# Development and Characterization of an Optical Contact-less fully Enclosed
Liquid Level Measurement System for JUNO and OSIRIS

E. Theisen  H. Th. J. Steiger,11footnotetext: Corresponding author. L.
Oberauer  O. Pilarczyk  M. Wurm

###### Abstract

Large scale neutrino experiments have high demands on the radiopurity of the
detector liquids used pursuing the goal to reduce background signals. The
liquid scintillator (LS) used for JUNO’s (Jiangmen Underground Neutrino
Observatory) 20 kt target will be monitored by the pre-detector OSIRIS (Online
Scintillator Internal Radiopurity Investigation System) with concern to JUNO’s
high radiopurity standards. During the filling of a complicated detector
construction like OSIRIS only slight differences in the liquid levels inside
the subsystems could lead to seriously harming hydrostatic overpressure. To
prevent damage to the detector, the filling level heights inside the tanks
have to be monitored with cm-precision. In OSIRIS the measurement will be
accomplished by the TruSense S310 infrared laser sensor enabling a contactfree
determination of the liquid level height to maintain the high scintillator
radiopurity. As a further advantage to former laser-based filling level
determination systems, the measurement will be performed without the
application of a guided float in direct contact to the liquid scintillator
whose chemical compatibility to the LS would be an additional concern.

In the present work we report on tests dedicated to the functionality of the
TruSense S310 laser sensor which verify its applicability as optical filling
measurement system for OSIRIS and comparable experimental setups. Furthermore
we introduce the intended concept for the integration of the laser sensor into
the OSIRIS liquid handling system (LHS) and discuss the design of the major
component parts of the laser sensor installation.

##  1 Motivation

Large LS detectors like JUNO (Jiangmen Underground Neutrino Observatory) have
high demands on the radioactive properties of their detector liquids. In
particular the radiopurity of the LS is a central aspect to achieve the ultra-
low backgrounds required for particle detection. In JUNO the 20 kt main
detector consisting of a 120 mm thin highly transparent acrylic hollow sphere
stores the target in a $\sim$ 35 kt water pool [JunoCDR]. Such large detector
constructions in general have complicated geometries and filling procedures
which makes it necessary to monitor filling level heights inside the
integrated tanks closely. Similar monitoring is also required in JUNO’s pre-
detector OSIRIS [OSIRIS] and major storage tanks of the liquid handling
system. Slightly different filling levels in subsystems during the filling of
these volumes could cause fatal damage to the detector due to potentially
occurring pressure differences. However, the determination of the filling
level heights must not compromise the quality of the LS used for the
experiment. A contamination in consequence of the filling level measurement
has to be ruled out in order to guarantee the compliance of the LS to the
strict radiopurity limits. This is why a laser-based filling level
determination applying the TruSense S310 laser sensor has been selected for
JUNO’s pre-detector OSIRIS which avoids direct contact between the monitoring
system and the target media. A comparable laser-based distance measuring
system has already been employed in the Double Chooz experiment to monitor the
liquid level heights inside the detector vessels. A precision of $\pm$0.5 mm
[Franke] could be achieved by the optical measurement. However, the wavelength
of the used laser sensor M10L/100 [MEL], provided by MEL-Mikroelektronik, was
in the visible range ($\lambda$ =650 nm). Due to the high intensity of the
light, the laser system could not be operated simultaneously with the detector
PMTs since the detected light levels threatened to damage the sensors.
Instead, an optically separated light path was realized in specific liquid
level measurement tubes connected to the corresponding detector vessels
[Franke]. Furthermore, it was not possible to realize a contact-less
measurement, since a guided float xxmadefrom? in direct contact to the liquids
had to be used as a target for the laser beam [Franke].
With regard to the requirements of upcoming large scale neutrino experiments
for a contact-less optical measurement of distances up to $\sim$50 m to highly
transparent liquids, the market for commercially available sensors was
investigated. The company Laser Technology Inc. supplied a prototype sensor
which measures the distance by means of a pulsed IR (infra-red) laser
($\lambda$ =905 nm [TruSense]) using the time of flight (TOF) technique. The
system exploits the laser pulse reflections at the liquid surface and thus
enables the determination of the level without the additional application of a
guided float. This is a major advantage in comparison to former proceedings,
as it excludes a contamination of liquid scintillator in consequence of the
direct contact to the float. The system now sold under the name TruSense S310
(shown in Figure 1) was selected as an option for JUNO and will be deployed in
the pre-detector OSIRIS. In this work we characterize the TruSense S310 laser
sensor system and report on dedicated test measurements to assess its
applicability for large scale neutrino experiments. Furthermore we discuss the
integration of the laser sensor into the OSIRIS LHS which is accomplished by a
closed system sealed by a protective window in order to prevent the entrance
of oxygen or radioactive gases into the detector vessel.
Closed system: no oxygen or radioactive gases, transparent window (acrylics,
macrolon)
Reflecting surfaces: no float required

