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Level measurement
TEAS
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Related topics
Limit value monitoring, continuous measurement, initial echo, multiple reflections, time of flight, sound
velocity, and acoustic impedance.
Principle
For the experiment, a level measuring system for a two-phase liquid tank is set up and a calibration
curve for the filling volumes is recorded. One task is to determine the minimum measurement volume
(“dead band”). For maximum tank filling, an ultrasonic limit switch is to be tested. Level measurements
play an important role for many industrial processes. Particularly for the automation of these processes,
level meters are used to determine the content volume of tank systems, silos, reactors, collecting reservoirs, etc.
Equipment
1
Basic set “Ultrasonic Echoscope”
consisting of:
1x Ultrasonic echoscope
1x Ultrasonic probe 1 MHz
1x Ultrasonic probe 2 MHz
1x Ultrasonic test block
1x Ultrasonic cylinder set
1x Ultrasonic test plates
1x Ultrasonic gel
13921-99
Fig. 1: Experiment set-up
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Level measurement
Tasks
1.
2.
3.
4.
5.
Set up a level measurement system for continuous measurements.
Determine the minimum detectable filling level (“dead band”) and the associated liquid volume.
Perform a volume calibration for two different liquids (water and oil) for the tank.
Record and analyse suitable ultrasound signals for a limit switch.
Perform a level measurement with a two-phase system (water-oil).
Instructions for use:
Use the 2 MHz probe (red) for all of the measurements concerning the determination of the filling level.
Connect it to the reflection port of the ultrasonic echoscope. Set the echoscope to the reflection mode.
Ensure that the measuring range in the software is set to “Full” so that times of flight up to 200µs can be
measured.
Adjust the transmission power (“OUTPUT”) and gain (“GAIN”) at the echoscope so that the echo amplitudes can be measured without problems. TGC is not required. The time of flight is measured by moving
one of the cursors to the leading edge of the echo signal. The height of the echo amplitude plays a less
important role is this context. However, all of the measurements should be performed approximately at
the same amplitude. The measurement value of the time of flight is displayed at the bottom of the
screen.
Maintenance:
Do not clean the ultrasonic probe with alcohol- or solvent-containing liquids. Gel residues can be removed with some washing-up liquid, water, and perhaps a soft brush.
Use only edible oil for the measurements since the measurements concerning a two-phase system produce an oil-water-mixture that must be disposed of
after the completion of the experiment.
Set-up and procedure:
Set-up and selection of the parameters for continuous measurements:
Set up a level measuring system in accordance
with the schematic drawing. Ensure that the ultrasonic probe is coupled with a copious amount of
coupling gel to a plane area of the tank bottom (Erlenmeyer flask) and that the ultrasound is applied
to the tank as perpendicularly as possible.
The optimum alignment of the probe can be identified with the aid of the maximum of the signal amplitude.
In order to find suitable device settings, fill
approximately 1 litre of water into the flask and
observe the echo signal. Then, adjust the parameters for the transmission power (“OUTPUT”) and
of the ultrasonic probe for monitoring limit values
gain (“GAIN”) at the echoscope. Ensure that the Fig. 2: Set-up
and for continuous measurements
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Level measurement
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echo signal is clearly visible. Then, measure the
time of flight up to the leading edge of the first echo
peak.
The shape of the echo strongly depends on the
thickness of the tank bottom. In this area,
numerous multiple echoes are produced since the
sound travels to and fro in the tank bottom.
Under adverse ambient conditions (vibrations of
the desktop), the echo amplitude can show strong
variations since the movements of the water
surface also change the reflection conditions.
Averaging the displayed signals (setting: Options –
Parameters – Scan Average = 5) makes the
measurements considerably easier.
Determination of the minimum level volume (“dead
band”):
After the tank has been completely emptied, the ultrasonic probe must be coupled to the tank once
more while the set device parameters are maintained. The left side of the display now shows the Fig. 3: Set-up of the level measuring system for continuous
measurement
initial echo and numerous multiple echoes.
The measuring range in which these echoes are located is only to a limited extent suitable for level
Fig. 4: Selection of suitable device parameters for continuous level measurement
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Level measurement
Fig. 5: Wall echoes of an empty tank
measurements since the echo of the liquid surface should be separated from the interference echoes, if
at all possible. The minimum amount of water at which the echo can be clearly discerned can be
determined by filling water in small steps into the tank.
Volume calibration for water:
In order to perform the volume calibration of the system, add water in steps of approximately 100 ml and
measure the time of flight up to the echo as well as the total volume. When the liquid level increases, the
signal amplitude may decrease considerably. This can be compensated by readjusting the probe in order
to ensure a perpendicular sound incidence and by increasing the gain (“GAIN”). Repeat the
measurements until the echo has left the display range.
Set-up as a limit switch:
Since the echo can no longer be observed due to the long time of flight when the tank is filled further, the
next step is to set up a limit switch configuration.
