1. Selection of types

instrumentThe selection of types must meet the requirements of process production. For example, whether remote transmission, automatic recording, or alarm are required; Whether the properties of the measured medium (such as temperature, viscosity, corrosiveness, degree of dirt, flammability and explosiveness) have special requirements for the instrument, and whether the on-site environmental conditions (such as humidity, temperature, magnetic field strength, vibration, etc.) have specific requirements for the instrument type. Therefore, selecting the appropriate instrument type according to process requirements is an important prerequisite for ensuring the normal operation and safe production of instruments.

For example, the spring tube of ordinary pressure gauges is mostly made of copper alloy (alloy steel is used for high-pressure gauges), while the material of the spring tube of ammonia pressure gauges is carbon steel (or stainless steel), and copper alloy is not allowed to be used. Because ammonia reacts chemically with copper and can explode, ordinary pressure gauges cannot be used for ammonia pressure measurement.

Oxygen pressure gauges can have the same structure and material as regular pressure gauges, except that oxygen pressure gauges must be oil free. Because oil entering the oxygen system can easily cause explosions. When calibrating the oxygen pressure gauge, oil should not be used as the working medium like ordinary pressure gauges, and the oxygen pressure gauge should be strictly avoided from coming into contact with oil stains during storage. If it is necessary to use existing pressure gauges with oil stains to measure oxygen pressure, they must be repeatedly cleaned with carbon tetrachloride before use and carefully inspected until there is no oil stain.

2. Determination of measurement range

In order to ensure that the elastic element can work reliably within the safe range of elastic deformation, when selecting the range of the pressure gauge, sufficient margin must be left according to the magnitude of the measured pressure and the speed of pressure change. Therefore, the upper limit value of the pressure gauge should be higher than the maximum possible pressure value in process production. According to the "Technical Regulations for Chemical Automation Design", when measuring stable pressure, the maximum working pressure should not exceed 2/3 of the measurement upper limit value; When measuring pulsating pressure, the maximum working pressure should not exceed 1/2 of the measurement upper limit value; When measuring high pressure, the maximum working pressure should not exceed 3/5 of the measurement upper limit. The minimum value of the measured pressure should generally not be less than one-third of the upper limit value of the instrument measurement. Thus ensuring a 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 parameter, these values cannot be directly used as the measurement range of the instrument. When selecting the upper limit value of the instrument scale, we should choose from the standard series specified by the country. The standard series of pressure gauge measurement range in China includes: -0.1-0.06, 0.15; 0-1,1.6,2.5,4,6,10X10 'MPa (where n is a natural integer) Can be positive or negative).

3. Selection of accuracy level

Based on the maximum allowable error of the production process and the selected instrument range, calculate the maximum allowable reference error of the instrument, and determine the accuracy of the instrument in the national prescribed accuracy level. Generally speaking, the more precise the instrument selected, the more reliable the measurement results will be. But it cannot be assumed that the higher the accuracy of the selected instrument, the better, because more precise instruments are generally more expensive, and the operation and maintenance are more complicated.

Example selection:

1. When used to measure special media such as viscosity or acidity or alkalinity, diaphragm pressure gauges, stainless steel spring tubes, stainless steel movements, stainless steel shells or rubberwood shells should be selected.

According to the different media being measured, there should be specified color codes on the pressure gauge, and the name of the special medium should be indicated. The oxygen gauge must be marked with the red "no oil" word, hydrogen gas with a dark green underline, ammonia with a yellow underline, and so on.

2. When installing against a wall, pressure gauges with edges should be selected; When directly installed on pipelines, pressure gauges without edges should be selected; When used for direct gas measurement, a pressure gauge with a safety hole on the back of the case should be selected. For the consideration of pressure measurement location and ease of observation and management, the size of the watch case diameter should be chosen.