##  2 Design of the System

The optical filling level measurement system as designed for the contactless
monitoring of the filling level height inside the acrylic vessel of OSIRIS
consists of several component parts with different functionalities. The
TruSense S310 laser sensor is the central element of the system, as it
accomplishes the technical realization of the measurement. But further
component parts are required to enable an appropriate integration of the laser
sensor into the OSIRIS liquid handling system (LHS). This section gives an
detailed description of the characteristics of the laser sensor devoted to the
optical filling level measurement and provides an introduction to the design
of the onsite sensor laser installation.

###  2.1 The TruSense S310 Laser Sensor

The contactless filling level determination inside the acrylic vessel of
OSIRIS (and in the liquid handling system of JUNO) is enabled by the
application of the TruSense S310 infrared laser sensor. It is provided by the
Laser Technology Inc. company and has been designed for non-contact fluid
level measurement of highly reflective and turbulent surfaces in distances in
the range from $\sim$ 0.5 m to 50 m to the sensor front window [TruSense]. The
system determines the distance to a target surface by measuring the return
time from the emission of an infrared laser pulse to the arrival of the
corresponding target reflex and comparing it to the speed of light (TOF
technique). The TruSense S310 applies a class 1 measurement laser emitting
xx(duration) pulses at a wavelength of 905 nm. The frequency of these pulses
can be costumized in the range from a minumum of 1 Hz to the maximum of 14 Hz,
which corresponds to the number of distance measurements performed per second.
In addition, the orientation of the beam path can be reviewed by a visible
class 2 alignment laser with a wavelength of 650 nm. [TruSense]

The TruSense S310 supports the application of three target modes using either
the first reflex (DF mode), the strongest reflex (DS mode) or the last reflex
(DL mode) caused by the target surface to determine the distance. Of course,
results obtained applying these different target modes can be interpreted
under consideration of the corresponding reflex used for the measurement.
However, the differences in our test results in consequence of the applied
mode were hardly identifiable and will be of minor importance with regard to
the usability of the different modes for the operation in OSIRIS or comparable
experimental setups.

The laser sensor was acquired in the OEM (original equipment manufacturer)
version without a housing.(xxdifferencebetweenhousingandeclosure) To prevent
damage to the electronics and optics of the system, it is mounted inside a
safety proved cylindrical enclosure with a diameter of $\sim$ 127 mm, a length
of $\sim$ 254 mm and a weight of $\sim$ 3.8 kg [TruSense]. The enclosure front
and back parts are covered with lids that can be unscrewed to gain access to
the interior. The front lid is equipped with a window in order to enable the
operation of the laser sensor inside the enclosure.

The TruSense S310 requires an input power of 12 - 24 VDC which can be provided
by a default power socket using the supplied Power COM Cable (PCC).
Furthermore the supplying company states a power draw of 1.8 W for the sensor
in operation and of 0.48 W for the sensor in standby mode [TruSense].

Figure 1: Left: Commercial photo of the TruSense S310 in the OEM version with
diffusor lens taken from the website of the supplying company [LaserTec_web].
Right: 3D-layout of the TruSense S310 mounted inside its enclosure. In this
illustration the enclosure front is screwed off the back part allowing a
vision of the sensor. The scheme is taken from user’s manual [TruSense].

