Considerations for the selection, installation, and use of industrial pressure gauges

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Considerations for selecting, installing, and using industrial pressure gauges; proper selection of pressure (differential pressure) measuring instruments. The proper selection of pressure measuring instruments involves determining the type of instrument, its range, accuracy and sensitivity, physical dimensions, as well as whether remote transmission is required and whether additional functions such as indication, recording, regulation, and alarm are needed. 　　Main criteria for selection: 1. The requirements of the manufacturing process regarding measurements, including range and accuracy. In static testing (or when changes are slow), it is specified that the maximum value of the pressure to be measured should be two-thirds of the full scale value of the pressure gauge ； In the case of pulsating (fluctuating) pressure, the maximum value of the pressure to be measured is taken as one half of the full scale value of the pressure gauge. 　　The accuracy grades of commonly used pressure measuring instruments include 0.4 grade, 1.0 grade, 1.5 grade, and 2.5 grade – a total of four grades. The choice should be made based on the accuracy requirements of the production process as well as economic considerations. The maximum allowable error of a gauge is the product of its range and the percentage corresponding to its accuracy class; if the error exceeds the accuracy requirement specified for the process, it is necessary to replace it with a pressure gauge of a higher accuracy class. 　　2. Properties of the medium under test, such as state (gas, liquid), temperature, viscosity, corrosiveness, degree of contamination, flammability, and explosibility. Such as oxygen gauges and acetylene gauges, which have an “oil-free” marking; they are corrosion-resistant pressure gauges, high-temperature resistant pressure gauges, diaphragm pressure gauges, etc., designed for use with special media. 　　3. On-site environmental conditions, such as ambient temperature, corrosion levels, vibration, and humidity. Such as shock-proof pressure gauges used in vibration environments. 　　4. Suitable for staff observation. Select instruments with different nominal diameters (external dimensions) based on the location of the measuring instrument and the lighting conditions. 　　II. Verification and calibration of pressure (differential pressure) measuring instruments Instruments must be verified and calibrated before use. Meters that are in long-term use should also be calibrated regularly, with the frequency depending on their level of usage and importance. When the instrument is equipped with a remote transmission system and secondary instruments, it should be calibrated together with those secondary instruments. 　　III. Proper installation of pressure (differential pressure) measuring instruments and related matters In fact, pressure measurement requires a measurement system to be implemented. To achieve accurate measurements, in addition to properly selecting and calibrating the instruments, it is also necessary to pay attention to the correct installation of the entire system. If only the instrument itself is accurate, its readings cannot fully represent the actual parameters of the medium being measured, because the errors of the measurement system are not equivalent to the errors of the instrument. 　　Proper installation of the system includes the correct positioning of the pressure tapping points, the proper routing of the connection ducts, and the accurate placement of the instruments. 　　1. Selection of the pressure tap location: (1) Avoid areas where the pipeline is bent, branched, or where vortices are formed in the flow. (2) When there are protruding objects in the pipeline (such as temperature sensing elements), the pressure tapping point should be located in front of them. (3) When it is necessary to take pressure readings near a control valve, if the pressure tapping point is located in front of the valve, the distance between them should be no less than 2 times the pipe diameter ； If the pressure tap is located behind it, the distance from the valve should be no less than 3 times the pipe diameter. (4) For large containers, the pressure tapping point should be located in an area where the fluid flow is steady and free of vortices. In short, the location of the pressure tapping points determined in the process flow should ensure that the desired process parameters can be measured. 　　2. Laying of the connection conduit The horizontal section of the connection conduit should have a certain slope to facilitate the removal of condensed liquid or gas. When the medium under test is a gas, the conduit should be inclined downward toward the pressure tapping point ； When the medium being measured is a liquid, the conduit should be inclined in the direction of the pressure measuring instrument ； When the parameter under measurement is a small differential pressure value, the inclination can be increased slightly further. Furthermore, if the conduit makes bends upward or downward, a device for draining condensed liquid should be installed at the lowest point or an exhaust device at the highest point, depending on the nature of the medium in the conduit, to ensure that the accuracy of measurements is not affected over an extended period due to the accumulation of condensed liquid or gas within the conduit. The condensed liquid or gas must be discharged regularly. 　　3. Precautions for the installation and use of pressure measuring instruments （1）The instrument should be installed perpendicular to the horizontal plane ； (2) The instrument measurement point and the instrument installation location are at the same horizontal level; otherwise, correction for additional height errors shall be considered ； (3) The distance between the instrument installation location and the measurement point should be as short as possible to avoid delayed indication ； (4) Ensure tight sealing; there should be no leaks, especially with flammable and explosive gas media as well as toxic and harmful media. 　　When the instrument is used under the following conditions, additional devices should be installed, but no additional errors should occur; otherwise, corrections should be considered. (1) To prevent the instrument from being damaged by the medium being measured, as well as by high viscosity or crystallization, an isolation device should be installed ； (2) To protect the instrument from the effects of sudden changes or pulsating pressures in the medium being measured, a buffer is installed. Especially during sudden increases and drops in pressure, pressure instruments are most prone to damage or failure, with the gauge tube even breaking and causing leaks ； (3) To ensure that the instruments are not affected by vibrations, pressure instruments should be equipped with vibration-damping devices and fixing mechanisms ； (4) To ensure that the instrument is not affected by the high temperature of the medium being measured, a liquid-filled elbow device should be installed ； (5) Special instruments designed for specific purposes must not be used for other purposes; measurements must not be taken without special and reliable devices, and it is absolutely forbidden to use ordinary pressure gauges to measure the pressure of special media ； (6) For newly purchased pressure measuring instruments, it is essential to conduct calibration before installation and use, in order to prevent the accuracy from being compromised due to vibrations during transportation, damage, or other factors.