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technical article
Knowledge about pressure gauges
2011-6-25 16:40:12

The pressure concept of a pressure gauge actually refers to the pressure in physics, that is, the magnitude of the pressure on a unit area.
Absolute pressure: The pressure is higher than the absolute zero based on the absolute zero.
Positive pressure: A pressure higher than atmospheric pressure based on atmospheric pressure.
Negative pressure (vacuum): A pressure lower than atmospheric pressure based on atmospheric pressure.
Differential pressure: The difference between two pressures.
Gauge pressure: A pressure that is greater or less than atmospheric pressure based on atmospheric pressure.
Pressure gauge : A gauge that is based on atmospheric pressure and is used to measure less than or greater than atmospheric pressure.
There are two ways to express the vacuum pressure and classified pressure: one is the pressure expressed using absolute vacuum as the reference, which is called absolute pressure; the other is the pressure expressed using atmospheric pressure as the reference, which is called relative pressure. Since the pressure measured by most manometers is relative pressure, relative pressure is also called gauge pressure. When the absolute pressure is less than the atmospheric pressure, it can be expressed by the value that the absolute pressure in the container is less than one atmosphere. It is called "vacuum degree". Their relationship is as follows: absolute pressure = atmospheric pressure + relative pressure and vacuum degree = atmospheric pressure-absolute pressure. The legal pressure unit in China is Pa (N / ㎡), which is called Pascal, referred to as Pa. Because this unit is too small, it is often used 106 times the unit MPa (megapascal)
Application of pressure gauges : In the process of industrial process control and technical measurement, mechanical pressure gauges have become more and more widely used due to their high mechanical strength and ease of production.
The elastic sensitive element in the mechanical pressure gauge is elastically deformed as the pressure changes. The mechanical pressure gauge adopts spring tube (Bourdon tube), diaphragm, diaphragm box and bellows and other sensitive components and is classified according to this. The measured pressure is generally regarded as relative pressure. Generally, the relative point is selected as atmospheric pressure. The elastic deformation of the elastic element under the action of the medium pressure is enlarged by the gear transmission mechanism of the pressure gauge, and the pressure gauge will display the relative value (high or low) relative to the atmospheric pressure.
The pressure value in the measurement range is displayed by the pointer, and the indication range of the scale is generally made 270 degrees.
Classification of pressure gauges: According to their measurement accuracy, pressure gauges can be divided into precision pressure gauges and general pressure gauges. The accuracy levels of precision pressure gauges are 0.1, 0.16, 0.25, and 0.4; the accuracy levels of general pressure gauges are 1.0, 1.6, 2.5, and 4.0.
Pressure gauges are divided into general pressure gauges , absolute pressure gauges , and differential pressure gauges according to the basis of their indicated pressure. General pressure gauges are based on atmospheric pressure; absolute pressure gauges are based on absolute zero pressure; differential pressure gauges measure the difference between two measured pressures.
Pressure gauges are divided into vacuum gauges , pressure vacuum gauges , micro pressure gauges , low pressure gauges , medium pressure gauges, and high pressure gauges according to their measurement ranges. Vacuum gauges are used to measure pressure values less than atmospheric pressure; pressure vacuum gauges are used to measure pressure values less than and greater than atmospheric pressure; micro-pressure gauges are used to measure pressure values less than 60,000 Pa; low pressure gauges are used to measure 0 to 6 MPa pressure ; Medium pressure gauge is used to measure the pressure value of 10 ~ 60MPa; High pressure gauge is used to measure the pressure value above 100MPa.
The housing of the shock-resistant pressure gauge is made of a fully sealed structure, and the housing is filled with damping oil. Due to its damping effect, it can be used in measuring places where the working environment is vibrating or the medium pressure (load) is pulsating.
The pressure gauge with electric contact control switch can realize alarm or control function.
The pressure gauge with remote transmission mechanism can provide electrical signals (such as resistance signals or standard DC current signals) required in industrial engineering.
The isolator (chemical seal) used in the diaphragm meter can isolate the measured medium from the instrument through the isolation diaphragm in order to measure the pressure of strong corrosion, high temperature, and easily crystallized media.
Elastic element of pressure gauge The elastic sensitive element in the mechanical pressure gauge is elastically deformed as the pressure changes. Mechanical pressure gauges use sensitive components such as spring tubes (bourdon tubes), diaphragms, capsules and bellows and are classified according to this. Sensitive components are usually made of copper alloy, stainless steel or special materials.
