Views: 0 Author: Site Editor Publish Time: 2022-08-12 Origin: Site
With the advancement of instrument technology, differential pressure transmitters of various brands are developing in the direction of high precision and intelligence. However, the advancement of technology has brought a misunderstanding to designers: when people choose differential pressure transmitters, they often only pay attention to the accuracy of the instrument itself, while ignoring other factors that affect the measurement accuracy.
In fact, what the user really needs is a true value that correctly reflects the measured value—measurement accuracy, not the accuracy of the meter itself. There are many factors that affect the measurement accuracy of a transmitter, some of which are also related to environmental conditions and installation methods. In the design, the accuracy of the product itself cannot guarantee the actual measurement effect. Designers should not only consider the influence of the actual environment on the accuracy of the product when choosing, but also pay attention to minimize the influence on the accuracy of the transmitter during the installation process.
1. Influence of turndown ratio
The range ratio of the differential pressure transmitter refers to the ratio of the maximum value to the minimum value that the transmitter can measure when the accuracy requirements are met. In general, the larger the turndown ratio, the lower the measurement accuracy. The larger the turndown ratio of the pressure transmitter, there are many advantages. Once this instrument is calibrated, it can be used in many different situations. However, in practice, if the turndown ratio is adjusted too high, it will bring a lot of measurement stability problems.
Most differential pressure transmitters are in the 10:1 turndown range, the accuracy is still the reference accuracy and is not affected by the turndown ratio. However, there are still a considerable number of differential pressure transmitters whose guaranteed reference accuracy has a turndown ratio of less than 10:1, even 3:1 and 2:1; in addition, different range models of the same type of product may lead to guaranteed parameter accuracy. There is a large difference in the turndown ratio of , and the user should pay special attention to this when calculating the accuracy affected by the turndown ratio.
2. The influence of temperature
In the process application of a differential pressure transmitter, the process temperature and ambient temperature will vary greatly, which must be different from the reference conditions for the transmitter test, which also means that the performance will be different from the introduced reference accuracy. The effect of temperature on the differential pressure transmitter is very small when it is close to a large range (ie, small turndown ratio), but it is very large when it is close to the minimum range (ie, large turndown ratio), and sometimes even exceeds our imagination.
According to the comprehensive influence formula of temperature on zero point accuracy and span accuracy provided in some product manuals, if the range ratio is larger than 100, the influence of nearly 10% can be obtained. Some modern smart pressure transmitter products add internal temperature sensors to correct for thermal effects caused by changes in ambient temperature.
3. The influence of static pressure
Similar to the effects of temperature and span, static pressure can have a significant impact on transmitter zero and span accuracy. For example, capacitive differential pressure transmitters are almost always affected by static pressure during the operation of a production process system. When the static pressure under working conditions exceeds a certain value, the error affected by the static pressure will exceed the accuracy of the transmitter itself. When calculating the channel accuracy of the measurement channel, if the static pressure influence error is added, the channel accuracy requirements may not be met. Therefore, during the calibration process of the capacitive differential pressure transmitter, the static pressure effect must be corrected.
4. The influence of installation inclination on the transmitter
The installation inclination refers to the angle between the center axis of the transmitter and the vertical line after the transmitter is installed on site.
During the differential pressure measurement, the detection element senses the differential pressure between the two sides by detecting the deformation and displacement of the central measuring diaphragm. Since the central measuring chamber is filled with filled silicone oil, when the differential pressure transmitter is tilted in the direction perpendicular to the plane of the measuring diaphragm, the filled silicone oil will exert pressure on the measuring diaphragm, causing the diaphragm to deform, resulting in a change in the output value. When the differential pressure transmitter is tilted in a direction parallel to the plane of the measuring diaphragm, the injection of silicone oil will not compress the measuring diaphragm, so it will not have an effect.
Transmitters of different ranges are affected differently by tilt. The smaller the range, the greater the impact; when the differential pressure transmitter is tilted back and forth, its output linearity and output value have no significant change, which does not affect its normal use; when the differential pressure transmitter is tilted to the left or right, its output value will drift in one direction according to the inclination. The larger the angle, the greater the effect, but the linearity does not change significantly.
5. Influence of height difference between measurement point and installation point
Due to the needs of the overall design of the industrial site, the differential pressure transmitter cannot be installed near the measuring point at the use site, but is introduced into a relatively concentrated place for maintenance and management through the pressure guiding pipe, which will cause the difference between the measuring point and the installation point. Height difference, the pressure medium in the pressure guiding pipe will generate additional pressure due to the height difference, resulting in impact and additional errors. In the application of the process industry, because the density of the medium is often very large, even if the height difference is small, the additional error introduced is also very large.