factors, it is additionally protected with siphons, buffer elements or other protection, as well as thermal insulation if necessary.
Classification by operating principle
Gas pressure gauges, depending on the mechanism for reading readings, are divided into:
- Deformation;
- Electrical;
- Deadweight piston;
- Liquid.
Each type has its own characteristic features.
Deformation type of pressure gauges
The principle and basis of operation of deformation class devices is that pressure acts on the sensitive element of the device, which is deformed. The level of pressure is determined by the degree of deformation.
The sensing elements in tubular-spring devices are tubular springs. These products are tubes bent in a circle with a transverse oval section. The gas affects the inner surface of the tube. During this exposure, the tube is deformed and changes its shape, approaching roundness.
One end of the tube is sealed and can be moved. The second is open and fixed with holders. When the spring tube is bent, the rings are also affected, which then unbend the spring. The sealed end of the spring moves in accordance with the pressure force. This movement is transmitted to the measuring scale.
When measuring pressures up to 40 bar, circular springs are used. At higher pressures, helical or spiral springs located in the same plane are used. The error of readings when measuring pressure using this method ranges from 1 to 4%.
Diaphragm and bellows sensing elements allow you to effectively measure small values of excess and vacuum pressure.
The bellows is made according to the principle of a plumbing bellows hose. It is a thin-walled metal tube made of movable transverse rings. Depending on the material and manufacturing parameters, the bellows can be more or less rigid.
Sensitive membrane elements have the greatest variety. The accuracy class of such devices does not exceed 1.5. Such devices have a protective system. In case of overload, the membrane rests against a special protective device.
Membrane boxes are often installed in devices that measure pressure and vacuum. Pressure gauges, draft gauges and pressure gauges with membrane boxes are produced with accuracy classes 1.5; 2.5 and measurement limit up to 25 kPa.
Flat diaphragms have a small displacement of the operating point, so they are most often used to convert pressure into force. They are unstable, but well calculated.
Corrugated membranes, together with similar boxes, are used to improve static performance. The former move better, but are difficult to calculate. The latter are used much more often due to their reduced rigidity.
To measure small pressure values, devices with flaccid membranes are used.
Devices need protection from high temperatures, as it negatively affects the elasticity and sensitivity of the main working elements.
Mechanical indicating pressure gauges
Many tube spring gauges are actually direct conversion devices. This means that the pressure is converted into displacement of the sensing element and the mechanical device in contact with it.
Under the influence of pressure, the free end of the spring moves, the driver acts on the gear sector, and the gear and the indicating arrow rotate.
Spring indicating pressure gauges are manufactured with a measurement range from 0.1 to 103 MPa and have various accuracy classes. Exemplary models are produced with accuracy classes of 0.15; 0.25; 0.4. Meters of the working category of increased accuracy - 1 and 0.6. General technical workers - with accuracy classes 1.5; 2.5; 4.
Electric contact pressure gauges
Structurally, it is a modification of the indicating pressure gauge. The essence of the work is that when the arrow reaches a threshold pressure value, the network closes.
The electrical circuit is closed and the alarm is triggered when the indicating arrow reaches one of the arrows with contacts. The accuracy class of such pressure gauges is 1.5. The measuring range corresponds to standard values.
To provide signaling or for the purpose of positional control, a pressure switch marked RD is used. They measure pressures in the range from 12 to 1600 kPa. The relay is adjusted to the upper and lower activation limits according to the readings of the control device, and it has a breaking power of 10 W.
Recording models of pressure gauges
The industry produces pressure gauges with a built-in indicator reading system, which records the values on a disk chart so that the dynamics of the indicators can then be monitored. One revolution can be completed in 8, 12, 24 hours. The movement occurs due to an electric motor or clock mechanism.
The operation of a pressure gauge recorder is based on the transmission of a signal by a large diameter tubular spring, which has a traction force. It transmits movement from the sensing element to the display system. Devices marked MTS record excess pressure values.
Such devices require operator control and have accuracy classes 1; 1.5; 2.5.
Bellows sensitive elements are used in self-recording differential pressure gauges, which can additionally be equipped with an alarm device and a pneumatic transducer. Such devices measure pressure in the range from 6.3 kPa to 0.16 MPa and have accuracy classes 1; 1.5.
Deadweight type pressure gauges
Such pressure gauges are often used as a standard when calibrating other measuring instruments. Their measurement range is very wide. Depending on the design of the device, it can start with serious vacuum values and end with redundancy of up to 2500 MPa. The accuracy class reaches maximum values up to 0.0015.
The principle of operation is to hold the cylinder in the piston in a specific state while calibration weights are applied on one side and the measured pressure is applied on the other. Depending on the weight of the loads, the amount of pressure created is judged.
The main working element of the device is the measuring column. Depending on the quality of its production, the accuracy and purity of the compounds, the magnitude of the error also changes.
Functionally, a deadweight pressure gauge consists of a pressure creation device, a measuring system and weights. The device is equipped with a rotating mechanism for increasing and decreasing pressure, as well as a pressure relief valve.
Pressure gauges with an unsealed piston are widely used. They have a gap between the piston and cylinder. The container under the piston is filled with oil, which is poured into the gap under pressure and lubricates the rubbing surfaces.
