Chemical Engineering September 2020 - 57

uid in the tank.
In a GWR installation, the instruFigure
2. GWR level instruments are normally
mounted on a tank roof through a flanged spud
come one of the most widely applied
solutions in interface applications.
GWR provides accurate and reliable
measurements in vessels with tight
geometry, in chambers, and in tanks
of all sizes. No compensation is necessary
for changes in the density,
dielectric properties, or conductivity
of the fluid. Moreover, changes
in pressure, temperature, and most
vapor-space conditions have no impact
on its measurement accuracy.
GWR instruments need minimal
maintenance because they have no
moving parts, and they can easily be
installed as a replacement for older
technologies, even while there is liqment's
transmitter is usually topmounted
(Figure 2), with the probe
extending to the full depth of the
vessel. A low-energy pulse of microwaves,
travelling at the speed of light,
is guided down the probe. At the
point of the liquid level, a significant
proportion of the microwave energy
is reflected up the probe to the transmitter
when the pulse encounters
the sudden change in dielectric constant
when passing from air to liquid.
The transmitter measures the time
delay between the transmitted and
received echo signal. An onboard
microprocessor then calculates the
distance to the liquid surface.
A portion of the pulse will continue
down the probe through the top
layer of liquid, and when it encounters
the change in dielectric constant
at the interface point, a second echo
can be detected from the top of the
second liquid at a point below the
initial liquid level.
This works as described provided
the liquid on top has a lower dielectric
constant than the lower liquid,
which is the case with oil on water
since most oil products have a very
low dielectric constant, <5 typically,
and water is >50.
In the rare cases where the product
lying on top has a higher dielectric
than the one below, top-down
measurement using GWR will be
ineffective. In this circumstance the
mounting position can be inverted
so the instrument is installed on the
tank bottom, but this will only be
able to determine the interface position
and not the top level.
Thin-layer measurement
The reason why the upper product
must be of a certain minimum
thickness when using GWR in interface
applications is to enable
the instrument to separate and
distinguish the echoes of the two
liquids (Figure 3).
For most GWR instruments available
today, the minimum thickness
must be between 6 and 8 in. (125
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Chemical Engineering September 2020

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