Level transmitter migration issue?
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Could you briefly explain the issue of level transmitter migration?
When the liquid level is at its lowest point, the output of the level transmitter is not 4mA (the lower limit of the transmitter); the adjustment performed in such a situation is known as zero shift.
When the measured liquid level is 0, the pressure on the positive-pressure side of the transmitter minus the pressure on the negative-pressure side yields a positive value; in this case, the instrument’s zero point needs to be shifted upward； Conversely, if a negative value is obtained, the instrument zero point needs to be shifted negative. The goal is for the transmitter to output 4mA when the liquid level is at zero.
It has never been migrated. A differential pressure of 0 at full water level is sufficient.
Since the positive and negative pressure levels are not at the same level, a differential pressure is generated within the pressure transduction tubes for positive (negative) pressure during measurement. The transmitter can provide accurate readings only starting from 0 Kpa; therefore, it is necessary to eliminate this differential pressure caused by the media in the positive (negative) pressure systems so that measurements can start from 0.
When using the differential pressure method to measure the liquid level in a sealed container, a balance tank is required on the negative pressure side, whose purpose is to balance the pressure inside that tank. Before measurement, fill the container with liquid; when the liquid level in the tank is at its lowest, the pressure on the positive-pressure side minus the pressure on the negative-pressure side gives the amount of migration, which is generally negative migration. When measuring an open-container, the positive-pressure side is connected, while the negative-pressure side is balanced with atmospheric pressure; therefore, (when the lowest liquid level is above the pressure sampling point) there is positive migration or no migration
Hang the upper and lower flanges of the level gauge onto the flanges of the tank; set the zero point to the value indicated by the transmitter