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EIPINI Chapter 4: Level Measurement Page 4-5
4.4 DIFFERENTIAL PRESSURE MEASUREMENT OF LEVEL
Perhaps the most frequently used technique of measuring level, is the method of
measuring differential pressure. The primary benefit of this method is that the
equipment measuring the differential pressure, can be externally installed or
retrofitted to an existing vessel. It can also be isolated safely from the process
using block valves for maintenance and testing.
There are, however, some disadvantages to this method. One vessel penetrations
near the bottom of the vessel is needed, where leak paths could be the cause of
many problems. Measurement errors also occur due to changes in liquid density.
Density variations are caused by temperature changes or change of product. These
variations must always be compensated for, to maintain accurate measurements.
4.4.1 Open Containers
To determine the fluid level H (Figure 4-7) in an open container, it is only necessary to
measure the difference between the pressure at the bottom of the container and
atmospheric pressure. In Figure 4-7 (a), the pressure difference is measured with a
differential pressure transmitter while in Figure 4-7 (b), a u tube manometer is used.
The pressure instruments may be installed with their zero lines level with the bottom
of the container or at a distance z, below the bottom of the container.
P atm
P atm
P atm
H
P atm
H
z
HP
LP
DP
transmitter
Zero line
z
HP
LP
h
U tube
manometer
Figure 4-7 (a)
Figure 4-7 (b)