I. Overview Pressure is one of the important parameters in industrial production. To ensure the operation of production parties, pressure must be monitored and controlled. However, it should be noted that the pressure mentioned here is actually the pressure in the physical concept. The force acting perpendicular to the unit area.
In the pressure measurement, the absolute pressure, gauge pressure, negative pressure, or vacuum degree is often used. The so-called absolute pressure refers to the total pressure exerted on the unit area of ​​the container by the measured medium, and is represented by the symbol pj. Instruments used to measure absolute pressure are called absolute pressure gauges. The average pressure generated by the air column on the ground is called atmospheric pressure and is represented by the symbol pq. The instrument used to measure atmospheric pressure is called a barometer. The difference between absolute pressure and atmospheric pressure. This is called gauge pressure and is represented by the symbol pb. That is, pb=pj-pq. When the absolute pressure value is less than the atmospheric pressure value, the gauge pressure is a negative value (ie, a negative pressure), and the absolute value of the negative pressure value, which is called a vacuum degree, is represented by a symbol pz. The instrument used to measure vacuum is called a vacuum gauge. Instruments that can measure both pressure and vacuum can be called pressure vacuum gauges.
Second, the pressure measurement and pressure gauge selection The principle of pressure measurement can be divided into liquid column type, elastic type, resistive type, capacitive type, inductive type and vibration frequency type and so on. The pressure gauge measures a wide range of pressures from ultra-vacuum, such as 133 x 10-13 Pa, to ultra-high pressure, 280 MPa. Pressure gauges can be divided into laboratory and industrial applications. There are a large variety of pressure gauges. Therefore, it is very important to select the pressure gauge well according to the measured pressure object.
1. Local pressure indication When the pressure is 2.6 kPa, diaphragm pressure gauges, bellows pressure gauges and Bourdon tube pressure gauges can be used. For near-atmospheric pressure detection, a diaphragm pressure gauge or bellows pressure gauge can be used.
2. Long-range pressure display If it is necessary to display a long-distance pressure, pneumatic or electric pressure transformers are generally used, and electrical pressure sensors can also be used. When the pressure range is 140~280MPa, high pressure pressure sensor should be used. A thermoelectric vacuum gauge can be used when measuring at high vacuum.
3. Multi-point pressure measurement When performing multi-point pressure measurement, a traveling pressure detector can be used.
If the measured pressure reaches the limit value and requires an alarm, various types of pressure gauges with alarm devices should be selected.
The correct choice of pressure gauge In addition to the above considerations, the following points need to be considered.
(1) Selection of range The instrument range is determined based on the measured pressure. For flexible pressure gauges, the maximum pressure should not exceed 3/4 of full scale when measuring steady pressure; the maximum pressure should not exceed 2/3 of full scale when measuring fluctuating pressure. The lowest measured pressure value should not be less than 1/3 of full scale.
(2) Accuracy selection Based on the maximum measurement error allowed for production, the accuracy level of the instrument is determined on the basis of economical and practical principles. General industrial pressure gauge 1.5 or 2.5 is sufficient, scientific or precision measurement using 0.5 or 0.35 precision pressure gauge or standard pressure gauge.
(3) Use environment and medium performance considerations Environmental conditions are harsh, such as high temperature, corrosion, humidity, vibration, etc., the properties of the measured medium, such as temperature, corrosion, easy crystallization, flammable, explosive, etc., In order to determine the type and model of the pressure gauge.
(4) Selection of outer dimensions of pressure gauge The surface of the pressure gauge indicated on-site shall have a general surface diameter of φ100mm. When the standard is high or the conditions of illumination are poor, the diameter of the surface is φ200-φ250mm, and the diameter of the panel-mounted pressure gauge is φ150mm. Or use a rectangular pressure gauge. See Table 2-1-13 for the specifications of commonly used elastic pressure gauges.
Third, the pressure sensor pressure sensor is an important part of the pressure detection system, by a variety of pressure sensitive components will be measured pressure signal into an easy to measure electrical signal for output, display pressure to the display instrument, or for control and alarm use .
1. Strain-type pressure sensors Strain-type pressure sensors measure changes in pressure by converting them into changes in resistance. Strain gauges are resistors made of metal conductors or semiconductors, and their resistance varies with the strain produced by the pressure.
2. Piezoelectric pressure sensor
The principle of the piezoelectric sensor is based on the piezoelectric effect of some crystalline materials. Currently widely used piezoelectric materials are quartz and barium titanate. When these crystals are mechanically deformed under pressure, they are on opposite sides. The opposite charge is generated on the device. This phenomenon is called the "piezoelectric effect."
3. Optical fiber pressure sensor Compared with traditional pressure sensors, optical fiber pressure sensors have unique advantages: they use light wave to conduct pressure information, are free of electromagnetic interference, have good electrical insulation, corrosion resistance, and no spark, they can be used in high pressure and easy to use. Measurement of pressure, flow, and liquid level in an explosive atmosphere. Its sensitive height, small size, and good flexibility make measurements possible by inserting into narrow spaces. Therefore, it is valued and developed rapidly.
Fourth, pressure transmitters need to display pressure in the control room instruments, the general choice of pressure transmitter or pressure sensor, for explosion hazardous locations, often use pneumatic pressure transmitter, explosion-proof electric type II or type III pressure transmitter For micro-pressure measurement, micro-differential pressure transmitter can be used; for viscous, easy to block, easy to crystallize and corrosive medium, diaphragm pressure transmitter with flange should be used; For strong corrosive medium measurement, 1151 series or 820 series pressure transmitters are also available.
The measuring force generated by the pressure measuring element of the pressure transmitter measurement section has a range of about 50 to 100 and a maximum of 150N. According to this requirement, the selection of the sensitive element is based on the bellows made. The structural principle is shown in Figure 2-1-12. When the measured pressure p enters the measuring chamber, the measured bellows is converted into a measuring force, which is used by the push rod on the main lever and transmitted to the pneumatic converting portion. Sensors for medium and high pressure measurements of 2.5 to 10 MPa and 10 to 60 MPa are generally made of chrome vanadium steel. The measuring principle is to use the radial force generated at the end of the end tube to act as a push rod 2. At the lower end of the main lever 3, the pneumatic switching part of the transmitter is actuated.
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