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Penetrating
pulse technology
A
safe, economical, non-contact, non-invasive, “through the wall”
digital technology for accurate level measurement
Leif Lindvall,
HiTECH
Technologies, Inc.,
Yardley,
Pa. |
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For
years, process industries have been seeking reliable techniques for
measuring or detecting liquid levels. This is now possible with digital
penetrating pulse technology. Its advantages include:
No physical contact
with the liquid because the acoustic energy transducers mount outside of
the vessel.
No possibility of leakage or contamination.
Measurement is independent of pressure and the presence of vapor and
foam on the surface of the liquid.
Measurement is not affected by toxic, aggressive or corrosive liquids.
Easily retrofits to existing vessels without welding.
No pressure vessel recertification required.
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Principle
of operation
The measurement principle is simple and easy to
understand. A special acoustic energy transducer, mounted in direct
contact with the vessel’s outside wall, emits a short pulse of acoustic
energy. The pulse penetrates the vessel wall and any tank lining and
travels through the liquid. Depending on the operating mode, the pulse is
either reflected back to the transducer or detected by another transducer.
The transit time, together with other application parameters, reveals the
actual level in the vessel. While the same technique can be applied to
both continuous level measurement and point-level detection,
implementation is somewhat different in each case. |
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Figure 1 |
Continuous
level measurement
The transducer is mounted in direct contact with the
bottom of the vessel (see Figure 1). The liquid/air or liquid/liquid
interface reflects the signal back to the device. The measured level in
the vessel is a function of the pulse’s transit time and other
variables, such as wall thickness, vessel shape, material characteristics
and temperature. A sophisticated algorithm built into the instrument
processes the variables and determines the liquid level.
Figure 2
shows the pulse position as a function of time. Part of the transmitted
pulse produces multiple reflections from the vessel’s inside wall. The
transducer detects these interfering reflections as early echoes and
eliminates them with a dead time, during which returning signals are
ignored. This means that levels cannot be measured down to zero
depth. |
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tolerance band is placed around the mean transmission time and only
measured values inside the expected range are included in the calculation
of further values. The transmission time is converted to depth on the
basis of the speed of sound. Because sonic velocity depends on the kind of
liquid and its concentration and temperature, a correction factor can be
programmed. Compensation also may be required if the operating temperature
changes substantially. |
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Figure 2 |
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Temperature
compensation
If the vessel always contains the same liquid,
transducers with
integral temperature sensors can eliminate the effect of temperature-
induced changes in the speed of sound. However, inaccurate level
measurements are still possible in a stratified liquid with different
temperatures in each layer.
Compensation with
reference measurement
If a vessel is filled with various liquids, the
concentration or chemical composition changes and the acoustic
transmission characteristics may change also. In these applications, a
second measuring channel of a dual channel device monitors the acoustic
characteristics of the liquid as a reference measurement. This is used
with the first channel to calculate the actual level. The reference signal
must travel through the vessel horizontally (see Figure 3).
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Figure 3 |
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Unit
conversion
Percentage level is the basis for subsequent
calculations and outputs. Converting the liquid level from percentage to
volume requires digitizing the vessel shape and its dimensions. These
variables are programmed into the device at set-up. This results in the
output being scaled in preferred volumetric units of measure, such as
gallons or cubic feet.
Measurement
response time
Instrument response time can be adjusted by programming the data update
intervals. Doing so affects the measured values, the local display panel,
the current outputs and the switching signals. |
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Point-level
detection
In point-level measurements, the pulse penetrates
the vessel wall, enters and radiates through the liquid. Depending on the
particular measuring mode being used, the reflected pulse may be received
by the same transducer or by a separate transducer mounted on the other
side of the vessel.
The controller software
has a point-level algorithm with a preset threshold value. A programmable
time integration function prevents intolerable jitter in the switched
output. The controller supports several operating modes:
Pulse-echo mode with
one sensor.
Pulse-echo mode with two sensors.
Pulse transmission with two sensors.
Dying out pulse with one sensor.