###  2.2 Beam Diameter and Diffuser Lens

The free aperture of the measurement laser features a width of $\sim$ 23 mm.
Together with the stated beam divergence of $\sim$ 3 mrad, this yields a beam
diameter of the $\sim$ 173 mm at the maximum measurement range of 50 m
[TruSense]. In case of OSIRIS, the beam path will be led through a pipe into
the interior of the acrylic vessel. In this setup the inner pipe diameter
provides a constraint to the maximally feasible beam spread, since reflections
at the inner pipe wall have to be excluded to ensure a proper measurement. For
level height measurements of turbulent liquid surfaces the Laser Technology
Inc. recommends the application of an additional diffuser lens attachable to
the front of the laser sensor. The impact of the diffusor lens on the laser
beam is characterized by an even larger spread of the beam diameter. This is
the reason why the optional application of the diffusor lens has to be taken
into account when selecting the appropriate inner diameter of the laser
pipe.xx(dimensionsofOSIRIS)

###  2.3 Integration into the OSIRIS LHS/LMS

The concept for the integration of the TruSense S310 into the OSIRIS LHS
foresees the installation of the laser sensor at the top of the water tank. At
this elevated position the system will be mounted onto a stainless steel pipe
reaching downwards into the volume of the acrylic vessel. Through this pipe
with an inner diameter of 4 inches, the sensor will send its laser pulses to
measure the distance to the liquid surface inside the acrylic vessel which
eventually will be converted into the filling level height. The connection
between laser sensor and laser pipe will be realized by the interface between
a DN125 flange topping the pipe and a corresponding counterpart flange to whom
the laser sensor will be attached. The latter so-called adapter flange was
specially designed to adapt the dimensions of the DN125 flange to the size of
a 4 inch NPT screw thread which is utilized to combine the back and front
parts of the laser sensor enclosure. The equipment of the adapter flange with
this thread enables a simple attachment of the TruSense S310 by screwing the
sensor carrying backpart of the enclosure to the adapter flange. In between
both flanges a protective window will be inserted which seals the inner volume
of the acrylic vessel to prevent a contamination of the present nitrogen
atmosphere with oxygen and radioactive gases. The connection of the flanges
will be fixed with eight M16 scews and sealed with flat gaskets at both sides
of the window pane. Figure 2 shows a schematical overview about the
integration of the TruSense S310 into the construction of OSIRIS and presents
the component parts dedicated to the laser sensor installation.

Figure 2: Left: 3D-Layout of OSIRIS showing the designated location of the
laser sensor installation at the roof of the OSIRIS water tank. Right:
Schematical illustration of the components parts required for laser sensor
installation indicating their intended assembly.

##  3 Sensor Communication and Software Development

For data transmission between user and sensor the TruSense S310 offers the
serial RS-232 ASC II communication pathway. This standard method can be
applied to read out the sensor ouput or to modify the sensor settings. In
addition, a standardized SDI-12 serial interface can be applied [TruSense],
but has never been used in the scope of the present work. Beside the
realization of the power supply, the PCC also establishes the connection
between laser sensor and a serial I/O device. The PCC is supplied with a DB9
pin serial connector which requires the application of RS-232-to-USB adapter,
if a connection to a standard USB port is desired.

Figure 3: Frontpanel screenshot of the LabVIEW software interface developed
and used for the communication with the TruSense S310 laser sensor.
Numeration: 1) Command input 2) Stop button with laser status display 3)
Adjustments of measurement frequency, target mode, time delay and wait period
4) Data storage to file 5) Communication settings 6) Error output 7) String
display of measurement results 8) Display of execution and measurement time 9)
Button to switch to the automatic conversion of the distances measured to
filling level heights. For this purpose, the entries of the laser mounting
height and vessel height are required. 10) Graphical filling level display 11)
Automatic plot of the distances recorded against the measurement time