Elastic sensor: spring tube (Bourdon tube) is divided into C tube, coil spring tube, spiral tube and other types. Generally, cold-work hardened material blank tubes are used, which have a high degree of plasticity in the annealed state, and obtains high elasticity and strength after cold working hardening and qualitative treatment. Under the pressure of the inner cavity of the spring tube, the elastic characteristics of the spring tube can be used to conveniently convert the pressure into the elastic displacement of the free end of the spring tube. The measuring range of the spring tube is generally 0.1MPa ~ 250MPa. The diaphragm sensitive element is a circular diaphragm with a wave. The diaphragm itself is located between the two flanges, or it is welded on the flange or its edge is sandwiched between the two. Between flanges. The diaphragm is under pressure from the measuring medium. In this way, the small bending deformation of the diaphragm can be used to indirectly measure the pressure of the medium. The amount of pressure is indicated by the pointer. Compared with Bourdon tube, the diaphragm has larger transmission force. Because the diaphragm itself is fixed around the edges, its vibration resistance is good. The diaphragm pressure gauge can achieve high over-pressure protection (for example, the diaphragm is attached to the upper flange). The diaphragm can also be added with a protective coating to improve corrosion resistance. Using open flanges, flushing, openings and other measures, diaphragm pressure gauges can be used to measure very viscous, unclean and crystalline media. The pressure range of the diaphragm pressure gauge is from 1600Pa to 2.5 MPa. The sensitive element of the capsule is composed of two pairs of diaphragms with a circular wave cross-section. The pressure of the measuring medium acts on the inside of the capsule cavity, and the resulting deformation can be used to indirectly measure the pressure of the medium. The magnitude of the pressure value is displayed by the pointer. Capsule pressure gauge is generally used to measure the micro pressure of gas, and has a certain degree of over-pressure protection. The stacking of several capsule sensitive elements will produce a large transmission force to measure extremely small pressures. The pressure measurement range of the capsule pressure gauge is from 250Pa to 60,000Pa. The pressure gauge's closing machine and working principle The open spring tube pressure gauge is composed of the case part, pointer, dial part, spring bend pipe, transmission mechanism part and pipe joints It is composed of five main parts. When the spring tube is subjected to the pressure of the medium, its movable end is stretched outward, and the pointer is driven to rotate by the transmission mechanism, and the pressure of the medium is indicated on the dial.
The number of amps in the circle below the pressure gauge scale is the accuracy grade mark of the pressure gauge. Indicates that the table's basic error does not exceed a percentage of full scale. The smaller the number, the higher the accuracy.
The selection principle of the pressure gauge The full scale scale of the pressure gauge is 1.5-3.0 times the maximum working pressure of the container or container system, preferably 2 times.
The diameter of the pressure gauge is not less than 100mm. It is best to have a red line scale indicating the highest working pressure of the container system on the dial.
The installation setting of the pressure gauge (1) should be set at a place where it is easy to observe and clean, and should not be affected by adverse factors such as radiant heat, freezing or vibration;
(2) The pressure gauge should be installed vertically. When the installation position is high, it can be tilted slightly forward for easy observation, but the tilt angle should not exceed 30 degrees;
(3) In one case, a buffer elbow should be installed in front of the pressure gauge to relieve the compressed air from rushing directly into the spring elbow, and also to condense water;
(4) A three-way plug or needle valve should be installed between the pressure gauge and the buffer elbow in order to replace and verify the pressure gauge.
4. Requirements for the use of pressure gauges Before the pressure gauges are installed, they shall be verified by a nationally recognized metrology department and a calibration certificate shall be issued. The pressure gauge in use should be calibrated at least once a year, and it must be replaced immediately when one of the problems such as the pressure indication is not working, the pointer is loose, the scale is not clear, the dial glass is broken, the pointer does not return to zero after pressure relief, and the seal is damaged.
Adjustment method for pressure indication value deviation Single coil spring tube pressure gauge is a widely used pressure instrument in industry. During the verification process, the problem of indication value deviation is common. The specific adjustment method is as follows:
1. The outliers are the same at each test point. After boosting, the pointer can be reinstalled at the first verification point other than zero to calibrate the indication.
2. The difference is a linear error. When the error gradually increases, move the indication adjustment screw outward to increase the arm length; otherwise, move it inward to decrease the arm length.
3. The displayed value difference is fast first (positive error) and then slow (negative error). Turn the movement counterclockwise to increase the included angle between the drawbar and the sector gear; otherwise, turn the movement clockwise to reduce the included angle. After adjustment. The error is a linear error, and then you can move the indication adjustment screw.
4. The indication is out of tolerance around half of the pressure. After boosting, the pointer can be reinstalled in the middle position. If the error cannot be eliminated, the angle between the lever and the sector gear should be adjusted for comprehensive adjustment.
5. Only one or two points are out of tolerance. Check the fit of the movement near this point. When the difference is positive, there are dirt and burrs on the gear meshing; when the difference is negative, the teeth are worn and injured, and they should be repaired or replaced.
6. Adjust the pressure gauge many times, but the back and forth difference is still too bad, and the pointer does not return to the zero position, the spring tube may be deformed and needs to be replaced again.
After the adjustment is completed, assemble all parts of the instrument, and perform the verification again according to the verification procedures.
Axial, axial mounting fixture, axial panel 6 Choice of dial scale of pressure gauge 7 Pressure gauge flange with flange diaphragm, capillary: direct type, remote type. Used for: power plants, petroleum, chemical plants, etc. 8 Accessories: siphons, throttles, dampers, overpressure protectors, joints 9 Oil and water are prohibited. Please indicate "No oil treatment" when measuring oxygen. Oil treatment "and corrosion resistance; please mark" No oil and water treatment "when measuring hydrogen.