The measurement of the gas pressure in the ventilation pipe shall be carried out in the pipe section with relatively stable air flow. During the test, the static pressure, dynamic pressure and total pressure of the gas shall be measured. The orifice for measuring the total gas pressure shall face the direction of the air flow in the air duct, and the orifice for measuring the static pressure shall be perpendicular to the direction of the air flow.
When the U-shaped pressure gauge is used to measure the total pressure and static pressure, the other end shall be connected with the atmosphere (when the inclined micro pressure gauge is used to measure the pressure in the positive pressure pipe section, one end of the pipe shall be connected with the atmosphere, and when the pressure is measured in the negative pressure pipe section, the open end of the container shall be connected with the atmosphere). Therefore, the pressure read out on the pressure gauge is actually the pressure difference between the gas pressure in the air duct and the atmospheric pressure (i.e. the relative pressure of the gas). Atmospheric pressure is generally measured with an atmospheric pressure gauge. Since the total pressure is equal to the algebraic sum of dynamic pressure and static pressure, only two values can be measured, and the other value can be obtained by calculation.
Shandong ventilation duct
The measurement of gas pressure (static pressure, dynamic pressure and total pressure) is usually to take out the pressure signal with the pressure measuring pipe inserted into the air duct and read it on the pressure gauge connected to it. The commonly used instruments are pitot tube and pressure gauge.
1 pitot tube
(1) Standard pitot tube
It is a double-layer concentric tube bent into 90 °, and its open end is connected with the inner tube to measure the total pressure; A small hole is opened on the outer wall near the pipe head to measure the static pressure. The pitot tube correction coefficient processed according to the standard size is approximately equal to 1. The standard pitot tube measuring hole is very small, and the ventilation pipe is easy to be blocked by dust. Therefore, this pitot tube is only suitable for measuring in relatively clean pipes.
(2) S-type pitot tube
It is composed of two identical metal tubes in parallel. During measurement, there are two openings in opposite directions. During measurement, the opening facing the air flow is equivalent to total pressure, and the opening facing the air flow is equivalent to static pressure. Due to the influence of the probe on the air flow, there is a large error between the measured pressure and the actual value, especially the static pressure. Therefore, the S-type pitot tube must be corrected with standard pitot tube before use. The dynamic pressure correction coefficient of S-type pitot tube is generally between 0.82 and 0.85. The measuring hole of S-type pitot tube is large and it is not easy to be blocked by dust in the ventilation pipe. This pitot tube is widely used in the monitoring of dust pollution sources.