1、 Overview

Pressure is one of the important parameters in industrial production. In order to ensure the operation of the production party, it is necessary to monitor and control pressure. However, it should be noted that the pressure referred to here is actually the pressure in the physical concept, which is the force acting vertically on a unit area.

In pressure measurement, absolute pressure, gauge pressure, negative pressure, or vacuum degree are commonly used. The so-called absolute pressure refers to the total pressure exerted by the measured medium on a unit area of the container, represented by the symbol pj. The instrument used to measure absolute pressure is called an absolute pressure gauge. The average pressure generated by the air column on the ground is called atmospheric pressure, represented 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 negative (i.e. negative pressure), and the absolute value of this negative pressure value is called vacuum degree, represented by the symbol pz. The instrument used to measure vacuum degree is called a vacuum gauge. An instrument that can measure both pressure and vacuum is called a pressure vacuum gauge.

2、 Measurement of Pressure and Selection of Pressure Gauges

The principle of pressure measurement can be divided into liquid column type, elastic type, resistance type, capacitance type, inductance type, and vibration frequency type, etc. The pressure gauge can measure a wide range of pressure, ranging from ultra vacuum such as 133 × 10-13Pa to ultra-high pressure of 280MPa. Pressure gauges can be divided into laboratory type and industrial application type structurally. There are various types of pressure gauges. Therefore, it is very important to choose a pressure gauge well based on the tested 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. When detecting low pressure close to atmospheric pressure, 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 sensing receiver should be used. When measuring under high vacuum, a thermoelectric vacuum gauge can be used.

3. Multi point pressure measurement

When conducting multi-point pressure measurement, a touring pressure detector can be used.

If the measured pressure reaches the limit value and requires an alarm, various pressure gauges with accompanying alarm devices should be selected.

In addition to the above considerations, the correct selection of a pressure gauge also requires consideration of the following points.

(1) The selection of range is determined based on the magnitude of the measured pressure to determine the instrument range. For elastic pressure gauges, when measuring stable pressure, the maximum pressure value should not exceed 3/4 of the full range; When measuring fluctuating pressure, the maximum pressure value should not exceed 2/3 of the full range. The minimum measured pressure value should not be less than 1/3 of the full range.

(2) The accuracy level of the instrument is determined based on the principle of economy and affordability, based on the maximum measurement error allowed by production. A pressure gauge of grade 1.5 or 2.5 is sufficient for general industrial use, while a precision pressure gauge or standard pressure gauge of grade 0.5 or 0.35 is used for scientific research or precision measurement.

(3) Considering the harsh environmental conditions, such as high temperature, corrosion, humidity, vibration, etc., and the performance of the tested medium, such as temperature, corrosiveness, easy crystallization, flammability, explosion, etc., to determine the type and model of pressure gauge.

(4) The selection of external dimensions for pressure gauges is generally indicated on site. The surface diameter of the pressure gauge is generally 100mm. In situations with high standards or poor lighting conditions, a surface diameter of 200mm~250mm is used. The diameter of the disc mounted pressure gauge is 150mm, or a rectangular pressure gauge is used. The commonly used specifications of elastic pressure gauges are shown in Table 2-13.

3、 Pressure sensor

Pressure sensors are an important component of pressure detection systems, which are converted into easily measurable electrical signals by various pressure sensitive components for output, displaying pressure values to display instruments, or for control and alarm purposes.

1. Strain type pressure sensor

Strain gauge pressure sensors convert changes in pressure into changes in resistance values for measurement. Strain gauges are resistance bodies made of metal conductors or semiconductors, and their resistance values change with the strain generated by pressure.

2. Piezoelectric pressure sensor

The principle of piezoelectric sensors is based on the piezoelectric effect of certain crystal materials. Currently, widely used piezoelectric materials include quartz and barium titanate. When these crystals undergo mechanical deformation under pressure, opposite charges are generated on their opposite sides, which 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 waves to transmit pressure information, are not affected by electromagnetic interference, have good electrical insulation, are corrosion-resistant, have no electric sparks, and can measure pressure, flow rate, liquid level, etc. in high-pressure, flammable and explosive environments. It is highly sensitive, small in size, flexible, and can be inserted into narrow spaces for measurement, thus gaining attention and rapid development.

4、 Pressure transmitter

Instruments that need to display pressure in the control room generally use pressure transmitters or pressure sensors. For explosive hazardous areas, pneumatic pressure transmitters, explosion-proof electric Type II or III pressure transmitters are often selected; For the measurement of micro pressure, micro differential pressure transmitters can be used; For media that are viscous, easy to block, easy to crystallize, and highly corrosive, it is advisable to choose a diaphragm type pressure transmitter with a flange; In atmospheric corrosive environments and measurement of highly corrosive media, 1151 series or 820 series pressure transmitters can also be used.

The measurement force generated by the pressure sensitive element in the measurement section of the pressure transmitter ranges from approximately 50 to 100, with a maximum of 150N. According to this requirement, the selection of sensitive components is based on the corrugated pipe made of, and its structural principle is shown in Figure 2-12. When the measured pressure p enters the measurement chamber, it is converted into a measuring force through the measuring bellows, which is used on the main lever through a push rod and transmitted to the pneumatic conversion part. The sensitive components for measuring medium and high pressure (2.5-10MPa, 10-60MPa) are generally made of chromium vanadium steel wrapped end tubes. Its measurement principle is to use the radial force generated at the end of the wrapped end tube to act on the lower end of the main lever 3 through the push rod 2, driving the pneumatic conversion part of the transmitter to operate.