As communication and read out software the Laser Technology Inc. recommends a
terminal emulation program, such as Tera Term [TruSense, TeraTerm]. However,
within the scope of this present work a user interface under the application
of the visual programming environment LabVIEW (Laboratory Virtual Instrument
Engineering Workbench [LabVIEW]) developed by the National Instruments
Corporation (NI) has been created. We benefited from the collection of sample
codes offered at the NI community support website which provided a basic
virtual environment (VI) to read out and write ASC II register values via a
serial RS-232 modbus [examplecode]. The VI offers a command line to enter
RS-232 ASC II serial commands and additional input boxes to manage the
communication settings (for further information, please refer to [TruSense]).
After the duration of an adjustable read delay, the response of the laser
sensor is displayed as a string output along with the number of bits read. In
addition, the sample code offers a display for error messages. Through the
consecutive integration of further features to the sample VI, a comprehensive
LabVIEW user interface has been developed enabling a convenient and more
sophisticated usage of the TruSense S310. The interface includes selection
boxes to choose the measurement frequency and the target mode for the upcoming
measurement. Depending on the frequency selected, the program automatically
calculates the means of the distances measured per second and displays the
measurement time. Results, means and the associated measurement times are
stored in lists that can be written to a text file and stored at a
costumizable location on the hard disk of the connected device. For a distance
measurement performed with 10 Hz over a period of 15 min the created text-file
requires a disk space of $\sim$ 275 kB which corresponds to $\sim$ 10 GB per
year for a measurement in continuous operation. Furthermore the LabVIEW
interface is equipped with a graphical display showing a live plot of the
means determined from the gathered data. In addition, we added a feature which
automatically converts the distances measured to filling level heights. Figure
3 presents the layout of the LabVIEW user interface and highlights its major
features.

##  4 Test Measurements and Lab Setups

We have performed multiple tests of the laser sensor functionality in order to
assess the feasibility of an optical filling level measurement devoted to JUNO
and OSIRIS under the application of the TruSense S310. These investigations
reviewed the precision of the laser sensor and the stability of the sensor
output in long-term measurements. Furthermore we ran tests dedicated to the
development of certain component parts required for the onsite sensor
installation including the design of the laser pipe, the adapter flange and
the protective window. In this section we report on the experimental setup
used for these examinations and discuss the test results in detail.

Figure 4: View of the laboratory setup built up to investigate the laser
sensor functionality. The acrylic tube with an inner diameter of 90 mm was
connected via a PFA hose and a stainless steel filling tube to a centrifugal
pump used to fill and drain the tube with liquids like water or LAB. The laser
sensor was mounted on an aluminium frame directly above the acrylic tube to
measure the liquid level inside the tube. Figure 5: Time evolution of the
means determined for each 100 measurement results. Each measurement was
performed with a frequency of 10 Hz. The colouring indicates the target mode
applied - blue: first reflex, red: strongest reflex, green: last reflex. The
range between largest and smallest single result is specified via the coloured
areas for the respective data set. The dashed gray lines show the accuracy
stated by the supplying company.

###  4.1 Laser Sensor Precision

The precision of a distance measurement performed by the TruSense S310 is
stated by the supplying company as $\pm 10$ mm [TruSense]. With the help of a
dedicated experimental setup we reviewed this statement in a series of
measurements. Fur this purpose, an acrylic tube was filled with water and
subsequently the distance to the liquid surface was measured with the laser
sensor mounted at an aluminium frame right above the tube. A photo of the
experimental setup used is presented by Figure 4. The distance measurement was
performed for a period of 15 min and with a measurement frequency of 10 Hz. In
order to investigate the impact of the target mode applied, we repeated the
distance measurement consecutively with each target mode. After an intervening
period of four days a rerun of the complete series of measurements was
executed. It was intended to investigate, whether the laser sensor is capable
of resolving the decrease of the filling level due to the evaporation of water
inside the acrylic tube with a diameter of 90 mm.

Figure 5 presents an evaluation of the results obtained showing a plot of the
time evolution of the means calculated for each 100 measurement results. For
all three target modes, separately plotted with different colours, the means
fluctuate within a range of a few millimeters around the respective total data
set average. The coloured areas indicate the widths between the largest and
smallest single measurement result of the respective data set. The largest of
these widths, associated with the measurement performed in strongest reflex
mode, suggests an accuracy of $\pm 5$ mm. The dashed gray lines illustrate the
precision statement by the Laser Technology Inc. indicating the $\pm 10$ mm-
range around the respective total data set averages. A comparison to the
ranges of the coloured areas shows that the precision stated by the supplying
company is a rather conservative estimate. Furthermore the even smaller mean
variations show that the precision can be further improved by averaging
multiple measurement results. This approach of improving the accuracy of the
laser sensor output is easily feasible, since stable means are realized within
seconds due to the possibility of measuring in a frequency up to 14 Hz. The
measurement results obtained in the second run indicate a significant decrease
of the filling level height which shows that the TruSense S310 measured the
distance to the liquid surface precise enough to resolve the evaporation of
water that had occurred during the intervening period.