Electrical gas meter
Such pressure gauges are used to convert direct or indirect gas pressure into an electrical parameter. The most common pressure gauges of this type are: strain gauge, capacitance and resistance devices. Pressure is measured in the range from 100 Pa to 1000 MPa. The devices are manufactured with accuracy classes from 0.1 to 2.5.
The operation of pressure gauges operating on the basis of the tensoresistive effect is to change the resistance value of the conductor due to deformation. They measure pressure in the range from 60 to 10 8 Pa with minimal error.
The flange mounting of the sensor and the special design of the device allows you to read pressure data in particularly aggressive environments with temperatures up to 300 °C. Used to measure pressure in systems with fast-flowing processes.
The sensitive element in such a device is a manganin wire, the resistance of which is easily measured by a balanced bridge.
The operation of capacitive pressure gauges is based on the effect of pressure on a membrane, which is a movable electrode. When the membrane moves, a change in the capacitance of the transducer follows. Characterized by significant temperature errors.
In capacitive pressure gauges, the deflection of the membrane is determined by an electrical circuit. Such devices are used in systems with rapid pressure changes.
Liquid measuring instruments
Pressure is determined by these devices by balancing the detected pressure with the pressure formed by the liquid column. In this way, you can measure small excess pressure, atmospheric pressure, vacuum level, pressure difference.
This group is represented by U-shaped pressure gauges, which consist of communicating vessels, and the pressure is determined by the liquid levels; compensation micromanometers; cup pressure gauges, which use a reservoir instead of a second tube; float, bell and ring differential pressure gauges.
In liquid measuring instruments, the working fluid is an analogue of the sensitive element.
Differential pressure gauges are usually equipped with alarms, flow meters, regulators and recording devices. Measurement range from 10 to 10 5 Pa. Depending on the liquids filling the device, the measurement limit changes.
Division by functional purpose
According to their intended purpose, the following types of pressure gauges used to measure gas pressure are distinguished:
Let's look at the features of each type.
Pressure gauges for general technical purposes
This type of pressure gauges is produced for the purpose of measuring vacuum and excess pressure values for general technical purposes. Various modifications of the devices allow them to be used in a wide variety of environments. They are used to measure pressure in production directly during technological processes.
Such pressure gauges can measure the pressure of gaseous media that are non-aggressive towards copper alloys at operating temperatures up to 150 °C. Typically, the body of the product is made of steel, and the mechanism parts are made of brass alloy.
General technical pressure gauges for low or high pressure gas are manufactured to be resistant to vibrations with a frequency in the range from 10 to 55 Hz, as well as a displacement amplitude of a maximum of 0.15 millimeters. They have several accuracy classes from 1 to 2.5.
Gas pressure gauges for general technical purposes with an electronic board on which the measurement data are displayed are gaining popularity. They are often equipped with converters, which automates technological processes. Pressure values are displayed on an electronic dial.
Group of special pressure gauges
Such devices are manufactured for a specific type of gas and the environment it creates. For systems with increased pressure, pressure gauges are made for high-pressure gas. Some gases are aggressive towards certain alloys, so it is necessary to use resistant materials when working with them.
Special pressure gauges are painted in different colors depending on the type of gas.
Propane pressure gauges are painted red, have a steel body and have the characteristics of general technical pressure gauges. The operating pressure of such devices is from 0 to 0.6 MPa. This is standard propane pressure. Operation is possible in the temperature range from - 50 to + 60 ° C. Working environment temperature up to + 150 °C. Often included with balloon reducers.
Ammonia pressure meters in cylinders and other containers are colored yellow. Units with multi-stage compression are equipped with a temperature scale. The pressure gauge components are made of materials resistant to ammonia vapors.
The acetylene pressure gauge is painted white. Manufactured as a pressure gauge for security systems from fat-free materials. Used to measure excess pressure in various acetylene distribution and generating systems. The body is made of steel, the internal components are made of brass alloy. The permissible temperature range is from - 40 to + 70 ° C.
The hydrogen pressure gauge turns dark green. The pressure gauge for other flammable gases is painted red. The measuring device for non-flammable mixtures is painted black. The oxygen pressure gauge is painted blue.
Reference devices for pressure measurement
This type of pressure gauge is designed to test, calibrate and adjust other instruments to ensure the highest possible measurement accuracy. Such devices are distinguished by a higher accuracy class in comparison with general technical ones. Working standards are divided into three categories.
Control pressure gauges, used to monitor the reliability of the readings of measuring instruments at the installation site, are also called high-precision pressure gauges. The operating measurement range is from 0-0.6 to 0-1600 bar for gaseous media.
Pressure gauges for conventional and composite gas cylinders must undergo a verification procedure at least once a year, unless other periods are specified in the documents for the device. Verification is carried out by accredited metrological organizations that have the status of legal entities. After verification, a certificate is issued and a stamp is placed.
The transmission mechanisms in the reference pressure gauges are machined at a higher gearing frequency. They are characterized by minimal friction in the pointer mechanism, as well as high sensitivity of the internal elements.
Standard pressure gauges with an accuracy class of 0.4 have a scale of 250 units, with an accuracy class of 0.15 or 0.25 they have a scale of 400 units with a division value of 1 unit. Operation of the device is possible at different temperatures depending on the housing filler. The ideal operating temperature is 20 °C.
The following article will introduce you to the specifics of refilling gas cylinders. All owners of country property not connected to a centralized gas supply should read it.
