HART HK51, Руководство по эксплуатации

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16
◆ Level measurement: The differential pressure transmitter used to measure
liquid level is actually the static head for measuring liquid column. This pressure
is determined by the level and specific gravity of liquid. The size is equal to
liquid level above the pressure tap multiplied by specific gravity of the liquid,
regardless of volume or shape of the container.
(a) Level measurement of open container
When measuring liquid level of open container, the transmitter is placed close
to bottom of the container to measure pressure corresponding to the liquid
level above it. Pressure at the tank level is connected to the high-pressure side
of the transmitter and the low-pressure side is open to the atmosphere. If the
lowest level of measured level changes is above the transmitter installation, the
transmitter shall be positively migrated.
(b) Level measurement of closed container
In a closed container, pressure of the container above the liquid affects pressure
at bottom of the container. Therefore, pressure at bottom of the container is
equal to liquid level multiplied by specific gravity of the liquid plus pressure of
the closed container.
In order to measure true level, pressure of the container shall be subtracted from
the measured bottom pressure of the container. Thus, open pressures tap on top
of the container and connect to the low-pressure side of the transmitter so that
the pressure in the container acts on both the high and low-pressure side of the
transmitter. As a result, differential pressure obtained is proportional to product
of the liquid level and specific gravity of the liquid.
◆ Dry pressure connection
If the gas above the liquid does not condense, the connection pipe on the low-
pressure side of the transmitter will remain dry. This condition is called a dry
pressure connection.
The method of determining measurement range of the transmitter is the same
as the method of open container liquid level.
◆ Wet pressure connection
If the gas on the liquid condenses, the liquid will gradually accumulate in the
pressure pipe on the low-pressure side of the transmitter, which will cause
measurement error. In order to eliminate this error, a certain liquid is previously
filled in the low-pressure side of the transmitter, which is called a wet pressure
connection.
(c) Reducing error
Pressure tube connects the transmitter to process pipeline and transfers pressure
at the pressure tap on the process line to the transmitter.
Causes of errors during pressure transmission are as follows:
Monocrystalline Silicon High Stability Transmitter