Selection of types The selection of instrument types mu […]
The selection of instrument types must meet the requirements of process production. For example, whether remote transmission is required,
Automatic recording or alarm; whether the properties of the measured medium (such as the temperature of the measured medium, viscosity, corrosiveness, degree of contamination, whether it is flammable and explosive, etc.) have special requirements for the instrument, and whether the on-site environmental conditions (such as humidity, temperature, magnetic field strength, vibration, etc.) requirements for instrument types, etc. Therefore, the correct selection of the instrument type according to the process requirements is an important prerequisite for ensuring the normal operation of the instrument and safe production.
For example, the spring tubes of ordinary pressure gauges are mostly made of copper alloys (alloy steels are used for high pressure), while the spring tubes of ammonia pressure gauges are all made of carbon steel (or stainless steel), and copper alloys are not allowed. Because ammonia reacts chemically with copper, it will explode, so ordinary pressure gauges cannot be used for ammonia pressure measurement.
The oxygen pressure gauge can be exactly the same as the ordinary pressure gauge in terms of structure and material, but the oxygen pressure gauge must be oil-free. Because oil enters the oxygen system, it is easy to cause an explosion. When the used oxygen pressure gauge is calibrated, oil cannot be used as the working medium like ordinary pressure gauges, and the oxygen pressure gauge should be strictly avoided to contact with oil during storage. If the existing pressure gauge with oil must be used to measure the oxygen pressure, it must be repeatedly cleaned with carbon tetrachloride before use, and carefully checked until there is no oil pollution.
In order to ensure that the elastic element can work reliably within the safe range of elastic deformation, when selecting the pressure gauge range, it is necessary to leave enough room according to the size of the measured pressure and the speed of the pressure change. Therefore, the upper limit of the pressure gauge It should be higher than the maximum pressure value possible in process production. According to the "Technical Regulations of Chemical Control Design", when measuring stable pressure, the maximum working pressure should not exceed 2/3 of the upper limit of measurement; when measuring pulsating pressure, the maximum working pressure should not exceed 1/2 of the upper limit of measurement; When measuring high pressure, the maximum working pressure should not exceed 3/5 of the upper limit of the measurement. Generally, the minimum value of the measured pressure should not be lower than 1/3 of the upper limit value of the instrument. So as to ensure the linear relationship between the output of the instrument and the input
After calculating the upper and lower limits of the instrument according to the maximum and minimum values of the measured parameters, the value cannot be directly used as the measuring range of the instrument. When we choose the upper limit value of the scale of the instrument, it should be selected from the standard series stipulated by the state. The standard series of pressure gauges in China are: -0.1-0.06, 0.15; 0-1, 1.6, 2.5, 4, 6, 10X10" MPa (where n is a natural integer. It can be positive or negative).
According to the maximum absolute error allowed by the process production and the maximum range of the selected instrument, calculate the maximum allowed reference error of the instrument, and determine the accuracy of the instrument in the accuracy level specified by the state. Generally speaking, the more precise the instrument selected, the more accurate and reliable the measurement results will be. However, it cannot be considered that the higher the accuracy of the selected instrument, the better, because the more precise the instrument is generally the more expensive, and the more troublesome it is to operate and maintain.
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