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Level measurement
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Fig. 6: Determination of the minimum filling level
For this purpose, the ultrasonic probe must
be coupled to the upper end of the tank
with a copious amount of ultrasound gel
and with the aid of the support stand. In
this case, too, the perpendicular alignment
of the probe is important. On the screen,
only the initial echo and the multiple
reflections of the wall will be displayed.
If one now continues to add more liquid, a
second echo appears when the probe position is reached. Continue to add more liquid until the echo amplitude has reached
its maximum. At that moment, the limit has
already been exceeded. Then, maximise
the signal with the “GAIN” setting and
specify the threshold at approximately 0.5
mV with the aid of the horizontal cursor.
Fig. 7: Determination of the minimum filling level
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Level measurement
Fig. 8: Limit switch “OFF”, limit not reached
Repeat the experiment and fill the tank up to the exact threshold value. Measure the total volume inside
the tank when the upper limit has been reached with the aid of a graduated cylinder.
Fig. 9: Limit switch “ON”, limit exceeded
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Level measurement
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Fig. 10: Limit switch “ON”, limit reached
Fig.11: Echo signal during the level measurement with oil
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Level measurement
Fig.12: Echo signal directly after the two-phase-system has been filled into the tank
Volume calibration for oil:
Determine the minimum filling volume and perform the volume calibration for edible oil (e.g. sunflower
oil) as described above.
Level measurement on a tank filled with a two-phase system:
Fill the tank approximately halfway with water. Then, add some edible oil until the tank is filled by
approximately 2/3 water and 1/3 oil. Couple the ultrasonic probe to the tank and align it in the same
manner as for the calibration measurements.
Directly after the liquids have been filled in, a distinct interface cannot be discerned yet since in the
beginning the two liquids form a droplet emulsion. As a result, it will be hardly possible to measure the
echo of the interface at this stage.
Then, you must wait until the emulsion has separated and a clear interface has formed. This process can
take one hour or longer. It can be sped up by cooling the vessel (refrigerator).
Then, align the probe once more and adapt the gain until the echo of the water-oil-interface becomes visible. Measure the times of flight of both of the echoes.
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Fig.13: Echo signal of the two-phase system after the separation
Theory and evaluation
Two different measurement tasks must be distinguished: If limit values are monitored, alarm signals are
triggered when the filling level falls below the minimum limit or when it exceeds the maximum limit. In the
case of a continuous measurement, the filling level is measured continuously and then used for various
purposes, e.g. for determining consumption values, control purposes, or monitoring and documenting
certain processes.
Apart from mechanical float systems as well as capacitive, optical, and electromagnetic sensors, numerous applications also use ultrasonic sensors for the level measurement.
Ultrasonic level measuring systems are particularly suitable for monitoring liquid levels. They can be
used in combination with nearly any medium, for several layered media, foaming media, and also for
highly aggressive liquids, since the measurement is performed from the outside through the tank wall.
Depending on the attenuation of the liquid and the required accuracy, ultrasonic frequencies between
40 kHz and 5 MHz are used.
During the ultrasonic level measurement, the echo signals of the liquid surface are recorded. Then, the
time of flight of the signal from the probe to the liquid surface and back is measured. During this process,
the signal passes through the tank wall and through the liquid. The measured time of flight depends on
the sound velocity of the wall cW as well as on the sound velocity of the liquid cFl.
t = 2( sW / cW + s Fl / c Fl )
For tank systems, it is actually not the filling height sFL that is of interest, but the filling volume.
In the case of tank systems with a regular shape and if the sound velocity of the liquid is known, the
filling volume can be calculated based on the time of flight and on the geometrical conditions. For most
systems, however, a volume calibration must be performed. Since the wall characteristics sW and cW
are constant at equal temperature, they do not have to be taken into consideration for the volume
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Level measurement
calibration.
The tank wall, however, produces multiple echoes that prevent the very low filling levels from being
measured. The measurement of the empty tank (Fig. 5) shows that the multiple echoes can be found
over a time of flight of 100µs and more. When the liquid is filled in, part of the ultrasonic energy is
coupled into the liquid so that the amplitude of the multiple echoes decreases. The area in which
measurements are impossible (i.e. the so-called “dead band”) becomes smaller. The amount of sound
energy that gets into the liquid depends on the sound impedance of the tank material and liquid. This
means that the minimum filling level depends on the acoustic characteristics of the liquids. The
measured minimum filling volume is 240 ml for water and 250 ml for oil.
For the calibration, the time of flight is measured for various filling volumes. Then, a calibration curve is
generated.
