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low-pressure side is connected with the atmosphere. If the lowest level of the
measured level changes is above the place where the transmitter is mounted,
the transmitter must make the positive transfer.
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Level measurement of closed containers
In the closed container, the pressure P0 of the container above the liquid
affects the measured pressure at the bottom of the container. Therefore, the
pressure at the bottom of the vessel is equal to the liquid level multiply by the
specific gravity of the liquid plus the pressure P0 of the closed container. In
order to measure the actual level, the pressure P0 of container should be
subtracted from the measured pressure at the bottom of container. For this
reason, open a pressure tap on the top of the container and get it connected to
the low-pressure side of the transmitter. Thus, the pressure in the container
acts on both the high and the low pressure sides of the transmitter at the same
time. As a result, the differential pressure obtained is proportional to the
product of the liquid level and the specific gravity of the liquid.
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Pressure connections
1) Dry pressure connection
If the gas above the liquid does not condense, the connecting tube on the
low-pressure side of the transmitter will remain dry. This condition is called the
dry pressure connection. The method of determining the measurement range
of the transmitter is the same as the method of determining the measuring
liquid level of opening container.
2) Wet pressure connection
If the gas above the liquid condenses, the liquid will gradually accumulate
in the pressure tube on the low-pressure side of the transmitter so that the
measurement error is caused. In order to eliminate this error, a certain liquid is
previously filled in the pressure tube on the low-pressure side of the transmitter,
