PSI's Guided Microwave level systems are the ideal solution for many particularly difficult level applications involving either liquids or solids.
Unaffected by most Dust, Vapor, and Foam or fluctuations in Temperature, Pressure, Humidity, or Dielectric Constant, the unit is a PSI answer for today's level monitoring requirements.
Unusual tank shapes are easily accommodated also.
No
empty & fill calibration is required as PSI Guided Microwave systems can be calibrated
without changing the product level.
FLEX 50 REFLEX
TYPICAL APPLICATIONS
LIQUIDS |
SOLIDS |
|
Chemicals |
Alumina |
Cement |
Liquefied Gases |
Cereals |
Coal |
Oil/Water |
Flour |
Fly Ash |
Petrochemicals |
Grain |
Gravel |
Solvents |
Plastic Granules |
Powder |
Waste |
Sand |
Stone |
Guided
Reflex Microwave (or radar) is based on the TDR principle (Time Domain Reflectometry), which is has been traditionally used in the continuity
measurement of communication cables.
Extremely
short low power microwave (or radar) pulses are guided along conductors – rods
or cables suspended in the vessel. At the point where the pulse meets the
surface of the material, the pulse is reflected back. This reflection enables
accurate level measurement in vessels, storage tanks, silos and hoppers.
The
intensity of the reflection is dependent on the dielectric constant of the
material. The higher the dielectric constant, the stronger the reflection will
be, e. g. up to a 80% reflection for water. The instrument measures the time
between pulse emission and reception.
Guided
Reflex Microwave has two different categories of level application dependent on
dielectric constant K of the materials being monitored:
1
– Products with a dielectric constant K > 1.8
With
these materials, the "direct mode", is utilized, meaning that the
reflection from the product surface is used directly for the measurement. Using
the "direct mode" both level and/or interface measurement are
possible:
a) Level measurement
The
wave is reflected from the first product surface and is received by the
receiver on the top of the tank. As the wave only propagates through the air
above the product at the speed of light, the return time of the wave pulse is
directly proportional to the distance between the top of the tank and the
surface of the product (level).
b) Interface measurement
Interface
measurement can only be made if the first layer has a lower dielectric constant
than the second and if the difference between the two dielectric constants is
larger than 10.
For
measuring the interface level we use the residual wave after the first
reflection. This part of the wave moves further downwards along the conductors
through the first product layer until reflected on the interface level. The
speed of this wave depends fully on the dielectric constant of the first
product. This means that we have to know the e r of this first layer to
determine the interface level. The instrument measures time between emission
and second reflection. Since we know the time between emission and first
reflection (from level measurement) the difference between the two gives the
transit time through the first layer.
The
thickness of the layer is directly proportional to the time and dielectric
constant .
If
the dielectric constant of a product is not known, start the measurement with
an approximate value for the dielectric constant and make adjustments after
calibration.
2
– Products with a dielectric constant K < 1.8
To
measure the level of low dielectric products a different procedure is needed
due to the poor reflection of the wave on the surface of the product. This
reflection is not strong enough to make a reliable level measurement. To
achieve reliable and precise measurement we use the Tank Bottom Following
Principle (TBF).
The
instrument has a short circuit at the bottom end at a precisely known length.
The downward wave will first go down through the air at a known speed and then
passes through the product at a speed depending on the dielectric constant of
the product.
The
instrument measures the time between emission and reception from the bottom. As
the exact the return time in air when no product is in the tank is known, then
the difference in time between the two is directly proportional to the product
level in the tank. As the pulse speed through the product depends on the
dielectric constant, this value has to be programmed into the unit in order to
calculate the level. Since this measurement is largely dependent on the
dielectric constant, its accuracy will be less than in the direct mode.