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I. Overview
Pressure is one of the important parameters in industrial production. In order to ensure the operation of equipment, pressure must be monitored and controlled, but it should be noted that the pressure mentioned here is actually the pressure in the physical concept, that is, the vertical action on the unit force on the area.
In pressure measurement, the division of absolute pressure, gauge pressure, negative pressure or vacuum degree is commonly used. The so-called absolute pressure refers to the total pressure of the measured medium acting on the unit area of the container, which is represented by the symbol pj. Instruments used to measure absolute pressure are called absolute pressure gauges. The average pressure created by a column of air on the ground is called atmospheric pressure, denoted by the symbol pq. The instrument used to measure atmospheric pressure is called a barometer. The difference between absolute pressure and atmospheric pressure. It 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 negative pressure), and the absolute value of this negative pressure value is called the degree of vacuum and is represented by the symbol pz. The instrument used to measure the degree of vacuum is called a vacuum gauge. An instrument that can measure both pressure and vacuum is called a pressure vacuum gauge.
2. Pressure measurement and selection of pressure gauge
The principle of pressure measurement can be divided into liquid column type, elastic type, resistance type, capacitive type, inductive type and vibration frequency type and so on. The pressure gauge can measure a wide range of pressure from ultra-vacuum such as 133 × 10-13Pa to ultra-high pressure 280MPa. The pressure gauge can be divided into laboratory type and industrial application type from the structure. There are many varieties of pressure gauges. Therefore, it is very important to select the manometer well according to the measured pressure object.
1. In situ pressure indication
When the pressure is 2.6Kpa, diaphragm pressure gauges, bellows pressure gauges and Bourdon tube pressure gauges can be used. For example, when the low pressure close to atmospheric pressure is detected, a diaphragm pressure gauge or a bellows pressure gauge can be used.
2. remote pressure display
If remote pressure display is required, pneumatic or electric pressure transformers are generally used, and electrical pressure sensors can also be used. When the pressure range is 140~280MPa, a high pressure pressure sensor should be used. Thermoelectric vacuum gauges can be used when measuring high vacuum.
3. Multipoint pressure measurement
For multi-point pressure measurement, a roving pressure detector can be used.
If the measured pressure reaches the limit value and needs to be alarmed, various pressure gauges with alarm devices should be selected.
In addition to the above considerations, the correct selection of pressure gauges also need to consider the following points.
(1) Selection of range
Determine the instrument range according to the size of the measured pressure. For elastic pressure gauges, when measuring stable pressure, the maximum pressure value should not exceed 3/4 of the full scale; when measuring fluctuating pressure, the maximum pressure value should not exceed 2/3 of the full scale. The minimum measured pressure value should not be lower than 1/3 of the full scale.
(2) Precision selection
According to the maximum measurement error allowed by production, the accuracy level of the instrument is determined in an economical and affordable way. 1.5 or 2.5 is sufficient for general industrial pressure gauges, and 0.5 or 0.35 precision pressure gauges or standard pressure gauges are used for scientific research or precision measurement.
(3) Consideration of using environment and medium performance
The environmental conditions are harsh, such as high temperature, corrosion, humidity, vibration, etc., and the performance of the measured medium, such as temperature, corrosiveness, easy crystallization, flammability, explosion, etc., to determine the type and model of the pressure gauge .
(4) Selection of external dimensions of pressure gauges
The surface diameter of the pressure gauge for on-site indication is generally φ100mm. In the case of high standards or poor lighting conditions, the surface diameter is φ200~φ250mm, and the diameter of the disk-mounted pressure gauge is φ150mm, or a rectangular pressure gauge. The specifications of commonly used elastic pressure gauges are shown in Table 2-1-13.
3. Pressure sensor
The pressure sensor is an important part of the pressure detection system. Various pressure sensitive elements convert the measured pressure signal into an easy-to-measure electrical signal for output, which displays the pressure value to the display instrument, or is used for control and alarm.
1. strain gauge pressure sensor
The strain gauge pressure sensor converts the change of pressure into the change of resistance value for measurement. The strain gauge is a resistor body made of metal conductor or semiconductor, and its resistance value changes with the strain generated by the pressure.
2. Piezoelectric pressure sensor
The principle of piezoelectric sensor is based on the piezoelectric effect of some crystal materials. At present, the widely used piezoelectric materials include quartz and barium titanate. When these crystals are mechanically deformed by pressure, the two opposite sides of the The opposite-sex charge is generated on it, which is called the "piezoelectric effect".
3. Optical fiber pressure sensor
Compared with traditional pressure sensors, optical fiber pressure sensors have unique advantages: using light waves to transmit pressure information, free from electromagnetic interference, good electrical insulation, corrosion resistance, no sparks, and can be used in high pressure, flammable and explosive environments. Measure pressure, flow, level, etc. It is highly sensitive, small in size, good in flexibility, and can be inserted into a narrow space for measurement, so it has been valued and developed rapidly.
4. Pressure transmitter
For instruments that need to display pressure in the control room, pressure transmitters or pressure sensors are generally used. For explosion-hazardous places, pneumatic pressure transmitters, explosion-proof electric type II or III pressure transmitters are often used; for micro-pressure measurement , a differential pressure transmitter can be used; for viscous, easy to block, easy to crystallize and strong corrosive media, a diaphragm pressure transmitter with a flange should be used; in atmospheric corrosive places and strong corrosive media measurement Among them, the 1151 series or 820 series pressure transmitters of Shanghai Haoying Measurement and Control Technology Co., Ltd. can also be selected.
