To meet different usage needs and environments, when deciding to use pressure gauges as pressure measuring instruments, it is necessary to strictly select the appropriate pressure gauge according to actual production needs in order to ensure that it can meet the requirements of production safety and process control.
The selection of pressure gauges should be based on the production requirements of the process, and specific analysis should be done according to the specific situation. Under the premise of meeting the process requirements, comprehensive consideration should be given to the principle of economy, and generally, the following issues should be considered:
The selection of the instrument type must meet the requirements of process production. For example, whether remote transmission, automatic recording, or alarm is required; whether the nature of the measured medium (such as the temperature, viscosity, corrosion, dirtiness, inflammability, and explosiveness of the measured medium) poses special requirements for the instrument; whether the environmental conditions on site (such as humidity, temperature, magnetic field strength, and vibration) require specific types of instruments. Therefore, correctly selecting the instrument type based on process requirements is an important prerequisite for ensuring normal instrument operation and safe production.
For example, the spring tube of a normal pressure gauge is mostly made of copper alloy (alloy steel for high pressure), while the materials of the spring tube of an ammonia pressure gauge are made of carbon steel (or stainless steel) and cannot be made of copper alloy because ammonia reacts chemically with copper and can cause an explosion. Therefore, normal pressure gauges cannot be used for ammonia pressure measurement.
The structure and materials of oxygen pressure gauges and normal pressure gauges can be exactly the same, but the use of oil should be prohibited for oxygen pressure gauges because the entry of oil into the oxygen system can cause explosions. When calibrating oxygen pressure gauges, oil cannot be used as the working medium for normal pressure gauges, and oxygen pressure gauges must be strictly kept away from oil contamination during storage.
If it is necessary to use pressure gauges with oil contamination to measure oxygen pressure, they must be repeatedly cleaned with carbon tetrachloride and carefully checked until no oil contamination is found before use.
In order to ensure that the elastic element can work reliably within the safe range of elastic deformation, a sufficient margin must be left when selecting the range of the pressure gauge according to the size of the measured pressure and the speed of pressure changes. Therefore, the upper limit value of the pressure gauge should be higher than the possible maximum pressure value in the production process.
According to the "Technical Design Regulations for Chemical Industry Automatic Control", when measuring stable pressure, the maximum working pressure should not exceed 2/3 of the measuring upper limit value; when measuring pulsating pressure, the maximum working pressure should not exceed 1/2 of the measuring upper limit value; when measuring high pressure, the maximum working pressure should not exceed 3/5 of the measuring upper limit value.
The minimum value of the measured pressure should not be less than 1/3 of the instrument measuring upper limit value, thereby ensuring the linear relationship between the output and input of the instrument.
After calculating the upper and lower limits of the instrument based on the maximum and minimum values of the measured parameters, this value cannot be directly used as the measurement range of the instrument. When selecting the upper limit value of the instrument's scale, it should be selected from the national standard series.
According to the maximum absolute error allowed by the production process and the selected instrument range, the maximum allowable reference error of the instrument is calculated to determine the accuracy of the instrument in the accuracy level stipulated by the country.
Generally, the more precise the selected instrument, the more accurate and reliable the measurement results. However, it cannot be assumed that the higher the accuracy level of the selected instrument, the better, because more precise instruments are generally more expensive and require more operation and maintenance.