1. The influence of small static pressure error
Differential pressure transmitters are usually calibrated when the low pressure end is open to the atmosphere, but almost all occasions where differential pressure transmitters are used include a certain value acting on the two ports of the transmitter's high and low pressure chambers. The static pressure. The main reason for the change of output of the differential pressure transmitter under static pressure is the mechanical deformation of the pressure taking part. Although the deformation of the transmitter casing and the silicone oil filled in the bellows is very small, the sensitivity of the instrument is too high , So the change is still significant. The following two methods are used to measure the static pressure error:
1) Single static pressure test
The nominal pressure is applied to the high and low pressure chambers respectively, and the basic error and return error are measured after the pressure is relieved. The value should meet the requirements of the appraisal regulations.
2) Static pressure test
At the same time, the rated pressure is applied to the high and low pressure chambers, and the change in the lower limit value of the output after the pressure is relieved shall meet the requirements of the verification regulations.
2. Reduce the impact of zero changes
Among the differential pressure transmitters, the installation position of the differential pressure transmitter has the greatest impact on the zero output, especially the micro differential pressure transmitter because of its small measuring range, the weight of the sensor element in the transmitter will also affect The output of the differential pressure transmitter can be obtained, so it is normal that the zero position change occurs when the differential pressure transmitter is installed. The way to eliminate the zero change is to make the pressure-sensitive element of the transmitter perpendicular to the direction of gravity during installation. If the installation conditions are restricted, the zero position of the transmitter should be adjusted to the standard value after installation and fixation. At the same time, the negative pressure chamber of the differential pressure transmitter should be installed with a water collector during installation, and the sewage should be drained frequently to prevent water accumulation from affecting the accuracy.
3. Use the migration principle to improve the liquid level measurement method
In order to be able to correctly indicate the height of the liquid level, the differential pressure transmitter must do some technical treatment-migration. Migration is divided into no migration, negative migration and positive migration. The relationship between measuring range, measuring range and migration is: measuring range = migration of measuring range. E.g. a measuring range is 30kpa, no migration, the measuring range is equal to 30kpa; b measuring range is 30kpa, migration is -30kpa, measuring range is -30kpa~0kpa; c measuring range is 30kpa, migration is 30kpa, measuring range is 30kpa~ 60kpa, it can be seen that the output and input characteristic curves of positive and negative migration are the characteristic curves without migration along the abscissa representing the input. The positive migration moves in the positive direction, the negative migration moves in the negative direction, and the distance moved is the migration amount. The essence of positive and negative migration is to adjust the differential pressure transmitter to change the upper and lower limits of the range, while the size of the range remains unchanged. From the perspective of the low pressure chamber, it can also be simply understood as positive migration, for example, the additional pressure P=ρgh (p is the additional pressure, Pa; ρ is the local acceleration due to the increase in the height difference of the pressure transmission medium in the low pressure chamber) , M/s²; h is the height difference of the pressure transmission medium, m) migration volume, and negative migration is like reducing the migration volume of ρgh in a low pressure chamber. Grasp the principle of measuring liquid level migration by differential pressure transmitter, and can make corresponding improvements to the liquid level measurement method according to the process conditions of the production equipment, the use conditions of the instrument and the surrounding environment, and the field instruments can be processed timely and accurately. There is a malfunction.
4, temperature characteristic compensation
Differential pressure transmitter has a large additional temperature error. When the ambient temperature differs greatly from the standard temperature, the temperature error can reach a level that cannot be ignored. During installation, the pressure opening of the differential pressure transmitter should be considered as low as possible to eliminate errors caused by temperature changes, and temperature compensation should be introduced when necessary.
5. Damping adjustment
Usually the damping adjustment can be carried out on site, according to the fluctuation of the output of the differential pressure transmitter. Since adjusting the damping does not affect the static accuracy of the transmitter, it is best to choose the shortest damping time constant to stabilize the fluctuation of the instrument output as soon as possible, so that the time constant can be directly changed between 0.2-1.67s. The method of on-site damping adjustment of the differential pressure transmitter is: the input pressure suddenly drops to 0 from the maximum value of the range, and the time required for the output current to drop from 20mA to 10mA is measured with a stopwatch, which is the damping time constant. When adjusting, a small screwdriver can be inserted into the damping adjustment hole (marked by D). When it is turned clockwise, the damping time will increase; otherwise, it will decrease.
usually analyzes the problems in the field verification of differential pressure transmitters, and provides some experience for guiding the field verification work and reducing measurement errors. In fact, due to the similarity between the differential pressure transmitter and the pressure transmitter measurement application, the method discussed is also applicable to the on-site verification of the pressure transmitter.