Dead band
Calibration
Water
Time of flight
Volume
µs
ml
19.2
240
21.5
25.4
36
45.4
51.8
61.9
69.3
73.9
78.5
86.1
93.4
103.2
113.6
123
134.8
148.5
168.7
179.8
188
196.1
260
340
500
650
740
900
1000
1060
1120
1220
1300
1410
1520
1620
1720
1840
2000
2070
2130
2170
Sunflower oil
Time of flight
Volume
µs
ml
20
250
22
28.5
32.6
36.5
39.5
44.1
47.5
53.1
58.6
69.3
76.3
84.3
91.9
101.9
111.4
118.7
127.4
143.7
150.2
166
290
380
450
510
560
630
680
770
850
1000
1100
1210
1300
1410
1510
1590
1670
1820
1880
2000
Fig. 13: Measurement results of the volume calibration
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Level measurement
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The calibration curves for water and oil are very similar since the sound velocities of the two liquids are
very close to one another. The curve for the sunflower oil has a slightly bigger slope. For the tank that is
used, the volume calibration curve can be fitted with a second-degree polynomial (quadratic equation).
V = k1t 2 + k 2 t + k 3
k1 - k3 are the coefficients of the quadratic equation and t is the measured time of flight.
The values of the constants are shown in the diagram. With this information, the filling volume can be
calculated for any given filling level based on the time of flight of the echo signal.
During the limit value detection, it is impossible at first to measure an echo since the ultrasound signal is
not transferred through the air. When the liquid level increases and reaches the position of the sound
probe, the sound is transferred to the opposite tank wall where it is reflected. The maximum echo amplitude is reached when at least half of the probe surface is covered by the liquid. By specifying an amplitude threshold, the maximum filling level can be fixed precisely. The measurements with a signal
threshold of 0.5 V led to a maximum filling volume of 2580 ml. For a fixed filling volume, the signal
threshold must be determined anew for every liquid since the echo amplitude also depends on the
acoustic characteristics of the liquid.
The measurements concerning the two-phase-system demonstrate the problems that may occur during
ultrasonic level measurements. In tank systems, this kind of layering is usually caused by deposition,
e.g. of water in fuel oil or diesel tanks.
The echoes at the interface can be very small or even disappear completely if the liquids are clearly
separated. This can particularly be the case during the filling of, or withdrawal from, the tank. The echo
signal is smaller if both liquids have similar ultrasound characteristics (sound impedance) as it is the case with water and sunflower oil. As a result, a higher signal gain must be selected. In each case, the level measuring system must be adapted to the measuring conditions and suitable measurement parameters must be defined.
The measurement of the two-phase system of water and sunflower oil led to the following times of flight
of the echoes:
Fig. 14: Volume calibration curves for a tank system filled with water and oil
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twater
toil =
Level measurement
= 99.1 µs
139.4 µs
The filling volumes of the two liquids can be calculated or read with the aid of the calibration curves.
The water volume is calculated based on the fitted polynomial
twater = 99.1 µs
Vwater = -0.0334 * t water2 + 18.106 * twater - 101,38
Vwater = 1364.91 ml
In order to calculate the oil volume, one must first calculate the corrected time of flight:
t*oil = t* + (toil – twater)
The time t* is the time of the flight of the signal in oil for an equivalent filling volume V'oil = Vwater. The
volume calibration for oil leads to the fitted polynomial
Voil = k1 * tÖl2 + k2 * toil +k3
Voil = -0.0346 * toil2 + 18.448 * toil – 111.27
Based on the general solution of the quadratic equation, the time of flight results as:
− k2 ± k2 − 4k1 (k3 − Voil )
.
2k1
2
t *1, 2 =
k1 - k3 are the coefficients taken from the calibration curve for oil. With the already calculated volume of
the water level measurement (1364.91 ml), the time t* can now be calculated:
Fig. 15: Volume determination based on the calibration curves
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Level measurement
t* = 98.05µs.
The second solution of the quadratic equation is a negative value and of no interest in this context. Then,
the correct time of flight of the echo of the oil surface t*oil can be calculated with the equation that is
stated above.
t*oil = t* + (toil – twater)
t*oil = 97.68µs + 139.4µs – 99.1µs
t*oil =138.35µs
The corrected total filling volume V*oil can now be calculated based on t*oil and the calibration for oil. In
accordance with the calibration curve, it is:
V*oil = -0.0346 * t*oil2 + 18.448 * t*oil – 111.27
V*oil =1778.73 ml
The quantity of oil in the tank, therefore, is:
Voil = V*oil-V'oil
Voil = 413.82 ml.
The differences in the acoustic characteristics of the tank system and of the water and sunflower oil are
very small so that it might be sufficient to determine the filling levels solely based on the water calibration. In this case, the error of the volume determination is approximately 1%. In order to simplify things,
the filling level can also be read off the diagram. This may be necessary in particular for tank systems
with complicated shapes since in this case the calibration curve cannot be fitted.
Calibration for
water and oil
Calibration for
water
Deviation
Filling volume:
water
1364.91 ml
1364.91 ml
0%
Filling volume:
oil
413.82 ml
408.64 ml
1.20%
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Level measurement
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