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Figure 4 |
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Pulse
Echo Method
In
the pulse-echo method with lateral transducer attachment, the transducer
is attached to the vessel at the height of the level switch point (see
Figure 4). If the vessel is filled with liquid to the specified level, the
transducer receives a reflected echo. If no liquid is present at this
level, there is no echo. The presence or absence of echo at the expected
point is evaluated by the link-up with the time window (gate) and the
instrument outputs the corresponding signal.
Application requirements
include:
Pipes or vessels with
parallel walls.
A minimum path length of eight in. for metal or glass and two in. for
plastic.
A maximum path length of 66 ft.
No agitator or internal fittings at the elevation of the level detection
point.
Pure (low-absorbing) liquids, free of bubbles and suspended solids.
This system can monitor
the level in ice silos of flake ice makers in refrigeration systems. It
can detect chemical reactions, such as crystallization or polymerization.
It can detect pigs in pipelines. |
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Pulse-echo
method with vertical sensor attachment
The transducer directs the pulse upward to within two or three degrees of
vertical. The surface of the liquid reflects the pulse back to the sensor
(see Figure 5). The gate setting sets the monitored filling level. The
temperature drift of the velocity of acoustic transmission and the time
needed for the pulse to return must be taken into consideration.
Applications include:
Substitute for
continuous level monitoring.
Vessels with height from approx. 2 in. to 66 ft.
No agitator or internal fittings in pulse path.
Liquids, free of bubbles and solids.
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Figure 5 |
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Pulse
transmission
For pulse transmission, two transducers are attached to the vessel at
opposite sides (see Figure 6). One functions as the transmitter; the other
as the receiver. This method requires a dual channel device.
Applications include:
Sound-absorbing
liquids laden with gas bubbles or contaminants, such as suspended
solids.
Level in pipes (requires special angle adapters).
Vessels with diameters to 66 ft.
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Figure 6 |
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Dying
down
Dying down involves evaluating signal decay. A sensor is attached to the
outside of the vessel at the same level as the liquid limit (see Figure
7). If no liquid is present at this level, the metal wall is not
attenuated and it vibrates for a long time under the impact of the
ultrasonic pulse. Any liquid present at this level eliminates the
vibration. Applications include:
Metal vessels with a
diameter of more than eight in. and wall thickness of 0.08 to 0.8 in.
Sound-absorbing liquids.
The liquid must not leave any residue on the vessel wall.
No reflecting wall or internal fittings in the vessel.
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Figure 7 |
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Transducers
One major breakthrough that enables penetrating pulse technology for
liquid level measurement is the acoustic energy transducers. Transducer
selection depends upon a number of factors including:
Vessel
geometry.
Material of construction.
The nature of the liquid.
The mounting
configuration and acoustic coupling between the transducer and vessel wall
is critical to accurate level measurement. Therefore, certain points need
to be observed when mounting a transducer. The surface at the point of
attachment must be extremely flat and smooth. Paint or other surface
treatments are detrimental and should be removed. Applying a special
acoustic mounting compound between the transducer and vessel can
compensate for some degree of surface unevenness.
Because there are numerous considerations involved in selecting the most
appropriate transducer and mounting configuration, discuss your specific
application in detail with a supplier trained in penetrating pulse
technology. |
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Start-up
Older systems require
an oscilloscope for installation, start-up and commissioning. Modern
technology features a built-in oscilloscope function to simplify the
process. Start-up requires only a notebook computer running Windows 95, 98
or 2000 (see Figure 8).
Although there are numerous factors involved in applying this technology
properly, penetrating pulse technology is a desirable level measurement
alternative, particularly in the biotech, chemical, food and beverage,
liquefied gas, oil pipeline and pharmaceutical industries.
For example, a
major chemical manufacturer is replacing its nuclear and radar level
systems with the penetrating pulse technology level systems to completely
eliminate the possibility of leakage and the bureaucratic nightmares of
licensing and testing.
Digital penetrating pulse technology is especially suitable for vessels
containing liquefied flammable gases because it can be installed without
welding or drilling. The devices can be attached to the vessels with a
special adhesive or stainless steel straps.
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Figure 8 |