###  4.2 Laser Pipe Diameter

A major concern with regard to the laser sensor performance is the occurrence
of beam reflexes at the inner pipe wall in consequence of the beam spread in
combination with a too narrow laser pipe. To investigate the minimum inner
laser pipe diameter required to ensure a proper laser sensor functionality, we
performed dedicated tests using two pipe sections with inner diameters of 7.5
cm and 10 cm respectively. These pipe sections were installed consecutively in
front of the TruSense S310 to perform distance measurements to a target at a
range of $\sim 2.4$ m and $\sim 2.5$ m respectively. As a reference the
measurements were repeated without the pipe section in front of the laser
sensor. We ran the measurements in each target mode with a frequency of 10 Hz
and for a period of 15 min.

Figure 6 illustrates the differences in the results obtained for the different
inner pipe diameters. The plot shows the comparisons between the mean values
of the complete data sets marked by the data points and the means of the
reference data indicated by the coloured lines. The ranges between largest and
smallest single measurement result are marked by the error bars and by the
coloured areas respectively. In case of the 10 cm pipe we see a remarkable
agreement of the results obtained with and without the pipe in front of the
laser sensor. Here, the laser beam seems to have propagated unaffectedly
through the pipe. On the contrary, the results obtained for the 7.5 cm pipe
show a clearly compromised laser sensor performance. The means of the
measurements with the pipe are significantly smaller than the reference means
and the ranges between largest and smallest measurement result are multiple
times larger. These observations suggests that beam reflexes at the inner pipe
wall indeed have occurred in case of the smaller pipe. Measuring in last
reflex mode, it was not even possible to gather usable data. These
measurements verified that a minimum pipe diameter of 10 cm is needed to
guarantee an undisturbed filling level measurement with the TruSense S310 and
therefore, in case of OSIRIS an inner diameter of 4 inches (= 10.16 cm) is
foreseen for the laser pipe.

Figure 6: Comparison of the measurement results obtained with and without the
pipe sections in front the laser sensor. The means of the respective complete
data sets are indicated by the data points, whereas the means of the
corresponding reference measurement are marked by coloured lines. The ranges
between largest and smallest single measurement results are specified by the
error bars and by the coloured areas respectively. The colouring indicates the
target mode applied - blue: first reflex (DF), red: strongest reflex (DS),
green: last reflex (DL). Left: Results for the pipe with an inner diameter of
10 cm. Right: Results for the pipe with an inner diameter of 7.5 cm.

###  4.3 Optimal Adapter Flange Depth

Designing the dimensions of the adapter flange it was of particular importance
to evaluate its optimal depth, since it defines the distance of the laser
sensor to the protective window. The optimization of this spacing is crucial
to prevent disturbing influences of the window to the outcome of the optical
filling level measurement. On this account, we performed tests to investigate
the impact of the window in different spacings to the laser sensor. The
laboratory setup was extended with a clamp holder to fix a window pane into
the beam path. The clamper holder was attached to a bar on the aluminium frame
which enabled the adjustment of its vertical position. In this way it was
possible to perform distance measurements to the liquid surface inside the
acrylic tube with spacings in the range from 2 to 16 mm. Figure 7 shows the
results obtained for windows with thicknesses of 2 mm and 10 mm respectively
marked by the coloured data points according to the target mode applied. For
the investigation of the influence of the window spacing, we performed two
different reference measurements. Indicated in green are on the one hand
measurement results gathered without a window placed into the beam path. On
the other hand the data presented in yellow show the results of measurements
under the application of the original enclosure window. The plots illustrate a
significant deterioration of the laser sensor performance with an increasing
window spacing. This observation can be made for both window thicknesses, but
in case of the thicker window the deterioration is even more apparent. Up to a
window spacing of 10 mm the stability of the laser sensor output is unaffected
and remains satisfying for each target mode. The small deviations to the
reference results without any window are of minor importance as they can be
corrected by adding a user offset. For larger window spacings the sensor
performance becomes increasingly unstable which is indicated by larger error
bars. The presented data evaluation suggests that the original enclosure is
situated in a distance to the laser sensor in the range of 4 to 7 mm, which is
the reason why a window spacing of 5 mm was considered as optimal for the
design of the adapter flange.

