Differential pressure transmitter

During use, maintenance and installation are often considered. The installation process may cause the instrument and the pressure point to be not on the same horizontal plane, affecting the measurement accuracy of the instrument. In order to achieve accurate measurement, the instrument is often migrated.

The value processed by the differential pressure transmitter is the differential pressure (p), and the differential pressure is zero and does not require offset. In use, when the measured value is the lower limit value, it should be output at 4mA. If it is not zero adjustment at this time, it should be adjusted through the zero adjustment screw so that the input differential pressure signal corresponds to the 4mA output. Regarding migration, it refers to the zero migration of the instrument. The difference from the zero adjustment number is whether the lower limit value is zero. It can be seen that the lower limit value is zero, which is the zero adjustment number, while the zero offset has positive and negative offsets.

Migration curve

When installing the instrument for measurement and detection, determine the instrument range. For example, the liquid level of the fluid medium in the storage tank, the density of the medium, the height of the storage tank, etc. The height of the storage tank refers to the installation distance between the flange of the high-pressure chamber of the instrument and the flange of the low-pressure chamber. When calculating; the pressure of the flange in the high-pressure chamber is calculated, and then the pressure of the flange in the negative pressure chamber is calculated. The pressure of the capillary filling liquid on the flange bellows is considered according to its length. Calculate the pressure of the high-pressure chamber and the low-pressure chamber, and subtract the pressure of the high-pressure chamber from the pressure of the low-pressure chamber. The migration problem of zero differential pressure should be ignored, and instead, the migration problem should be considered. For example, if the calculated differential pressure is positive, it will migrate positively.

Assume that the range is 20 and the differential pressure is 5. If there is no migration, the upper limit of the instrument is 20 and the lower limit is 0. It means that when the differential pressure is zero, the instrument outputs a 4mA signal. At this time, through theoretical calculation, the calculated differential pressure is 5. If the range is set according to the upper and lower limits, there is no offset, and when the differential pressure signal is 5, 8mA is output, resulting in a high output, which should be 0mA. So it is impossible to adjust the output from 8mA to 4mA with the zero adjustment screw, which is not caused by zero drift. Therefore, to make a positive migration, set the lower limit of the instrument to 5 and the upper limit to 25. At this time, when the differential pressure is 5, the output of 4mA is considered normal.

From the above statement, it can be seen that the zero drift changes the upper and lower limits of the instrument parameter setting, but the measurement range remains unchanged. Calibration is a job that cannot be ignored during use. Due to the influence of the working environment, the zero drift of the instrument must be calibrated. Before calibration, it is understood that the entire zero adjustment has almost no effect on the full scale, but the full scale will have an impact on the zero point, and the zero point is about 1/5 of the full scale. So to compensate for the impact, overshoot 25%. Before calibration, adjust the damping to zero, adjust the zero, and then adjust to the full scale. Generally, you need to prepare a communicator, such as Bt200 communicator, 375 communicator or 475 communicator, to see what protocol the differential pressure transmitter supports. The former is the Brown protocol communicator, and the latter is the Hart protocol communicator. Secondly, it is best to prepare a multimeter to detect the output signal at any time.

If the range is 0-100Kpa, the output is 4mA when the input is 0, and the output is 19.7mA when the input is 100. At this time, adjust the range and adjust the output value according to the calculation results. 19.7(20-19.7)x 1.25=20.075 mA, so adjust the measurement range to 20.075 mA. I found out why 0.075 mA is 20 mA more. Don't panic, its increase is only 1/5 of the increase in amplitude. Therefore, as long as the zero point is adjusted to 4 mA, the measurement range is correct. After the zero point and span are solved, check whether the middle scale is accurate. For example, if the output of 25, 50 and 75 is accurate, fine-tuning is required. Note: After the calibration is completed, remember to restore the damping.