Correct selection of instrument type according to product process requirements is an important prerequisite for ensuring normal operation and safe production of the instrument. Generally, the following three aspects should be considered when selecting the instrument type: (1) According to the requirements of the product process in industrial production. For example, whether remote transmission, automatic recording or alarm is required; (2) whether special metrics are applied to the measuring instrument according to the physical and chemical properties of the measured medium (such as corrosion, temperature, viscosity, degree of contamination, flammability and explosive properties, etc.) Requirements are selected; (3) According to the site environmental conditions (such as high temperature, electromagnetic field, vibration and on-site installation conditions), there are special requirements for the type of instrument.
1. Determination of the measurement range of the meter
In order to use the instrument reasonably and economically, the range of the meter cannot be selected too much, but in order to ensure the measurement accuracy, the minimum value of the measured pressure is not less than 1/3 of the full scale of the meter. At the same time, in order to prolong the service life of the elastic element and avoid permanent deformation of the elastic element due to excessive long-term stress, the upper limit of the pressure gauge should be higher than the measured maximum value (1/2 to 1/3 of the range), leaving margin.
2, the selection of instrument accuracy
Instrument accuracy is determined based on the maximum measurement error allowed in the process of production. It is not considered that the higher the accuracy of the selected instrument, the better. Under the premise of meeting the process requirements, the instrument with lower precision and cheaper and more durable should be selected as much as possible. Example of instrument type selection: The outlet pressure of a compressor is 25~28MPa, and the absolute error of measurement is not more than 1MPa. The process requires on-site observation, and high and low limit alarms. Try to select a pressure gauge correctly, indicating the model, accuracy and measurement range. The required conditions are: the measured pulsation pressure is 25-28 MPa, the absolute error is ≤1 MPa, and the local observation and high and low limit alarms are used. When the instrument is selected, considering the pulsating pressure has a great influence on the life of the instrument, so the upper limit of the selected instrument is: P1=Pmax * 2=28* 2=56MPa If the measuring range of the selected pressure gauge is 0-60MPa, then: 25MPa/60MPa >l/3, the minimum value of the measured pressure is not less than 1/3 of the full scale, and the lower limit also meets the requirements. In addition, according to the requirements of measurement error, the requirements for the allowable error of the instrument can be calculated as: 1/60 * 100% = 1.67%, so the instrument with the accuracy class of 1.5 can meet the error requirement.
