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 Radar
(sometimes referred to as microwave) uses high frequency electromagnetic
waves in the GHz range that are beamed downward from a sensor
located on top of the vessel. The sensor receives back a portion
of the energy that is reflected off the surface of the measured
medium. Travel time for the signal (called the time of flight)
is used to determine level. The noninvasive radar technologies
are frequency modulated continuous wave (FMCW) and pulse radar.
The invasive type uses a cable or rod as a wave-guide and extends
down into the tank's contents to near its bottom.
In the frequency-modulated continuous wave (FMCW) technology,
a sensor oscillator on top of the tank sends down a linear frequency
sweep, at a fixed bandwidth and sweep time. The reflected radar
signal is delayed in proportion to the distance to the level surface.
Its frequency is different from that of the transmitted signal,
and the two signals blend into a new frequency proportional to
distance. That new frequency is converted into a very accurate
measure of liquid level.
The second noninvasive radar technology, pulsed radar or pulsed
time of flight operates on a principle very similar to that of
the ultrasonic pulsed method. Where ultrasonic sensors transmit
its pulses and calculate the level based on the speed of sound,
pulsed radar also transmits it pulses towards its liquid level
target at the speed of light. The transit time of the pulse’s
return is calculated based on the speed of light to obtain a level
measurement.
Consequently
conditions that affect the speed of sound used in ultrasonic level
measurement such as variations in pressure, vacuum, temperature
do not affect the speed of light used in radar level measurement.
Guided
wave radar (GWR) is an invasive method that uses a rod or cable
to guide the microwaves as it passes down from the sensor into
the material being measured. The basis for GWR is time domain
reflectometry (TDR).
The
sensor transmits a microwave pulse along the surface of a stainless
steel cable. When the pulse reaches the measured material, the
pulse is reflected back up the cable to the sensor. The pulse
transmit time is measured and used to calculate the distance to
the product surface.
Radar Level Measurement Advantages:
- Non
contact
- Immune
to most vapors/ physical characteristics of media
- Hi
accuracy
- Ignores
vapor space changes
Radar Level Measurement Disadvantages:
- Expensive
- Cannot
be used for solids due to poor reflectivity
- Deposits
on antenna, multiple reflection can affect measurement
- Cannot
measure interface
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