Figure 7: Comparison of the measurement results illustrating the impact of
different window spacings. The colouring of the data points indicates the
target mode applied - blue: first reflex, red: strongest reflex, green: last
reflex. The green indicators show the results of the reference measurements
without a window inside the beam path, whereas the yellow indicators specify
the results under the application of the original enclosure window. Left:
Results for a window thickness of 2 mm. Right: Results for a window thickness
of 10 mm.

###  4.4 Window Material and Thickness

The properties of the window pane used to seal the interior of the acrylic
vessel against disruptive contaminations must also be considered to guarantee
a correct laser-based filling level measurement. From the tests described in
section sec:adapterflange further conclusions could have been drawn, in
particular with regard to the material used for the window production and the
pane thickness which also affects the performance of the laser sensor
significantly. Figure fig:thickness shows a rearrangement of the measurement
results obtained under the application of the different windows inside the
beam path in different spacings to the laser sensor. In this case, the plots
show the impact of the window thickness on the laser sensor performance for
the spacings of 4 mm and 7 mm respectively. The results of the reference
measurement performed without a window inside the beam path is indicated in
green, whereas the results gathered under the application of the original
enclosure window are presented in yellow. On the one hand, the results show
the already observed worsening of the laser sensor performance with an
increasing window thickness in the range of 2 to 6 mm in comparison to the
reference results for both spacings. On the other hand, the results obtained
with the 8 mm and 10 mm thick windows indicate a surprising adaption to the
reference results, suggesting an improvement of the laser sensor performance.
This observation can be explained by the material of the window panes
associated with these measurements. These two thicker panes were made out of
macrolon, whereas the thinner windows were produced out of acrylic. Comparing
the results obtained with panes of both materials, the conclusion can be drawn
that macrolon has the better optical properties for the production of the
protective window, since the corresponding results are more similar to the
reference results. However, in case of OSIRIS further tests will have to be
performed to make a final decision, since the chemical compatibility of the
material of macrolon with the liquid scintillator is yet to be verified.

Figure 8: Comparison of the measurement results illustrating the impact of the
window thickness on the lasers sensor performance. The colouring of the data
points indicates the target mode applied - blue: first reflex, red: strongest
reflex, green: last reflex. The green indicators show the results of the
reference measurements without a window inside the beam path, whereas the
yellow indicators specify the results under the application of the original
enclosure window. Left: Results for a window spacing of 4 mm. Right: Results
for a window spacing of 7 mm.

###  4.5 PMT safety

To ensure that the laser sensor can be mounted directly in JUNO’s CD or the AV
of OSIRIS without being hazardous to the PMTs [PmtPaper], one Hamamatsu
H12860HQE PMT (type to be used in both detectors) was irradiated while in
operation. The laser irradiated the photo cathode through 12 cm highly
transparent acrylic glass, what corresponds to the thickness of JUNO’s CD
acrylic sphere. The sensor and the acrylics were placed in a distance of
$\sim$1 m to the PMT. For the irradiation test the laser was operated without
diffuser lenses resulting in a beam divergence of 3 mrad and with the highest
possible measurement repetition rate of 14 Hz. The arrangement, which can be
considered as worst case scenario for the usage in JUNO and OSIRIS, is shown
on the right side of Figure XXX. The radiation directly hitting the
photocathode caused pulses between $\sim$160 p.e. and $\sim$200 p.e. (see
Figure XXX left). IR radiation reflected at the interfaces between acrylic
glass and air, which reaches the photocathode, causes a second peak in the
spectrum at about 17 p.e.. Due to several reflections and scattering on the
liquid and acrylics in the detector the intensity of the laser in the CD is
always distributed over several PMTs. Although the events caused by the laser
are intense pulses, they do not harm the functionality of the PMT. Since the
intensity in JUNO and OSIRIS will be distributed over a larger area and
furthermore the water buffer lie between the tubes and the acrylic vessels, it
can be assumed that even continuous operation of the laser, with the
photomultipliers switched on, does not pose a risk of destruction.

