The pressure transmitter
is a kind of on-site instrument which directly contacts the measured medium. It is often used in high temperature, low temperature, corrosion, vibration, impact, and other environments. Therefore, in the selection of pressure transmitter, besides the measurement range and accuracy of general instruments, the working environment and the medium under test are also important indicators. Generally speaking, the selection of the pressure transmitter is mainly based on:
General pressure transmitters have a certain range of adjustment, especially for different pressure transmitters. It is better to set the range to 1/3~2/3 to guarantee accuracy.
To determine the maximum pressure in the system, it is necessary to select a transmitter with a pressure ranged about 1.5 times larger than the maximum. This is because in many systems, especially in water pressure measurement
and processing, there are peak values and continuous irregular fluctuations, which can destroy the pressure sensor. Although we can use a buffer to reduce the pressure burr, it will inevitably reduce the response speed of the sensor. Therefore, the pressure range, accuracy, and stability should be fully considered when choosing a transmitter.
Accuracy is the key performance index of any pressure measurement equipment
, and it is also an important factor to be considered in equipment selection and maintenance. According to the range of pressure measurement and the maximum absolute error allowed technologically, first, the measurement range is selected, and then the maximum absolute error allowed is calculated to find the maximum reference error allowed by the instrument. In the end, remove the ± and %. The numerical value is accuracy. If there is no such level, we will depend on the accuracy level. Pressure transmitters have different precision levels in different countries.
For example, in China and the United States or other countries, the accuracy of the target is the best part of the linearity of the sensor, that is, the accuracy between 10% and 90% of the measurement range as we usually call it, while the accuracy of the European standard is the worst part of linearity, that is, the accuracy between 0-10% and 90-100% of the measurement inverse. If the precision of the European standard is 1%, the precision of Chinese standard is 0.5%.
At present, the precision index of most pressure differential pressure transmitters
is 0.1% or 0.075%, and the precision index is 0.05% or 0.025%. But the accuracy mentioned above is only provided by the manufacturer of the differential pressure transmitter.
In the practical application of the user's production, if expressed by the overall performance error, only a very small part of the overall performance error in field application may reach the target provided by the manufacturer. In most cases, the two are far from each other. The main factors affecting accuracy in the practical application include range ratio, temperature effect on zero points, temperature effect on range, static pressure effect on zero point and static pressure effect on range. The pressure transmitter only needs to consider the effects of temperature and range ratio, while the differential pressure transmitter needs to consider all the effects.
In addition, the factors affecting the accuracy include a power supply, vibration, installation location, etc., but these factors are relatively small and need to be considered according to actual needs.
If a relatively clean fluid is measured, a standard pressure transmitter can be used directly.
If the measured medium is high viscosity, easy to crystallize and corrode strongly, then the isolated transmitter must be selected. The corrosion of its medium to the metal of the bellows should be taken into account when selecting. The material of bellows must be selected properly, otherwise, the outer bellows will be corroded and the flange will be corroded soon after use, resulting in equipment and personal accidents. The material of diaphragm box of the transmitter includes ordinary stainless steel, 304 stainless steel, 316L stainless steel, tantalum diaphragm box and so on.
In general, the material of the contact medium of pressure transmitter
is 316 stainless steel. If the measuring medium is not corrosive it, then basically all pressure transmitters are suitable for the measurement of medium pressure.
If the measuring medium is corrosive to 316 stainless steel, chemical seal should be used, which can not only measure the pressure of the medium, but also effectively prevent the contact between the medium and the liquid receiving part of the pressure transmitter, thus protecting the pressure transmitter and prolonging the life of the pressure transmitter.
Because the signal of the pressure transmitter
is converted through the part of the feeder circuit, the medium temperature of the pressure transmitter is - 30 to + 100 degrees in general. If the temperature is too high, condensation bending is usually used to cool the medium.
Because of various acquisition needs, there are many kinds of output signals of pressure transmitters on the market, mainly 4-20 mA, 0-20 mA, 0-10 V, 0-5 V and so on. However, 420mA and 0-10V are commonly used. Of the output signals listed above, only 4-20mA is a two-wire system with grounding or shielding lines, while the others are three-wire systems.
In practice, some applications (liquid level measurement) need to migrate the measurement range of the transmitter and calculate the measurement range and migration amount according to the site installation position. At present, the intelligent transmitter has been quite popular. Its characteristics are high accuracy, wide adjustable range, convenient adjustment, good stability, and should be considered more in the selection. In addition, the process connection interface of the differential pressure transmitter
and the supply voltage of the pressure transmitter should also be considered explosion-proof and protection level if used on special occasions.