Figure 9: Left: Charge spectrum of the PMT obtained from the IR laser pulses
shining through 12 cm of highly transparent acrylic glass. The peak at
$\sim$180 p.e. corresponds to radiation directly hitting the photo cathode,
while the left peak is caused by IR photons undergoing previous reflections on
the interfaces between acrylics and air. Right: Shown is the arrangement used
for the PMT irradiation test. The laser sensor was placed at a distance of
$\sim$1 m to the central part of the PMT photo cathode. Within the laser beam
a 12 cm thick block of highly transparent acrylics was placed to simulate the
impact of JUNO’s CD sphere. Note, in JUNO the IR light will be further
attenuated by $\sim$2 m of water between the PMTs and the acrylic sphere.

##  5 Tests in the Double Chooz Near Detector

During the dismantling of the Double Chooz (DC) near detector, the opportunity
arose for a sensor test under operating conditions perfectly matching the
foreseen usage for the laser in the OSIRIS detector and in the dedicated
liquid level measurement pipes of the JUNO CD. The level meter was operated
during one hour in the Neutrino Target (NT) chimney. The measured values for
the distance to the NT’s highly transparent LS are depicted in Figure YYY. The
obtained distance to the LS of (2019.2 $\pm$ 0.1(stat.)) mm is in well
agreement with the expectation (200 $\pm$ 1.5) cm calculated from the live
readings of the hydrostatic pressure sensors on the bottom of the NT. The
stability of the sensor readings is remarkable, since the sensor mount had no
vibration decoupling. During the test, no contradictions could be found
regarding the usability of the sensor in JUNO and OSIRIS.

Figure 10: Distance to the LS surface in the DC near detector NT measured with
a sampling rate of 1 Hz during a sensor runtime of 1 h. The data shown in the
plot are the mean values of 10 sampling points.

ToDo: Referenzen für Double Chooz raussuchen. Gibt es was besseres zum Tamago
und dem LMS in DC als: M. Franke, The neutrino experiment Double Chooz and
data analysis with the near detector, PhD Thesis, Technische Universität
München (2016)

##  6 Conclusions

The filling level determination inside the acrylic vessel of OSIRIS will be of
great importance to prevent damage to the pre-detector in consequence of the
potential build-up of hydrostatic pressure differences. To maintain the
radiopurity of the inserted liquid scintillator, the measurement will be
performed under the application of the TruSense S310 laser sensor. This
optical liquid level measurement system measures the filling level in a
contactless way using infrared laser pulses to determine the distance to the
liquid surface with the TOF technique. The measurement is executed without the
usage of a guided float in direct contact to the liquid scintillator, as it
was the usual proceeding in former experiments. We have performed several
tests to review the functionality of the TruSense S310 and to assess its
applicability for JUNO and OSIRIS. Averaging single measurement results, we
were able to state the precision of the filling level measurement performed by
the laser sensor as $\pm 5$ mm which fully complies to the requirements of
OSIRIS. The long-term stability of the laser sensor performance is also
guaranteed as a consequence of the possibility to perform high frequent
measurements in which stable means are realized within seconds. Furthermore we
introduced the concept dedicated to the integration of the optical filling
level measurement system into the OSIRIS LHS and presented the design of
several component parts needed for its onsite installation. In particular we
evaluated the optimal dimensions of the adapter flange used to realize the
interface between sensor and the OSIRIS construction. This interface will be
sealed by a protective window whose design also was reviewed with concern to
its thickness and material. In addition, we have stated the minimum inner
diameter of the pipe establishing a line-of-sight connection from the sensor
into the detector vessel as $10$ cm $\approx 4$ inches on the basis of the
results of dedicated tests. Furthermore we verified that a simultaneous
operation of the TruSense S310 and PMTs is feasible, as running PMTs are not
harmed by the irradiation of the infrared laser pulses emitted by the laser
sensor.

## Acknowledgments

This work benefited substantially from the support by the ETAP Engineer Rainer
Orthegraven and ETAP Technician Karl-Heinz Geib. Therefore, we are very
greatful.
We are also very greatful for the help during the PMT irradiation tests
performed in the AG Lachenmaier of the University of Tübingen by M.Sc. Tobias
Sterr.
The development of this liquid level monitoring system was funded by the DFG
Research Unit FOR2319 “JUNO” and the Maier Leibnitz Laboratorium (MLL).